This science treats of the nature, weight, and pressure of the air, and the effects arising from it.

The air is that thin transparent fluid body in which we live and breathe. It encompasses the whole earth to a considerable height; and, together with the clouds and vapours that float in it, is called the atmosphere. The air is justly reckoned among the number of fluids, because it has all the properties by which a fluid is distinguished. (See HYDROSTATICS.) For it yields to the least force impressed, its parts are easily moved among one another, it presses according to its perpendicular height, and its pressure is every way equal.

That the air is a fluid, consisting of such particles as have no cohesion betwixt them, but easily glide over one another, and yield to the slightest impression, appears from that ease and freedom with which animals breathe in it, and move through it without any difficulty or sensible resistance.

But it differs from all other fluids in the three following particulars. It can be compressed into a less space than what it naturally possesses, which no other fluid can. 2. It cannot be congealed or fixed, as other fluids may. 3. It is of a different density in every part, upward from the earth's surface, decreasing in its weight, bulk for bulk, the higher it rises; and therefore must also decrease in density. 4. It is of an elastic or springy nature, and the force of its spring is equal to its weight.

That air is a body, is evident from its excluding all other bodies out of the space it possesses; for, if a glass jar be plunged with its mouth downward into a vessel of water, there will but very little water get into the jar, because the air of which it is full keeps the water out.

As air is a body, it must needs have gravity or weight; and that it is weighty, is demonstrated by experiment. For, let the air be taken out of a vessel by means of the air pump; then, having weighed the vessel, let in the air again; and upon weighing it, when re-filled with air, it will be found considerably heavier. Thus, a bottle that holds a wine quart, being emptied of air, and weighed, is found to be about 17 grains lighter than when the air is let into it again; which shows that a quart of air weighs 17 grains. But a quart of water weighs 14625 grains; this divided by 17, quotes 860 in round numbers; which shews, that water is 860 times as heavy as air near the surface of the earth.

As the air rises above the earth's surface, it grows rarer, and consequently lighter, bulk for bulk. For since it is of an elastic or springy nature, and its lowermost parts are pressed with the weight of all that is above them, it is plain that the air must be more dense or compact at the earth's surface, than at any height above it; and gradually rarer the higher up. For, the density of the air is always as the force that compresseth it; and therefore, the air towards the upper parts of the atmosphere being less pressed than that which is near the earth, it will expand itself, and thereby become thinner than at the surface of the earth.

Dr Cotes has demonstrated, that if altitudes in the air be taken in arithmetical proportion, the rarity of the air will be in geometrical proportion. For instance,

| Altitude | Air | |----------|-----| | 7 | | | 14 | | | 21 | | | 28 | | | 35 | | | 42 | | | 49 | | | 56 | | | 63 | | | 70 | | | 77 | | | 84 | | | 91 | | | 98 | | | 105 | | | 112 | | | 119 | | | 126 | | | 133 | | | 140 | |

And hence it is easy to prove by calculation, that a cubic inch of such air as we breathe, would be so much rarified at the altitude of 500 miles, that it would fill a sphere equal in diameter to the orbit of Saturn. The weight or pressure of the air is exactly determined by the following experiment.

**The Toricellian Experiment.**

Take a glass tube about three feet long, and open at one end; fill it with quicksilver; and putting your finger upon the open end, turn that end downward, and immerse it into a small vessel of quicksilver, without letting in any air; then take away your finger, and the quicksilver will remain suspended in the tube 29½ inches above its surface in the vessel; sometimes more, and at other times less, as the weight of the air is varied by winds and other causes. That the quicksilver is kept up in the tube by the pressure of the atmosphere upon that in the basin, is evident; for, if the basin and tube be put under a glass, and the air be then taken out of the glass, all the quicksilver in the tube will fall down into the basin; and if the air be let in again, the quicksilver will rise to the same height as before. Therefore the air's pressure on the surface of the earth, is equal to the weight of 29½ inches depth of quicksilver all over the earth's surface, at a mean rate.

A square column of quicksilver, 29½ inches high, and one inch thick, weighs just 15 pounds, which is equal to the pressure of air upon every square inch of the earth's surface; and 144 times as much, or 2160 pounds, upon every square foot; because a square foot contains 144 square inches. At this rate, a middle-sized man, whose surface may be about 14 square feet, sustains a pressure of 30240 pounds, when the air is of a mean gravity: a pressure which would be insupportable, and even fatal to us, were it not equal one very part, and counterbalanced by the spring of the air within us, which is diffused through the whole body, and reacts with an equal force against the outward pressure.

Now since the earth's surface contains (in round numbers) 200,000,000 square miles, and every square mile 27,878,400 sq. ar. feet, there must be 5,575,680,000,000,000 square feet on the earth's surface; which multiplied by 2160 pounds (the pressure on each square foot) gives 12,043,468,800,000,000,000 pounds for the pressure or weight of the whole atmosphere.

When the end of a pipe is immersed in water, and the air is taken out of the pipe, the water will rise in it to the height of 33 feet above the surface of the water in which it is immersed; but will go no higher; for it is found that a common pump will draw water no higher than 33 feet above the surface of the well; and unless the bucket goes within that distance from the well, the water will never get above it. Now, as it is the pressure of the atmosphere, on the surface of the water in the well, that causes the water to ascend in the pump, and follow the piston or bucket, when the air above it is lifted up, it is evident, that a column of water 33 feet high, is equal in weight to a column of quicksilver of the same diameter 29½ inches high; and to as thick a column of air, reaching from the earth's surface to the top of the atmosphere. See Hydrostatics.

**Of the Barometer.**

In serene calm weather, the air has weight enough to support a column of quicksilver 21 inches high; but in tempestuous stormy weather, not above 28 inches. The quicksilver, thus supported in a glass tube, is found to be a nice counterbalance to the weight or pressure of the air, and to show its alterations at different times. And being now generally used to denote the changes in the weight of the air, and of the weather consequent upon them, it is called the barometer, or weather-glass.

The mercury will stand at the same height either in an inclined barometer or in an erect one.

If the mercury at any time stands at the height of 30 inches in the barometer D, (Plate CXLV. fig. 6.) then by inclining this barometer into the position E, the perpendicular height of the mercury will not be altered; for it will still stand at the height of 30 inches; so that if the level AB is 30 inches from the surface FG, the mercury will stand at this level, either in the erect tube D or in the inclined one E. Now here it is evident, that if NL is the height of the mercury when the tube is erect, and NM is the height of the mercury in the same tube or an equal one when it is inclined, there must be more mercury in the inclined tube than there is in the erect one. For we may consider NL as the side, and NM as the diagonal, of a right-angled parallelogram. But the diagonal of a right angled parallelogram is longer than the side. Therefore, though either L or M are at the same perpendicular distance from FG, yet NM will be longer than NL. Since then the column of mercury is longer in the inclined tube than in the erect one, there will be more mercury in the inclined than in the erect one. The question therefore is, How the pressure of the atmosphere can support a greater quantity of matter in one situation of the tube than in the other. We cannot say, that though in the inclined tube there is a greater quantity of mercury than in the erect one, yet a part of this greater weight will be supported by the side of the tube as by an inclined plane. The true answer is, that the column of air which supports the mercury in the inclined tube, is greater than the column which supports it in the erect one. The height of the column of air is indeed the same in both cases; for in either case it is equal to the height of the atmosphere. But the base of the column of air, and consequently its weight, is greater when the tube is inclined than when it is erect. For the base of the column of air which supports the mercury in the tube, is equal to as much of the stagnant mercury's surface as the base of the tube covers. Now, if the diameters of the tubes D and E are equal; the base of the inclined tube E will cover a greater part of the surface FG than the erect tube D covers, or the base of the tube E will be greater than the base of the tube D. For the contents of the inclined tube are greater than those of the erect one, as has been shewn already. But the column of mercury in each of the tubes are cylinders of the same height. Therefore their bases are as their contents. Eucl. b. XII. prop. ii. Since then the contents or the weights of mercury in each tube are as their bases, when their heights are equal; and the base of the column of air, which supports the mercury, and consequently the weight of this column, is proportional to the base of the tube; it follows, that the weight of the air will always be proportional to the weight of the mercury when it stands at a given height, whether the tube is inclined or erect.

Though we have here shewn, that the contents of the inclined tube are greater than those of the erect one, and consequently that their bases must be unequal, since their perpendicular heights are the same; yet it will not be improper to shew what we mean by the base of the inclined tube, or rather of the inclined column of mercury, and why this should be greater than the base of the erect one. Now by the base of the inclined column we do not mean the bottom of the inclined tube, but the lowest horizontal section of it. Thus, if we consider the surface FG as a plane passing through the two tubes D and E, this plane will cut the tube D perpendicularly, and the tube E obliquely. But a perpendicular section of a cylinder is a circle, and an oblique section of it is an ellipse. Therefore the base of the erect column is circular, and the base of the inclined tube is elliptical.

Now, by the supposition, the two tubes have equal diameters, and consequently the shorter axis of the elliptical base will be equal to the diameter of the circular one.

There is another sort of inclined barometer, such as one as ABR, (ibid., fig. 7.) which is erect for 28 inches from A to B, and then is inclined from B to C. The mercury will stand at the same height in this barometer, as if it had been a flat one AS; for the column of air pressing at the base A would be the same in either case: and though there is more mercury in the tube ABR than there would be in the tube ABS; yet, supposing the mercury to stand at the same level DC in either case, the pressure of the mercury downwards will in either case be the same. For, the pressure of fluids is as their base and perpendicular height: and here the base A is the same, and the perpendicular height is the same, whether the tube is erect all the way up as AS, or is inclined at the top as ABR.

The advantage which is proposed by these diagonal barometers, as they are called, is to make the variation of the mercury greater, and consequently more apparent, upon a given change in the weather. Thus suppose AB or 23 inches to be the least height of the mercury, and AD or 31 inches to be the greatest height of it: then the whole variation will be within the compass BD, or 3 inches. But if the barometer, instead of being erect at the top, is inclined into the position BC; then, as the mercury stands at the same perpendicular height in this diagonal barometer as in an erect one, AB will be the least height, and ABC will be the greatest height, since D and C are on the same level or at the same perpendicular distance from A. Now though BD, one side of the parallelogram, is but 3 inches long; yet BC may be 30 inches long, or more; and consequently since AB is the least height, and ABC is the greatest height, the variation of the mercury will be much greater than in an erect barometer; in particular, if BC is 30 inches long, the variation will be 30 inches instead of 3, or will be 10 times greater in the diagonal barometer than it would have been in an erect one.

The barometer stands at the same perpendicular height, whether the tube is large or small.

If the mercury stands at the same height either in the large tube C or in the small tube D, there must be more mercury in the large one than in the small one. But since the heights are equal, the quantities of mercury contained in these tubes will be as their bases. Now since the columns of air, by which the mercury is supported in these tubes, are as the respective bases of the tubes, the columns of air will be proportional to the weights in each tube, when the perpendicular heights are equal.

But though the heights of the mercury would be the same in small tubes as in large ones, if, as we must suppose in the proposition, the mercury moved equally free in both; yet in fact, upon any change of weather, the variation will be greater in a large tube than in a small one: because, in a large tube, the weight of mercury is so great, that the motion of it will not be hindered by any attraction or repulsion of the glass upon it; whereas, in a small tube, where the weight of mercury is less, the action of the glass is considerable in proportion to that weight, and consequently the variations will be less upon a given change of the weather.

The barometer will commonly be low in rainy weather.

From what has been said already about the barometer, it appears, that the mercury will be low when the weight of the atmosphere is diminished; and such a diminution of the atmosphere will occasion rain. Therefore, since rain is occasioned by the same cause that makes the mercury fall, the barometer will commonly stand low in rainy weather.

The barometer is the lowest of all in violent storms of wind.

When the air moves horizontally with a great velocity, as it does in violent storms of wind, its weight, or rather its pressure downwards occasioned by its weight, will be diminished. For as any heavy body may have such a velocity given it, when it is thrown down horizontally, as may either carry it quite off from the earth's centre, or with a velocity as will make it move round the earth in a circle without either departing from the centre or approaching to it; so every degree of velocity given to the air will make it tend or press less towards the centre; and for this reason, as the mercury in the barometer will be less pressed, the height of it in the tube will be less in storms than it is any other time.

When a storm of wind is over, the mercury will rise very fast:

Because as the horizontal velocity of the air ceases, the pressure downwards will be suddenly restored, and consequently the mercury in the barometer will keep rising as this pressure is restored.

Of the Thermometer.

The variations of different thermometers are seldom equal, upon equal variations of heat or cold.

A thermometer is a well-known instrument for estimating different degrees of heat or cold. It consists of a tube or stem, with a hollow ball at one end of it. The cavity of the ball, and part of the tube, is filled with spirits of wine, or with linseed oil, or with mercury. The upper end of the tube is commonly sealed hermetically. But in sealing this end, the liquor in the thermometer is raised by heating it till it almost fills the tube; so that when it is sealed, and the liquor contracts again as it cools, there will be a vacuum left in the upper part of the tube. Any of these fluids will rarify by heat, and will contract again when they cool; and consequently in warm weather, the spirits, or the oil, or the mercury, whichever the thermometer is made of, will stand higher than in cold weather.

Thus far thermometers may be said to vary alike: they will either rise or sink from the same causes. But then, upon an equal increase of heat, they seldom vary equally, though they are made of the same liquor. One thermometer made with spirits of wine may vary upon an equal increase of heat much more than another that is made with the same sort of spirits: so that if one rises an inch, another may rise but \( \frac{1}{2} \) or \( \frac{1}{4} \) inch.

The variation of a thermometer is directly as the capacity of of the ball; and inversely as the base of the stem. First, If the base of the stem or cylindrical tube is given, the variation, when the spirits are equally warmed, will be directly as the capacity of the ball. For when the spirits are equally warmed, and consequently are equally rarefied in the balls of two different thermometers, whatever proportion the bulk of the spirits in one ball bears to the bulk of the spirits in the other ball before they were rarefied, the same proportion these bulks will still bear to each other after they are rarefied. Thus, if one ball is double the other, and consequently the bulk of spirits in one is double the bulk of spirits in the other before they were warmed; then, upon being warmed equally, their densities will diminish equally. But if their densities diminish equally, their bulks will still have the same proportion to each other; or the bulk of spirits in one thermometer will still be double the bulk in the other. But if the bulks continue in the same proportion to each other, after they are swelled as they were before; the spirits must swell in proportion to their respective bulks, or the spirits in one must swell twice as much as in the other. But if the spirits swell in this proportion, and by swelling rise into equal tubes in each, they must rise twice as high in the tube of one of these thermometers as they do in the tube of the other. And so, in all other instances, the spirits, upon being equally warmed, will swell in proportion to their bulk, that is, in proportion to the capacity of the ball that contains them. But the heights, to which they rise in equal tubes, will be as the increase of their bulk. Therefore the heights to which they rise, or the variations in equal degrees of heat, will be as the capacity of the ball, when the tubes are equal. We have here supposed that the spirits in the balls of the thermometers are equally heated quite through. In sudden changes of heat and cold, it will be otherwise: for the spirits in a small ball will be sooner heated quite through than in a large one. And consequently, if the heat does not last long enough to warm the spirits in a large ball as much as they are warmed in a small one, the spirits will not be equally rarefied in both, and will not swell in proportion to their respective bulks; but those in the small ball will swell more in proportion than those in the large one. Secondly, If the balls are equal, the variations will be inversely as the bases of the stems. For if the balls are equal, then, upon being equally heated, the spirits contained in them will swell equally; and consequently equal quantities will rise into the stems. Now the spirits which rise into a cylindrical stem are a cylindrical column. But the heights of equal cylinders are inversely as their bases. Therefore, when the balls are equal, and equal cylinders of spirits rise into the stems, the heights to which they rise, or the variations, will be inversely as the bases of the stems.

An universal scale may be made, by which the variations of different thermometers may be compared with one another.

Let the ball of a thermometer be put into water when it is beginning to freeze, or, which is the same as to heat or cold, into snow when it is beginning to melt; and let the place where the fluid in the thermometer stands be marked. The place where the fluid stands in such a trial is the freezing point. Let the ball of the same thermometer be put into water just hot enough to let wax, that swims upon it, begin to coagulate. This again is another determinate degree of heat, and is to be marked upon the thermometer.

Divide the distance between these two points into 110 equal parts; and each of these parts we call a degree. Now a thermometer often sinks lower than the freezing point; because the cold is frequently more intense than what is just sufficient to make water freeze: for this reason, the scale must not begin from the freezing point. This point, therefore, should not be marked 0, nor should the point where melted wax begins to coagulate be marked 110. In this scale, which from the inventor is called Fahrenheit's scale, the freezing point is marked 32; and then the point, where melted wax begins to coagulate, being 110 degrees above it, must be marked 142. When the length of a degree is thus found in one part of the scale, 32 degrees of the same length are set off below the freezing point, and as many such degrees as we please are set off above the point where melted wax begins to coagulate. If the thermometer is made with spirits of wine, only 33 degrees need be set off or marked above 142: and then the scale will begin from 0; 32 degrees will be the freezing point; 142 will be the point where melted wax begins to coagulate; and 142+33=175 degrees will be the highest point marked in the scale. The reason why no higher degree need be marked in a scale applied to a thermometer made with spirits, is, that at this degree of heat the spirits will boil, and consequently the thermometer would burst. But if the thermometer is made with mercury, the scale should contain at least 212 degrees from the bottom to the top, or 32 degrees below the freezing point, and 180 above it. The heat of boiling water, at the middle height of the mercury in the barometer, or in the middle weight of the atmosphere, will raise the mercury in the thermometer to 212 degrees, or 180 degrees above the freezing point. A thermometer made with mercury will not burst in such a degree of heat as this; for mercury requires a greater degree to make it boil.

In thermometers with such a scale, or, as they are called, in Fahrenheit's thermometers, the greatest degree of heat in the external parts of the human body is commonly about 96. Boerhave imagined that air, if its heat exceeded 80 or 90 degrees at most, would be destructive to the life of animals. But in this he was mistaken. For in the year 1732 the thermometer in Pennsylvania was at the height of 96 or 97; and in the year 1734 the height of it at Petersburg was 98 degrees. The thermometer in our own climate is scarce ever higher than 78 degrees, and seldom lower than 18; so that we may reckon 48 degrees to be the middle temperature of our air.

The variations of different thermometers, though they are not equal, may be compared with one another by Fahrenheit's scale. For each degree upon different thermometers is proportional to their respective variations; and consequently, though in equal heats one may vary more than another, yet each will vary an equal number of degrees. Thus, if, upon any given increase of heat, one thermometer will vary twice as much as another, then the distance between the freezing point and the point where melted wax begins to coagulate will be twice as great, or 110 degrees will be twice as long, in one as in the other. Therefore each degree will be twice as long in the former thermometer as in the latter. But by the supposition, one of these thermometers in a given degree of heat will vary twice as much as the other does; and consequently, whatever heat raises the former one degree, will likewise raise the latter one degree. If the ball of a thermometer is dipped into hot water, the fluid in the thermometer will sink a little before it begins to rise.

Not only fluids, such as spirits, oil, or mercury, but likewise glass, or iron, or almost any hard bodies, will expand when they are heated, and will contract again when they grow cold. Now, when the ball of a thermometer is dipped into hot water, the heat will be communicated to the glass of which the ball is made, before it is communicated to the fluid contained in the ball. By this means the ball will be expanded, and the capacity of it will be increased, so that some of the fluid will sink out of the item into it. But when the ball has been long enough in the water for the fluid within it to be heated, this fluid will be expanded; and then it will rise into the item, and will continue to rise as the heat increases.

Of Sound.

Sound, in the body that produces it, is a trembling motion: this motion is communicated to the air, and the air conveys it to the ear.

When any elastic body is struck, so as to produce a sound, the body, or some part of it, is made to vibrate. This is evident to sense in the strings of a violin or harpsichord; for either the eye may see, or the hand may feel, the trembling of the strings, when by striking them they are made to sound. See Musick.

If a bell is struck by its clapper on the inside, the bell is made to vibrate. The base of the bell is a circle; but by striking any part of this circle on the inside, the part which is struck will fly out a little way, so that the diameter, which passes through this part of the circular base, will become longer than another diameter which crosses this at right angles. Therefore by the stroke the base will be changed into an ellipse, whose longer axis will pass through the part against which the clapper struck. But the elasticity of the bell will restore the figure of the base, and will make the part which was forced out of its place return back. This part in returning will acquire velocity in the same manner as an elastic string would in the same circumstances. And since it acquires velocity in returning to its place, it will not stop at that place, but will over-run it. Thus the circular figure of the base will be changed into an ellipse again; only now the shorter axis will pass through the part that was first struck. If the bell was to be struck at first by a hammer on the outside, the part struck would move inwards; and such a motion would likewise change the base into an ellipse: only in this case the shorter axis of the ellipse would pass through that part where the blow was given. The elasticity of the bell will restore its figure; and as the part which was struck will acquire velocity in returning to its proper situation, the acquired velocity will not suffer it to rest there, but will carry it farther out from the opposite side; and the base will by this means be again changed into an ellipse, having the longer axis at that part where the blow was hit given. Thus we have seen, that wherever the bell is struck, the parts of it will perform one vibration; the part, which is struck will yield to the blow; the elasticity of the bell will bring it back to its former situation; in returning, it will acquire velocity; and as far as the blow had driven it one way, so far the acquired velocity will carry it the other. But since, after one vibration is thus performed, the figure of the base will be elliptical; the parts of the bell will vibrate a second time; and so on, in the same manner that an elastic string vibrates.

The same stroke which makes a bell vibrate makes it found too; and as the vibrations decay, the sound grows weaker. Our senses may convince us that the parts of a bell are in a trembling or vibratory motion whilst the bell sounds: for if we lay our hand upon it, we may feel it jar; or if small straws or pieces of paper are thrown upon it, we may see that the jarring or trembling of the bell will put them in motion.

But the air must convey this vibratory motion to the ear; for otherwise, though the founding body is made to vibrate, no sound will be heard. Thus if a bell is rung in the receiver of an air-pump, the sound will grow weaker as the air is exhausted; and when all the air is drawn out of the receiver, no sound at all will be heard. When the air is admitted again into the receiver, the sound will at the first entrance of the air begin to be heard, and will grow louder as more air returns. If the bell was to be rung in like manner in a vessel where the air is condensed, the sound of it would be much louder than it is in common air. And accordingly, when divers are let down to any great depth of water, because the air in the diving-bell is much condensed, they seem to one another to speak much louder than usual.

The intensity of sound, at different distances from the founding body, is inversely as the squares of the distances.

Sounds may differ from one another, both in respect of their tone, and in respect of their intensity or strength. In respect of their tone, they are distinguished into grave and acute; in respect of their intensity, they are distinguished into loud or strong, and low or weak. The tone of any sound depends upon the time that an impression continues, and is not altered by the distance of the ear from the founding body. But the intensity or strength of any sound depends upon the force with which the particles of air, as they are condensed, strike the ear; and this force is found to be different at different distances, so that a sound which is very loud if we are near the body that produces it, would be weaker if we were farther from it, and our distance from it may be so great that we cannot hear it at all.

The proportion in which the intensity of sound decreases, as the distance of the ear from the founding body increases, is this: If the different distances at which the ear is placed are to one another as 1, 2, 3, 4, 5; then the squares of those distances are 1, 4, 9, 16, 25; and the intensity of sound will be inversely as these squares, or as the reciprocals of these squares; that is, the strength of the sound will decrease in the same proportion with the fractions, \( \frac{1}{1}, \frac{1}{4}, \frac{1}{9}, \frac{1}{16}, \frac{1}{25} \), which are the reciprocals of 1, 4, 9, 16, 25. This is what we mean when we say that the intensity of sound is inversely as the squares of the distances, or that it decreases in the departure of the ear from the founding body in the same proportion that the squares of the distances increase.

The intensity of sound decreases as the vibrations in the founding body grow weaker.

If an elastic string was to stop all at once, the sound produced by it would cease immediately. But if the vibrations of the string decay gradually, the sound will likewise keep growing weaker, till it becomes too weak to be heard. The string performs all its vibrations from the first to the last in equal times, and consequently each pulse that the string produces is produced in an equal time, and upon that account each pulse from the first to the last will have the same thickness. But when the thickness of the pulse is given, the quantity of air, or number of particles, by which the ear is struck, will likewise be given; and the moment with which it is struck, or the intensity of the sound, will be as the velocity with which the particles move. Now the velocity of the string is successively communicated to the particles of air, as they are made to vibrate. Therefore, as the velocity of the string decays, the velocity of the particles, and consequently the intensity of the sound, will likewise decay.

The intensity of sound is increased by a speaking-trumpet.

When a man speaks without such a trumpet, the pulses, as they are produced, dilate themselves in all directions, or the motion is immediately communicated to the air all round him. But if he speaks in a trumpet, his voice, that is, the motion produced by his voice, is confined to the small portion of air contained in the trumpet. For this reason, as there are fewer particles to be moved than there are when the motion dilates itself immediately in all directions, the motion that is communicated will be greater, and consequently, when the voice comes out of the trumpet, its intensity or strength will be greater, than it would have been if it had been propagated in all directions at first.

Sound moves with the same velocity at all distances from the sounding body.

The sound of a cannon, or of a bell, moves at the rate of 1142 feet in a second at all distances from the gun or the bell. If it moves at this rate for the first mile, it would move just at the same rate for the second mile: so that a person who is within one mile of the cannon when it is discharged, will hear the report just as soon again as another who is at the distance of two miles. The velocity of the sound does not decrease as it is propagated forwards, but continues the same from the first to the last. This property of sound has been proved by repeated experiments.

When sound strikes against an obstacle, it will be reflected.

By sound we here mean the pulses in the air, which are, properly speaking, the causes of sound. If these pulses in their progress strike upon any obstacle, such as a rock, a thick wood, or the side of a building, the air, which is condensed at the obstacle, is prevented from expanding itself forwards, or from propagating the sound beyond the obstacle. Therefore, in expanding itself, the motion, which would otherwise have been propagated forwards, will be returned from the obstacle; and a person, who is placed so as to receive the pulses in their return, will hear the sound by reflection. Such a reflected sound is called an echo.

The number of syllables which an echo repeats distinctly depends upon the distance of the obstacle from whence the sound is reflected. The syllables that we hear distinctly repeated are those which are returned after we have done speaking. Therefore, if the obstacle is so near to us, that the first syllable we speak will be returned before we can speak a second, no reflected sound at all can be heard distinctly; because the direct and reflected sound, or the voice and the echo, will be confounded with one another. If the obstacle is at such a distance, that five syllables may be spoken before one will be returned; then if we speak a sentence consisting of ten syllables, the first five will be reflected whilst we are speaking the five last, so that in speaking the five last syllables the voice will be confounded with the echo of the five first, and we shall hear the echo of only the five last syllables distinctly, because these only will be returned after we have done speaking. But if the obstacle is at such a distance, that we may speak ten syllables before the first of them will return to the speaker; then if we speak a sentence of only ten syllables, we shall have done speaking before the echo begins, and consequently we may hear the whole sentence distinctly repeated by the echo.

Sometimes the same sound is frequently repeated by an echo. This happens when there are several obstacles at different distances. For though there are several obstacles, yet if all of them are at the same distance, the sound will be returned from them all at once and the same time; and consequently the several reflected sounds will be heard together, and will make but one echo. But if the obstacles are at different distances, each will return the sound at a different time, and as many echoes will be heard as there are obstacles that produce them.

The Diving Bell.

The air in a diving-bell is compressed by the weight of the atmosphere before the bell is let down into the water. But when it has sunk 35 feet below the surface, the air contained in it is compressed by the weight of the atmosphere as before, and by the weight of 35 feet of water besides, which is equivalent to another atmosphere. Therefore the compressing force at this depth is doubled, and consequently the air in the bell will then be twice as dense as the common air that we breathe. As much air, likewise, as just fills the bell, when it is at the surface of the water, will, at the depth of 35 feet, only fill half of it; for as the compressing force is doubled, the same quantity of air will be reduced to half its usual dimensions. For this reason, the water would rise into the bell, through the base or bottom of it, which is always open, and would fill the other half of it, if there was not a contrivance for bringing down additional air enough to force out this water, and to keep the whole capacity of the bell full of air. However, the air which fills it will, at the depth of 35 feet, have twice the density that common air has; and at the depth of 70 feet, where it will be compressed by the weight of another atmosphere, it will have triple the density of common air.

We shall here give a short account of the contrivance for bringing down additional air to the diving-bell; because it will serve to show, that if a vessel full of air is sunk into water, and the water communicates with the air in the vessel, then the pressure upon that air will be so much the greater as the vessel is sunk farther below the surface of the water. The contrivance is this. A barrel is made use of, which has one bung-hole in the lower part of it, and another in the upper part. A leather pipe is fastened to the hole in the upper part; and this pipe is so long, that, when it hangs down on the outside of the barrel, its orifice reaches below the bung hole in the lower part. If this barrel, by the help of weights fastened to it, is made to sink with its bottom downwards, the water, by pressing against the lower bung hole, will condense the air contained in the barrel: for, notwithstanding this pressure, none of the air can escape through the upper hole, because it is kept in by a greater pressure against the orifice of the leathern pipe which hangs below the bottom of the barrel, and consequently, being deeper in the water, sustains a greater pressure than what acts against the lower bung-hole. If the barrel is let down in this manner, till it gets below the bell, and then the end of the leathern pipe is lifted up into the bell; the lower bung-hole will then be more pressed than the orifice of the pipe; and therefore the air contained in the barrel will be driven up through the pipe, and will be received into the bell. And because the barrel is deeper in the water than the bell is, the water will press more against the base of the barrel to force the air out of it than it does against the base of the bell; for which reason the air will rush out of the barrel with force enough to drive out any water which had risen into the bell whilst it was descending.

By the same contrivance, fresh air is brought down to the bell as often as there is occasion for it. The air, which has been heated by frequently breathing it, is let out through a stop cock in the top of the bell and rises in bubbles to the surface of the water, whilst fresh air is received from the leathern pipe of a barrel contrived in the manner already described.

Air necessary for the life of animals.

All common air is impregnated with a certain kind of vivifying spirit or quality, which is necessary to continue the lives of animals; and this, in a gallon of air, is sufficient for one man during the space of a minute, and not much longer.

This spirit in air is destroyed by passing through the lungs of animals; and hence it is, that an animal dies soon after being put under a vessel which admits no fresh air to come to it. This spirit is also in the air which is in water; for fish die when they are excluded from fresh air, as in a pond that is closely frozen over. And the little eggs of insects stopped up in a glass, do not produce their young, though assisted by a kindly warmth. The seeds also of plants mixed with good earth, and inclosed in a glass, will not grow.

This enlivening quality in air is also destroyed by the air's passing through fire; particularly charcoal fire, or the flame of sulphur. Hence smoking chimneys must be very unwholesome, especially if the rooms they are in be small and close. See Smokes.

Air is also vitiated, by remaining closely pent up in any place for a considerable time; or perhaps, by being mixed with malignant fumes and particles flowing from the neighbouring bodies; or lastly, by the corruption of the vivifying spirit; as in the holds of ships, in oil-cisterns, or wine-cellars, which have been shut up for a considerable time. The air in any of them is sometimes so much vitiated, as to be immediate death to any animal that comes into it.

Air that has lost its vivifying spirit is called damp, not only because it is filled with humid or moist vapours, but because it deadens fire, extinguishes flame, and destroys life. The dreadful effects of damps are sufficiently known to such as work in mines.

The atmosphere is the common receptacle of all the effluvia or vapours arising from different bodies; of the fumes and smoke of things burnt or melted; the fogs or vapours proceeding from damp watery places; and of effluvia from sulphurous, nitrous, acid, and alkaline bodies. In short, whatever may be called volatile, rises in the air to greater or less heights, according to its specific gravity.

When the effluvia which arise from acid and alkaline bodies meet each other in the air, there will be a strong conflict or fermentation between them; which will sometimes be so great, as to produce a fire; then if the effluvia be combustible, the fire will run from one part to another, just as the inflammable matter happens to lie.

Any one may be convinced of this, by mixing an acid and an alkaline fluid together, as the spirit of nitre and oil of cloves; upon the doing of which, a sudden ferment, with a fine flame, will arise; and if the ingredients be very pure and strong, there will be a sudden explosion.

Whoever considers the effects of fermentation, cannot be at a loss to account for the dreadful effects of thunder and lightening; (see Electricity:) For the effluvia of sulphurous and nitrous bodies, and others that may rise into the atmosphere, will ferment with each other, and take fire very often of themselves; sometimes by the affluence of the sun's heat.

If the inflammable matter be thin and light, it will rise to the upper part of the atmosphere, where it will flash without doing any harm; but if it be dense, it will lie nearer the surface of the earth, where taking fire, it will explode with a surprising force; and by its heat rarefy and drive away the air, kill men and cattle, split trees, walls, rocks, &c. and be accompanied with terrible claps of thunder.

The heat of lightening appears to be quite different from that of other fires; for it has been known to run through wood, leather, cloth, &c. without hurting them, while it has broken and melted iron, steel, silver, gold, and other hard bodies. Thus it has melted or burnt a slender sword, without hurting the scabbard; and money in a man's pocket, without hurting his cloaths: the reason of this seems to be, that the particles of the fire are so fine, as to pass through soft loose bodies without dissolving them; whilst they spend their whole force upon the hard ones.

It is remarkable, that knives and forks which have been struck with lightening have a very strong magnetical virtue for several years after.

Much of the same kind with lightening, are those explosions, called fulminating or fire-damps, which sometimes happen in mines; and are occasioned by sulphureous and nitrous, or rather oleaginous particles, rising from the mine, and mixing with the air, where they will take fire by the lights which the workmen are obliged to make use of. The fire being kindled will run from one part of the mine to another, like a train of gunpowder, as the combustible matter happens to lie. And as the elasticity of the air is increased by heat, that in the mine will consequently swell very much, and so, for want of room, will explode with a greater or less degree of force, according to the density of the combustible vapours. It is sometimes so strong as to blow up the mine; and at other times so weak, that when it has taken fire at the flame of a candle, it is easily blown out.

Air that will take fire at the flame of a candle may be produced thus. Having exhausted a receiver of the air-pump; let the air run into it through the flame of the oil of turpentine: then remove the cover of the receiver; and holding a candle to that air, it will take fire, and burn quicker or slower, according to the density of the oleaginous vapour.

When such combustible matter, as is above-mentioned, kindles kindles in the bowels of the earth, where there is little or no vent, it produces earthquakes, and violent storms or hurricanes of wind when it breaks forth into the air.

An artificial earthquake may be made thus. Take 10 or 15 pounds of sulphur, and as much of the filings of iron, and knead them with common water into the consistence of a paste: this being buried in the ground, will, in 8 or 10 hours time, burst out in flames, and cause the earth to tremble all around to a considerable distance.

From this experiment we have a very natural account of the fire of mount Ætna, Veluvius, and other volcano's, their being probably set on fire at first by the mixture of such metallic and sulphureous particles.

Of the Air-Pump.

The air-pump being in effect the same as the water-pump, (see Hydrostatics,) whoever understands the one will be at no loss to understand the other.

Having put a wet leather on the plate LL of the air-pump, (Plate CXLV. fig. 8.) place the glass receiver M upon the leather, so that the hole i in the plate may be within the glass. Then, turning the handle F backward and forward, the air will be pumped out of the receiver; which will then be held down to the plate by the pressure of the external air or atmosphere. For, as the handle (fig. 9.) is turned backwards, it raises the piston de in the barrel BK, by means of the wheel F and rack Dd; and as the piston is leathered too tight, as to fit the barrel exactly, no air can get between the piston and barrel; and therefore, all the air above d in the barrel is lifted up towards B, and a vacuum is made in the barrel from e to b; upon which, part of the air in the receiver M (fig. 8.) by its spring, rushes through the hole i, in the brass plate LL, along the pipe GCG (which communicates with both barrels by the hollow trunk IHK (fig. 9) and, pushing up the valve b, enters into the vacant place be of the barrel BK. For, wherever the resistance or pressure is taken off, the air will run to that place, if it can find a passage.—Then, as the handle F will be turned forward, the piston de will be depressed in the barrel; and, as the air which had got into the barrel cannot be pushed back through the valve b, it will ascend through a hole in the piston, and escape through a valve at d; and be hindered by that valve from returning into the barrel, when the piston is again raised. At the next raising of the piston, a vacuum is again made in the same manner as before, between b and e; upon which more of the air, which was left in the receiver M, gets out thence by its spring, and runs into the barrel BK, through the valve B. The same thing is to be understood with regard to the other barrel AI, and as the handle F is turned backwards and forwards, it alternately raises and depresses the pistons in their barrels, always raising one whilst it depresses the other. And, as there is a vacuum made in each barrel when its piston is raised, every particle of air in the receiver M pushes out another, by its spring or elasticity, through the hole i and pipe GG, into the barrels, until at last the air in the receiver comes to be so much dilated, and its spring so far weakened, that it can no longer get through the valves; and then no more can be taken out. Hence there is no such thing as making a perfect vacuum in the receiver: for the quantity of air taken out at any one stroke, will always be as the density thereof in the receiver: and therefore it is impossible to take it all out, because, supposing the receiver and barrels of equal capacity, there will be always as much left as was taken out at the last turn of the handle.

There is a cock & below the pump-plate, which being turned lets the air into the receiver again; and then the receiver becomes loose, and may be taken off the plate. The barrels are fixed to the frame Eee by two screw-nuts N, which press down the top piece E upon the barrels; and the hollow trunk H (in fig. 9.) is covered by a box, as GH in fig. 8.

There is a glass tube lmmmm open at both ends, and about 34 inches long; the upper end communicating with the hole in the pump-plate; and the lower end immersed in quicksilver at n in the vessel N. To this tube is fitted a wooden ruler mm, called the gage, which is divided into inches and parts of an inch, from the bottom at n (where it is even with the surface of the quicksilver) and continued up to the top, a little below l, to 30 or 31 inches.

As the air is pumped out of the receiver M, it is likewise pumped out of the glass tube lmmmm, because that tube opens into the receiver through the pump-plate; and as the tube is gradually emptied of air, the quicksilver in the vessel N is forced up into the tube by the pressure of the atmosphere. And if the receiver could be perfectly exhausted of air, the quicksilver would stand as high in the tube as it does at that time in the barometer: for it is supported by the same power or weight of the atmosphere in both.

The quantity of air exhausted out of the receiver on each turn of the handle, is always proportionable to the ascent of the quicksilver on that turn; and the quantity of air remaining in the receiver, is proportionable to the defect of the height of the quicksilver in the gage, from what it is at that time in the barometer.

EXPERIMENTS WITH THE AIR-PUMP.

I. To show the resistance of the air.

There is a little machine, consisting of two mills, a and b, (ibid. fig. 10.) which are of equal weights, independent of each other, and turn equally free on their axes in the frame. Each mill has four thin arms or sails fixed into the axis; those of the mill a have their planes at right angles to its axis, and those of b have their planes parallel to it. Therefore, as the mill a turns round in common air, it is but little resisted thereby, because its sails cut the air with their thin edges: but the mill b is much resisted, because the broad sides of its sails move against the air when it turns round. In each axle is a pin near the middle of the frame, which goes quite through the axle, and stands out a little on each side of it; upon these pins, the slider d may be made to bear, and so hinder the mills from going when the strong spring c is set on bend against the opposite ends of the pins.

Having set this machine upon the pump-plate LL (fig. 8.) draw up the slider d to the pins on one side, and let the spring c at bend upon the opposite ends of the pins; then push down the slider d, and the spring acting equally strong on each mill, will set them both going with equal forces and velocities: but the mill a will run much longer than the mill b, because the air makes much less resistance against the edges of its sails than against the sides of the sails of b. Draw up the slider again, and set the spring upon the pins as before; then cover the machine with the receiver M (fig. 8.) upon the pump-plate, and having exhausted the receiver of air, push down the wire P (through the collar of leathers in the neck q) upon the slider; which will disengage it from the pins, and allow the mills to turn round by the impulse of the spring; and as there is no air in the receiver to make any sensible resistance against them, they will both move a considerable time longer than they did in the open air; and the moment that one stops, the other will do so too.—This shews that air resists bodies in motion, and that equal bodies meet with different degrees of resistance, according as they present greater or less surfaces to the air, in the planes of their motions.

2. Take off the receiver M (fig. 11.) and the mills; and having put the guinea a and feather b upon the brass flap c, turn up the flap, and shut it into the notch d. Then, putting a wet leather over the top of the tall receiver AB (it being open both at top and bottom) cover it with the plate C, from which the guinea and feather tongs cd will then hang within the receiver. This done, pump the air out of the receiver; and then draw up the wire f a little, which by a square piece on its lower end will open the tongs cd; and the flap falling down, as at c, the guinea and feather will descend with equal velocities in the receiver; and both will fall upon the pump-plate at the same instant. N.B. In this experiment, the observers ought not to look at the top, but at the bottom of the receiver; in order to see the guinea and feather fall upon the plate; otherwise, on account of the quickness of their motion, they will escape the sight of the beholders.

II. To show the weight of the air.

1. Having fitted a brass cap, with a valve tied over it, to the mouth of a thin bottle or Florence flask, whose contents are exactly known, screw the neck of this cap into the hole i of the pump-plate; then, having exhausted the air out of the flask, and taken it off from the pump, let it be suspended at one end of a balance, and nicely counterpoised by weights in the scale at the other end; this done, raise up the valve with a pin, and the air will rush into the flask with an audible noise; during which time, the flask will descend, and pull down that end of the beam. When the noise is over, put as many grains into the scale at the other end as will restore the equilibrium; and they will show exactly the weight of the quantity of air which has got into the flask, and filled it. If the flask holds an exact quart, it will be found, that 17 grains will restore the equipoise of the balance, when the quicksilver stands at 29½ inches in the barometer; which shews, that when the air is at a mean rate of density, a quart of it weighs 17 grains; it weighs more when the quicksilver stands higher, and less when it stands lower.

2. Place the small receiver A (fig. 8.) over the hole i in the pump-plate; and upon exhausting the air, the receiver will be fixed down to the plate by the pressure of the air on its outside, which is left to act alone, without any air in the receiver to act against it; and this pressure will be equal to as many times 15 pounds, as there are square inches in that part of the plate which the receiver covers; which will hold down the receiver so fast, that it cannot be got off, until the air be let into it by turning the cock k; and then it becomes loose.

3. Set the little glass AB (fig. 12.) (which is open at both ends) over the hole i upon the pump-plate LL, and put your hand close upon the top of it at B: then upon exhausting the air out of the glass, you will find your hand pressed down with a great weight upon it; so that you can hardly release it, until the air be readmitted into the glass by turning the cock k; which air, by acting strongly upward against the hand as the external air acted in pressing it downward, will release the hand from its confinement.

4. Having tied a piece of wet bladder b (fig. 13.) over the open top of the glass A (which is also open at bottom), set it to dry, and then the bladder will be tight like a drum. Then place the open end A upon the pump-plate, over the hole i, and begin to exhaust the air out of the glass. As the air is exhausting, its spring in the glass will be weakened, and give way to the pressure of the outward air on the bladder, which, as it is pressed down, will put on a spherical concave figure, which will grow deeper and deeper, until the strength of the bladder be overcome by the weight of the air; and then it will break with a report as loud as that of a gun.—If a flat piece of glass be laid upon the open top of this receiver, and joined to it by a flat ring of wet leather between them; upon pumping the air out of the receiver, the pressure of the outward air upon the flat glass will break it all to pieces.

5. Immerse the neck cd (fig. 14.) of the hollow glass ball eb in water, contained in the phial aa; then set it upon the pump-plate, and cover it and the hole i with the close receiver A; and then begin to pump out the air. As the air goes out of the receiver by its spring, it will also by the same means go out of the hollow ball eb, through the neck cd, and rise up in bubbles to the surface of the water in the phial; from whence it will make its way, with the rest of the air in the receiver, through the air-pipe GG and valves a and b, into the open air. When it has done bubbling in the phial, the ball is sufficiently exhausted; and then, upon turning the cock k, the air will get into the receiver, and press upon the surface of the water in the phial, as to force the water up into the ball in a jet, through the neck cd, and will fill the ball almost full of water. The reason why the ball is not quite filled, is because all the air could not be taken out of it; and the small quantity that was left in, and had expanded itself so as to fill the whole ball, is now condensed into the same state as the outward air, and remains in a small bubble at the top of the ball; and so keeps the water from filling that part of the ball.

6. Pour some quicksilver into the jar D (fig. 15.) and set it on the pump plate near the hole i; then set on the tall open receiver AB, so as to be over the jar and hole; and cover the receiver with the brass plate G. Screw the open glass tube fg (which has a brass top on it at b) into the syringe H; and putting the tube through a hole in the middle of the plate, so as to immerse the lower end of the tube e in the quicksilver at D, screw the end b of the syringe into the plate. This done, draw up the piston in the syringe by the ring I, which will make a vacuum in the syringe below the piston; and as the upper end of the tube opens into the syringe, the air will be dilated in the tube, because part of it, by its spring, gets up into the syringe; and the spring of the undilated air in the receiver acting upon the surface of the quicksilver in the jar, will force part of it up into the tube: for the quicksilver will follow the piston in the syringe, in the same way, and for the same reason, that wa... ter follows the piston of a common pump when it is raised in the pump-barrel; and this, according to some, is done by suction. But to refute that erroneous notion, let the air be pumped out of the receiver \(AB\), and then all the quicksilver in the tube will fall down by its own weight into the jar; and cannot be again raised one hair's breadth in the tube by working the syringe: which shews, that suction had no hand in raising the quicksilver: and, to prove that it is done by pressure, let the air into the receiver by the cock \(k\) (fig. 8.) and its action upon the surface of the quicksilver in the jar will raise it up into the tube, although the piston of the syringe continues motionless. If the tube be about 32 or 33 inches high, the quicksilver will rise in it very near as high as it stands at that time in the barometer. And, if the syringe has a small hole, as \(m\), near the top of it; and the piston be drawn up above that hole, the air will rush through the hole into the syringe and tube, and the quicksilver will immediately fall down into the jar. If this part of the apparatus be air tight, the quicksilver may be pumped up into the tube to the same height that it stands in the barometer; but it will go no higher, because then the weight of the column in the tube is the same as the weight of a column of air of the same thickness with the quicksilver, and reaching from the earth to the top of the atmosphere.

7. Having placed the jar \(A\) (fig. 16.) with some quicksilver in it, on the pump-plate, as in the last experiment, cover it with the receiver \(B\) then push the open end of the glass-tube \(d\) through the collar of leathers in the brafs neck \(C\) (which it fits so as to be air-tight) almost down to the quicksilver in the jar. Then exhaust the air out of the receiver, and it will also come out of the tube, because the tube is close at top. When the gauge \(mm\) shews that the receiver is well exhausted, push down the tube, so as to immerse its lower end into the quicksilver in the jar. Now, although the tube be exhausted of air, none of the quicksilver will rise into it, because there is no air left in the receiver to press upon its surface in the jar. But let the air into the receiver by the cock \(k\), and the quicksilver will immediately rise in the tube; and stand as high in it, as it was pumped up in the last experiment.

Both these experiments shew, that the quicksilver is supported in the barometer by the pressure of the air on its surface in the box, in which the open end of the tube is placed: and that the more dense and heavy the air is, the higher does the quicksilver rise; and, on the contrary, the thinner and lighter the air is, the more will the quicksilver fall. For, if the handle \(F\) be turned ever so little, it takes some air out of the receiver, by raising one or other of the pistons in its barrel: and consequently, that which remains in the receiver is so much the rarer, and has so much the less spring and weight; and thereupon, the quicksilver falls a little in the tube; but upon turning the cock, and re-admitting the air into the receiver, it becomes as weighty as before, and the quicksilver rises again to the same height.

—Thus we see the reason why the quicksilver in the barometer falls before rain or snow, and rises before fair weather; for, in the former case, the air is too thin and light to bear up the vapours, and in the latter too dense and heavy to let them fall.

N. B. In all mercurial experiments with the air-pump, a short pipe must be screwed into the hole \(i\), so as to rise about an inch above the plate, to prevent the quicksilver from getting into the air-pipe and barrels, in case any of it should be accidentally spilt over the jar; for if it once gets into the pipes or barrels, it spoils them, by loosening the folder, and corroding the brass.

8. Take the tube out of the receiver, and put one end of a bit of dry hazel-branch, about an inch long, tight into the hole, and the other end tight into a hole quite through the bottom of a small wooden cup: then pour some quicksilver into the cup, and exhaust the receiver of air; and the pressure of the outward air, on the surface of the quicksilver, will force it through the pores of the hazel, from whence it will descend in a beautiful shower into a cup placed under the receiver to catch it.

9. Put a wire through the collar of leathers in the top of the receiver, and fix a bit of dry wood on the end of the wire within the receiver; then exhaust the air, and push the wire down, so as to immerse the wood into a jar of quicksilver on the pump-plate: this done, let in the air; and upon taking the wood out of the jar, and splitting it, its pores will be found full of quicksilver, which the force of the air, upon being let into the receiver, drove into the wood.

10. Join the two brasfs hemispherical cups \(A\) and \(B\) (fig. 17.) together, with a wet leather between them, having a hole in the middle of it; then screw the end \(D\) of the pipe \(CD\) into the plate of the pump at \(i\), and turn the cock \(E\), so as the pipe may be open all the way into the cavity of the hemispheres; then exhaust the air out of them, and turn the cock a quarter round, which will shut the pipe \(CD\), and keep out the air. This done, unscrew the pipe at \(D\) from the pump, and screw the piece \(Eb\) upon it at \(D\); and let two strong men try to pull the hemispheres afunder by the rings \(g\) and \(h\), which they will find hard to do; for if the diameter of the hemispheres be four inches, they will be pressed together by the external air with a force equal to 188 pounds. And to shew that it is the pressure of the air that keeps them together, hang them by either of the rings upon the hook \(P\) of the wire in the receiver \(M\) (fig. 8.), and upon exhausting the air out of the receiver, they will fall afunder of themselves.

11. Place a small receiver \(O\) (fig. 8.) near the hole \(i\) on the pump-plate, and cover both it and the hole with the receiver \(M\); and turn the wire so by the top \(P\), that its hook may take hold of the little receiver by a ring at its top, allowing that receiver to stand with its own weight on the plate. Then, upon working the pump, the air will come out of both receivers; but the large one \(M\) will be forcibly held down to the pump by the pressure of the external air; whilst the small one \(O\), having no air to press upon it, will continue loose, and may be drawn up and let down at pleasure, by the wire \(PP\). But, upon letting it quite down to the plate, and admitting the air into the receiver \(M\), by the cock \(k\), the air will press so strongly upon the small receiver \(O\), as to fix it down to the plate; and at the same time, by counterbalancing the outward pressure on the large receiver \(M\), it will become loose. This experiment evidently shews, that the receivers are held down by pressure, and not by suction, for the internal receiver continued loose whilst the operator was pumping, and the external one was held down; but the former became fast immediately, by letting in the air upon it.

12. Screw the end \(A\) (fig. 18.) of the brasfs pipe \(ABF\) into the hole of the pump-plate, and turn the cock \(e\) until the pipe be open; then put a wet leather upon the plate \(cd\), which which is fixed on the pipe, and cover it with the tall receiver GH, which is close at top; then exhaust the air out of the receiver, and turn the cock e to keep it out; which done, unscrew the pipe from the pump, and set its end A into a basin of water, and turn the cock e to open the pipe; on which, as there is no air in the receiver, the pressure of the atmosphere on the water in the basin will drive the water forcibly through the pipe, and make it play up in a jet to the top of the receiver.

13. Set the square phial A (fig. 21.) upon the pump-plate; and having covered it with the wire-cage B, put a close receiver over it, and exhaust the air out of the receiver; in doing of which, the air will also make its way out of the phial through a small hole in its neck under the valve b. When the air is exhausted, turn the cock below the plate, to re-admit the air into the receiver; and as it cannot get into the phial again, because of the valve, the phial will be broke into some thousands of pieces by the pressure of the air upon it. Had the phial been of a round form, it would have sustained this pressure like an arch, without breaking; but as its sides are flat, it cannot.

To shew the elasticity or spring of the air.

14. Tie up a very small quantity of air in a bladder, and put it under a receiver; then exhaust the air out of the receiver, and the small quantity which is confined in the bladder (having nothing to act against it) will expand itself so by the force of its spring, as to fill the bladder as full as it could be blown of common air. But upon letting the air into the receiver again, it will overpower the air in the bladder, and press its sides almost close together.

15. If the bladder so tied up be put into a wooden box, and have 20 or 30 pounds weight of lead put upon it in the box, and the box be covered with a close receiver; upon exhausting the air out of the receiver, that air which is confined in the bladder will expand itself so, as to raise up all the lead by the force of its spring.

16. Take the glass-ball mentioned in the fifth experiment, (fig. 14.), which was left full of water all but a small bubble of air at top; and having set it with its neck downward into the empty phial aa, and covered it with a close receiver, exhaust the air out of the receiver, and the small bubble of air in the top of the ball will expand itself, so as to force all the water out of the ball into the phial.

17. Screw the pipe AB (fig. 18.) into the pump-plate, place the tall receiver GH upon the plate cd, as in the twelfth experiment, and exhaust the air out of the receiver; then, turn the cock e, to keep out the air; unscrew the pipe from the pump, and screw it into the mouth of the copper vessel CC (fig. 22.) the vessel having first been about half filled with water. Then turn the cock e (fig. 18.) and the spring of the air which is confined in the upper vessel will force the water up through the pipe AB in a jet into the exhausted receiver, as strongly as it did, by its pressure on the surface of the water in a basin, in the twelfth experiment.

18. If a fowl, a cat, rat, a mouse, or bird, be put under a receiver, and the air be exhausted, the animal will be at first oppressed as with a great weight, then grow convulsed, and at last expire in all the agonies of a most bitter and cruel death.

19. If a butterfly be suspended in a receiver, by a fine thread tied to one of its horns, it will fly about in the receiver, as long as the receiver continues full of air; but if the air be exhausted, though the animal will not die, and will continue to flutter its wings, it cannot remove itself from the place where it hangs in the middle of the receiver, until the air be let in again, and then the animal will fly about as before.

20. Pour some quicksilver into the small bottle A (fig. 19.) and screw the brass collar c of the tube BC into the brass neck b of the bottle, and the lower end of the tube will be immersed into the quicksilver, so that the air above the quicksilver in the bottle will be confined there, because it cannot get out about the joinings, nor can it be drawn out through the quicksilver into the tube. This tube is also open at top, and is to be covered with the receiver G and large tube EF, which tube is fixed by brass collars to the receiver, and is close at the top. This preparation being made, exhaust the air both out of the receiver and its tube; and the air will by the same means be exhausted out of the inner tube BC, through its open top at C; and as the receiver and tubes are exhausting, the air that is confined in the glass bottle A will press to by its spring upon the surface of the quicksilver, as to force it up in the inner tube as high as it was raised in the ninth experiment by the pressure of the atmosphere; which demonstrates that the spring of the air is equivalent to its weight.

21. Screw the end C (fig. 20.) of the pipe CD into the hole of the pump-plate, and turn all the three cocks d, G, and H, so as to open the communications between all the three pipes E, F, DG, and the hollow trunk AB. Then, cover the plates g and h with wet leathers, which have holes in their middle where the pipes open into the plates; and place the close receiver I upon the plate g: this done, shut the pipe F by turning the cock H, and exhaust the air out of the receiver I. Then turn the cock d, to shut out the air; unscrew the machine from the pump; and having screwed it to the wooden foot L, put the receiver K upon the plate h: this receiver will continue loose on the plate as long as it keeps full of air; which it will do until the cock H be turned to open the communication between the pipes F and E, through the trunk AB; and then the air in the receiver K, having nothing to act against its spring, will run from K into I, until it be so divided between these receivers, as to be of equal density in both; and they will be held down with equal forces, to their plates by the pressure of the atmosphere, though each receiver will then be kept down but with one half of pressure upon it that the receiver I had when it was exhausted of air; because it has now one half of the common air in it which filled the receiver K when it was set upon the plate; and therefore a force equal to half the force of the spring of common air will act within the receivers against the whole pressure of the common air upon their outsides. This is called transferring the air out of one vessel into another.

22. Put a cork into the square phial A, (fig. 21.) and fix it in with wax or cement; put the phial upon the pump-plate with the wire-cage B over it, and cover the cage with a close receiver. Then, exhaust the air out of the receiver; and the air that was corked up in the phial will break the phial outwards by the force of its spring, because there is no air left on the outside of the phial to act against the air within it.

23. Put a shrivelled apple under a close receiver, and exhaust the air; then the spring of the air within the apple will plump it out, so as to cause all the wrinkles disappear; but but upon letting the air into the receiver again, to press upon the apple, it will instantly return to its former decayed and shriveled state.

23. Take a fresh egg, and cut off a little of the shell and film from its smallest end; then put the egg under a receiver, and pump out the air; upon which, all the contents in the egg will be forced out into the receiver, by the expansion of a small bubble of air contained in the great end, between the shell and film.

34. Put some warm beer in a glass; and having set it on the pump, cover it with a close receiver, and then exhaust the air. Whilst this is doing, and thereby the pressure more and more taken off from the beer in the glass, the air therein will expand itself, and rise up in innumerable bubbles to the surface of the beer; and from thence it will be taken away with the other air in the receiver. When the receiver is near exhausted, the air in the beer, which could not disentangle itself quick enough to get off with the rest, will now expand itself so, as to cause the beer to have all the appearance of boiling; and the greatest part of it will go over the glass.

25. Put some warm water in a glass, and put a bit of dry wainscot or other wood into the water. Then, cover the glass with a close receiver, and exhaust the air; upon which, the air in the wood having liberty to expand itself, will come out plentifully, and make all the water to bubble about the wood, especially about the ends, because the pores lie lengthwise. A cubic inch of dry wainscot has so much air in it, that it will continue bubbling for near half an hour together.

Of Winds.

As the air is a fluid, subjected to the same laws of gravitation as other fluids, it necessarily has a constant tendency to preserve an equilibrium in every part; so that, if by any means whatever it is rendered lighter in any one place than another, the weightier air will rush in from every side towards this place, till as much be there accumulated as makes it of an equal weight with the rest of the atmosphere: It is these currents of air which are called winds.

Many are the causes which may vary the weight of the atmosphere, and occasion particular topical winds.

Although other causes may occasion winds in certain circumstances, yet their principal and most universal cause is the sun, which warms the air to a much greater degree in some places of the atmosphere than in others; and as the air is susceptible of a great degree of expansion by heat in those places where it is heated to any considerable degree, it is expanded so much as to become lighter than the air in those places where it is colder; so that the weightier cold air from all the circumjacent parts rushes towards this point to restore the equilibrium which had been destroyed. So that if there be any particular part upon the earth's surface where the sun acts constantly with greater force than on any other part, a current of air will constantly flow from these towards the warmer region: but the sun acts with greater force upon those parts of the earth which are nearest the Equator, than those which approach towards either Pole; so that we might naturally expect that a wind would constantly blow from the polar regions towards the Equator; which is really found to be the case in the Torrid Zone, where the influence of the sun overcomes almost all the other lesser causes which produce the variable winds in our more northerly regions. However, even in the Torrid Zone, these north and south winds are varied in different ways.

Although the heat of the equatorial region is greater than any other; yet as the sun acts perpendicularly in his diurnal course upon one point of the equator only at one time, and immediately passes over it; and as the air retains the heat communicated to it by the sun but for a short time, cooling gradually as he retires, and continuing still to decrease till his influence again returns the following day; the degree of heat upon this great circle must be very different in different parts, and perpetually varying in every point; which must in some measure tend to disturb those winds coming from the polar regions, which we have already mentioned. To comprehend clearly what will be the effects of this rotation, let us consider what effect it would naturally produce upon the equator with regard to wind, supposing no other cause should interrupt it. And here we must observe, that as the point upon which the sun acts with the greatest power is constantly moving from east to west, the air to the east of that point over which the sun has more lately passed will be more rarefied than that to the west, and will naturally flow towards that point from east to west with greater velocity than from west to east, as the cool air to the west of that point will be interrupted in its motion towards it by the motion of the sun meeting it. Hence therefore it follows, that from the diurnal motion of the earth from west to east a constant east wind would always be produced, were it not obstructed by other causes. But as there is a constant stream of air flowing from the polar towards the equatorial regions, a composition of these two currents of air acting at the same time will produce a north-east wind in all parts of the northern hemisphere, and a south-east wind in all parts of the southern one. These winds are known by the name of the general trade-winds.

If there were no inequalities on the surface of our globe, and if it were composed of a substance perfectly homogeneous, this wind would invariably take place at all times on every part of the earth's surface; but as this is not the case, it is liable to several very considerable variations. In all those regions towards the poles, as the influence of the sun is there but weak, other lesser causes occasion particular winds, and disturb that regularity which at first view we might expect, so that the general trade wind does not invariably take place beyond the 28th or 30th degree of latitude; and the regions between that and the poles have nothing but variable winds. Even in the Torrid Zone, there are many causes which in particular places alter this direction of the wind; so that the genuine trade-winds do not take place except in the Atlantic and Pacific oceans on each side the equator to the distance of 28 or 30 degrees, and in the greatest part of the Indian ocean to the south of the Equator as appears, more distinctly upon the Map; see Plate CXLVI., where the course of the winds are marked by the direction of the darts, the darts pointing in the same direction as the wind blows.

Having thus explained the nature and causes of the general trade wind, we now proceed to take notice of the principal deviations which take place in the Torrid Zone. The general trade-wind, when thus altered at particular seasons, is known by the name of monsoons. There are other variations, which, although as general, are yet of smaller and more limited influence. These are known by the name of breezes; and as they blow periodically from the sea, they are denominated sea or land breezes, and take place more... or less in every sea coast within the tropics. As the causes of the monsoons will be more clearly comprehended after the nature of these breezes is explained, we shall first consider them.

The sea and land breezes of the Torrid Zone are gentle periodical winds regularly shifting twice every day, and blowing from the sea towards the land during the daytime, and from the land towards the sea in the night. These breezes do not blow with an equal degree of force throughout the whole day and night, but are perpetually varying, being always strongest about mid-day and midnight, and becoming gradually weaker till the time of change in the evening and morning; about which time the air continues for a short space perfectly calm: but in a little the breeze begins to be felt on the side opposite to that from which it blew last, so faint at first as hardly to be perceived; but by degrees acquiring greater strength, it goes on increasing for five or six hours, after which it again as gradually sinks and dies away. They always blow directly off or towards the shore, and never extend their influence to a great distance from it, although this is varied by particular circumstances in different places; as they never extend so far from the points of capes and promontories, as in deep bays; nor upon the windward, as lee-shores.

These breezes are produced by the same cause which gives rise to the trade-wind, viz., the heat of the sun. In these warm regions the days and nights are nearly of an equal length throughout the whole year; the sun rising high in the daytime, and descending almost perpendicularly at night; which occasions a much greater variation between the heat of the day and night than is experienced in the more temperate climates; and it is this great difference between the heat of the night and day which produces the breezes. For the rays of the sun are reverberated from the land during the daytime, much more powerfully than from the sea, whose surface is constantly evaporating; and the air above the land is rendered much warmer, and consequently more rarified, than above the sea; so that a current of air necessarily takes place at that time from the sea towards the land, increasing and diminishing in strength as the heat increases or declines. But when the sun descends below the horizon, the evaporation from the surface of the sea is stopt, or greatly diminished, and the cold which it occasioned is of consequence removed: the reverberation of the sun's rays from the surface of the earth is likewise removed, and the air above the land quickly resumes its natural degree of cold, which is always greater than the sea, when the influence of the sun is withdrawn; so that the air above the sea becomes warmer during the night than that above the land, and a current of air is of course established from the land to the sea, which forms the land-breeze, which acts as uniformly, although less powerfully, than the sea-breeze; blowing at first gently as the air begins to cool, and gradually gathering strength as the sun retires below the horizon; till his influence begins to be felt again in the morning, when it gradually gives place to the more powerful influences of the sea-breeze. These breezes are not, however, entirely confined to the Torrid Zone. They are even felt in more northern regions: the sea-breeze in particular being almost perceptible during the summer season along the coasts of the Mediterranean and the Levant, both on the African, and European and Asiatic shores, as within the tropics. Even in our own colder climate, the effects of this are often sensibly felt during the summer season; although, from the length of the day and shortness of the night, the difference between the heat of these is far less than in warmer climates; And although the shortness of our nights prevents us from feeling a nocturnal breeze, similar to the land-breezes of the Torrid Zone; yet in every serene evening we have an opportunity of observing a phenomenon, proceeding from a similar cause with that which occasions them in warmer climates. For as the waters retain their heat longer than the earth after the sun withdraws, the moisture which was raised during the heat of the day to a small distance from the earth's surface is quickly condensed by the cold of the evening, and falls down in copious dews; whereas that which is above the surface of the water is more slowly condensed, by reason of the heat which that element retains longer, and hovers at a small distance above it in the form of a dense vapour, which slowly subsides as it loses its heat. This is the cause of those low mists which are so often seen hovering above the surface of rivers and other waters in the evenings towards the end of summer.

It was already observed, that in the Indian ocean the general trade-wind only took place in some parts to the south of the Equator. To the north of the line, and in some places to the south of it in that ocean, the general trade-wind only blows regularly for six months, and during the other six months the wind blows in a direction entirely opposite. It is these winds, which shift thus regularly, which are called Monsoons, although they are also sometimes called trade-winds.

At the Equator the days and nights are always of an equal length throughout the whole year; so that the heat being thus equally divided, it never arrives to such an intense degree as to be insupportable to the inhabitants. And as there is no vicissitude of seasons at the Equator, so at the Poles they never experience the more pleasing vicissitudes of day and night; the sun never setting during the summer season, nor rising above the horizon during the winter; and although the day decreases in length as we recede from the pole, from six months to twenty-four hours; yet in all high latitudes the sun descends for such a short space below the horizon, and in such an oblique direction, that the difference between the heat of the day and night is but very inconsiderable. From which it follows, that during this season, when the sun continues to act with such uninterrupted influence upon the surface of the earth, the air will then be rarified more above the dry land than upon the surface of the water; so that a wind would naturally set in at that time from the sea towards the land, similar to the diurnal sea-breezes in the warmer climates; and on the contrary, during the winter season, the air in these northern regions being colder above the land than the water, the winds will naturally blow from the land towards the sea, similar to the land-breezes of the Torrid Zone. But as the influence of the sun, although of longer continuance, is in general more languid in climates of a high latitude than in those near the line, it is not to be expected that these effects will follow with the same regularity as in the Torrid Zone: being more apt to be interrupted by lesser causes which affect the atmosphere and produce winds in different directions. Yet these are not so totally interrupted, but that we can easily trace their effects even in our own cold climate: For during the summer season, the large continent to the east of us, being more heated than the Atlantic ocean westward, pro- A View of the General and

N. B. The arrows among the

N. B. The exact point where the S. E. anc of the wind on each side of the equator, at 6 o' as to us more probable, that this point varies acc vations are wanting to confirm this.

Neither is it easy to ascertain, with precision, to the South of the equator, extend west from duces a general tendency of the current of air towards the east, inasmuch that westerly winds are observed to prevail more than any other, not only here, but in all the frontier countries on the continent, during the summer season. And easterly winds become again more prevalent in the winter and spring. On the contrary, it is observed in North America, that the easterly winds prevail more in summer than at any other time; and the westerly winds always prevail during the cold months of winter. The same effects take place with a greater degree of constancy in other parts of Europe, particularly in Greece, and the countries in that neighborhood; as the ancient Greeks have particularly remarked, that the winds blew from the south during the heat of summer, particularly about the dog-days, and from the north during the colder weather of winter.

Any attentive reader, who has accompanied us thus far, will readily see, that the monsoons which take place in the Indian ocean proceed from the same general cause. For when the sun, in his annual course, has crossed the line, and comes to act very strongly upon the extensive countries of Arabia, Persia, China, and the other parts of India, these become heated to a much higher degree than the ocean to the south of them; and the air above these extensive countries being so much rarefied, naturally draws the wind towards that place, which, by overcoming the general trade-wind, produces the southerly monsoons which take place in all those seas during the months of April, May, June, July, August, and September. But when the sun has again retreated towards the southern hemisphere, this great degree of heat in these countries subsides, and the genuine trade-wind again resumes its natural course; forming what they call the northerly monsoon, which blows in the months of October, November, December, January, February, and March; and as the continent of Asia now assumes a greater degree of cold than the Atlantic or Pacific oceans in the same latitude, it produces a brisker and more steady gale during the continuance of this monsoon, than is ever experienced in the general trade wind.

Having thus explained the nature of the monsoons in general, we shall proceed to consider the particulars which influence the direction of these in those parts where they take place. In all that part of the Indian ocean which lies between the island of Madagascar and Cape Comorin, the wind blows constantly from the W. S. W. between the months of April and October; and in the opposite direction from the month of October till April, although with some variation in different places, as these winds are neither so strong nor constant in the Bay of Bengal as in the Indian ocean. And it is likewise remarkable, that the S. W. winds in these seas are generally more southerly on the African side, and more westerly on the Indian, as appears distinctly in the map. But these variations are not repugnant to the general theory. For it is sufficiently known, that high lands in every part of the globe are much colder than low and flat countries; and as that part of Africa is very high and mountainous, the cold in these regions is much greater than in the more flat countries of Arabia and India; so that the wind naturally blows from these cold regions, in the summer season, towards the warmer continent of Asia; which occasions these inflections of the wind to the eastward which take place in these seas during the summer months; and is still farther assisted by the peninsula of India, the kingdom of Siam, and the islands of Sumatra and Java, on the eastern part of this ocean, lying so much farther south than the kingdom of Arabia and Persia; so that these, being more heated than the ocean to the westward, naturally draw the wind towards them, and produce the easterly variation of the monsoon which takes place in this part of the ocean, while the warm and sandy deserts of Arabia draw the winds more directly northward near the African coasts.

In the eastern parts of the Indian ocean, beyond the island of Sumatra, through the gulf of Siam and bay of Tonquin, and along the southern parts of China, and among the Philippine islands, &c., to the north of the Equator, the monsoons observe a different direction, blowing nearly due south and north. Here the greatest part of the warm continent is to the west of this district, which makes the wind naturally assume this direction. A little to the east of this, among the Mariannine islands, the general trade-wind takes place, there being no continent to the north of them to occasion monsoons.

The monsoons are as regular in the eastern part of the Indian ocean to the south of the Equator, as they are to the north of it; as here a northern monsoon sets in from the month of October till April, and a southern from April till October. And here, as to the north of the line, we find the direction of the monsoons varying according to particular circumstances in different places: for about the island of Sumatra, and towards the west end of Java, the monsoons set in nearly from the north and south; but toward Celebes and Timur, they begin to tend a little more to the east and west, gradually declining as they approach the coast of New Guinea, near to which the northerly monsoon from October till April blows from the N. W. and the opposite monsoon from the S. E. between October and April. The reader will easily perceive that these monsoons are occasioned by the continent of New Holland and Guinea, which being heated by the sun when in the southern signs, draws the wind towards that in the summer season, in the same manner as the continent of Asia produces the monsoons to the north of the line. And it is likewise sufficiently plain, that the inflection of these periodical winds about Celebes and Timur is occasioned by that part of the continent called New Guinea jutting out so near to the Equator to the east of these, and drawing the wind toward that quarter.

These are the most general and extensive monsoons which take place in our globe. But there are other periodical winds, which occur in particular places in these warm regions, that deserve particular attention.

In the Red Sea, the monsoon shifts as regularly as in other places; but being influenced by the coasts, it tends a little more to the north and south than in the Indian ocean.

On the south coast of Africa, to the south of C. Corrientes, and about the southern parts of the island of Madagascar, the regular trade-wind from the S. E. takes place between October and April; but from April till October the wind blows from the W. or N. W., and is at that season exceedingly cold. This is evidently occasioned by a cause already taken notice of: for, notwithstanding the high and cold nature of this continent, yet when the sun is to the south of the line, his powerful influence at that season so far abates their natural degree of cold, as not to interrupt the general trade-wind between the months of October and April. But when he returns to the northern hemisphere, the high mountains of Africa resume their native coldness, and repel the general trade-winds by their cold and more powe- ful blast, so as to produce the intemperate monsoon which here takes place between the months of April and October.

From Mozambique, to cape Guardafuy, at the mouth of the Red Sea, the monsoons are a little more irregular than in the other parts of the Indian ocean. For it is observed, that between October and January the winds are variable, although chiefly from the north. In January the N. E. monsoon sets in, and continues regular till the month of May. From May till October the winds again become variable, but blow chiefly from the southern points; but in the months of June, July, and August, there are frequent calms, especially about the bay of Melinda, which sometimes continue for several weeks together, and extend only about a hundred leagues from shore.

Before we can explain the cause of this irregularity clearly, it will be necessary to attend to the direction of the wind on each side of this track at each particular season. In the months of October, November, and December, the winds are here variable, but chiefly from the north. Now during these three months, the wind to the south of this beyond C. Corrientes blows from the S. E. at the Red Sea, and all to the north of this the wind during this season of the year is from the N. E. And as the sun is then perpendicular to the bay of Melinda, these opposite winds here meeting and opposing one another, and being both of them swept in their course westward by the cold regions of Africa near the Mountains of the Moon, will naturally produce the variable winds here observed according as the one or the other of these three balancing powers shall predominate: although, as the coast here runs away towards the south-west, it is natural to expect that the northerly wind, which follows the same direction, should more frequently prevail than those which are opposed by it; especially when we consider, that the island of Madagascar, now beginning to be warmed by the influence of the sun, will concur in drawing the wind to the southward; and when the continent of Africa is more heated in the months of January and February, it does not oppose the easterly monsoon, so that the winds become then more fixed than before. But in the months of June, July, and August, the wind to the south of C. Corrientes is from the N. W.; and near the Red Sea, and throughout the northern part of the Indian ocean, the S. W. monsoon is then in its greatest vigour; so that on each end of this district the wind is blowing in an opposite direction; from which results these calms about Melinda, which we just now mentioned.

This much may suffice for the shifting winds on the African and Asiatic coasts. As to America, the only places where the wind shifts regularly are, the bays of Honduras and Campeachy on the east, and that of Panama and some parts on the coast of Mexico on the west, with a small track upon the coast of Brazil. In the south part of the bay of Honduras, between C. Gratia de Dios and C. la Bela, the common trade-wind between E and NE blows between March and November; from October till March, there are westerly winds; not constant or violent, but blowing moderately sometimes two or three days, or a week; and then the easterly breeze may again prevail for an equal length of time. The reason of the peculiarity here observed is this. During the summer season, the high land on the Isthmus of Darien is so much warmed as not to interrupt the course of the general trade-winds. But when he retires to the southern hemisphere, the cold upon the Isthmus at that season becomes so great, as to condense the air to such a degree as to repel the trade-wind for some time: but not being cooled to such an intense degree as in some of the larger continents, the trade-wind, at times overcomes and repels these land breezes in its turn, and produces the phenomena above described. And that this is really the case, appears evident from this circumstance, that the land-breezes are most prevalent and of longest duration in the coldest months of December and January; before and after which two months, the trade-wind being generally checked, only a day or two about the full or change of the moon. As these western breezes on this coast take their rise from the same cause as the diurnal land-breeze in warm climates, they may be considered as land-breezes of two or three days continuance, and forming an intermediate step between the land-breezes and monsoons. Although the influence of these breezes is felt farther off at sea than the common diurnal breeze, yet they do not extend a great way, being seldom felt above twenty or thirty or forty leagues from shore; and about C. La Vela, which is much exposed to the east wind, these breezes seldom extend above eight or ten leagues from shore. Land breezes of the same nature, and proceeding from similar causes, are also experienced in the winter season in the bay of Campeachy which are there known by the name of Sumajenta winds. Beyond C. la Vela these western breezes are not felt, which is undoubtedly occasioned by the whole of that coast as far as C. St Augustine being too much exposed to the general trade-wind, which here sweeps along the coast with so much violence as almost totally to repel the weaker influence of the breezes. But between C. St Augustine and St Catherine's island, or a little farther, we again meet with a variation of the wind at different seasons, as it is here observed to blow in an E or NE direction from September till April; and from April till September from the SW. This variable wind, or monsoon, like the others on this coast, extends but for a very short way from shore, and is evidently occasioned by the same causes as the other periodical winds. For, in the summer-months, (which in this climate is between September and April,) the land of the continent being heated by the sun, draws the trade-wind from its common course of SE, a little to the westward; and as the coast here tends towards the SW, the wind in some measure (as it always does) follows the same direction, and produces this ENE monsoon. But, in the winter, when this region becomes more cool, the east wind is repelled by the dense cold air from the mountains; by which means it is bent to the northward, and is forced along the coast to C. St Augustine; where meeting with no further hindrance, it again falls in with the general trade-wind, and is carried along with it in its proper direction.

We have purposely omitted mentioning the winds on the west coasts of Africa and America, till the others were explained, as the causes of the peculiarities here observed will be now more easily comprehended. On the coasts of Chili and Peru, in America, from 25° or 30° of south latitude to the line; and on the parallel coast of Angola, &c., in Africa, the wind blows all the year from the south, varying in its direction a little in different places according to the direction of the coast, towards which it always inclines a little. But whatever is the direction at any one place, it continues the same throughout the whole year without any any variation, and always blows from some farther point. But there is this difference between this wind upon the coasts of Chili and Angola, that it extends much farther out to sea upon the former than upon the latter.

In order to explain the cause of this singular phenomenon, it is necessary to recollect, that the general trade-wind is produced by the concurrence of two separate causes. One is the great heat of the equatorial region, by which alone would be produced a constant north or south wind. The other is the diurnal revolution of the earth, which would cause a perpetual tendency of the air in these warm regions from east to west. From the concurrence of these two causes result the general trade-wind, which would constantly blow from the SE or NE, as we have already demonstrated. But if any one of these two causes, in any particular place, is prevented from producing its full effect, while the other continues to exert its influence, the general direction of the wind will be varied, and it will assume another. Thus, if the east wind was prevented from acting in any particular place, while nothing interrupted the north or south wind, it is evident that the air would rush towards the equator in that direction which was nearest and easiest, whether that should be pointing eastward or westward. Now as the high mountains in the internal parts of Africa and America interrupt the course of the east wind near the surface of the earth, while these coasts of which we now treat are entirely open to the south, the wind naturally rushes along the coasts of Chili and Angola from north to south; and as the low lands near the shore, in these warm regions, is generally warmer than the sea, the wind will naturally point in towards the shore, as is generally observed to happen.

This, then, is the obvious cause of the south wind which always prevails upon the coasts of Chili and Peru, as well as along the shores of Angola, Leango, &c. But it is only near the shore that this can take place; nor can it extend to a great height above these low and fertile regions. For as the internal parts of these countries are exceedingly high; but more especially the Andes of America, which experience a perpetual degree of cold more intense than some polar regions ever experience; the air must here be condensed to a very great degree, and send forth from these high regions a perpetual wind to every side, which occasions almost all the peculiarities that have been remarked in these climates: For by opposing the general current of the trade-wind upon the eastern part of these continents, they produce these deluges of rain which supply the immense rivers of the Amazonas, La Plata, &c., these do not, like the Nile and Gambia, swell only at a particular season, and then shrink into a diminutive size again, but continue throughout the whole year, with less variation of size, to pour their immense floods of water into the ocean. These cold winds likewise stretching to the westward, at a considerable distance above the warmer regions of the sea-coast, at length descend as low as the ocean, and form the general trade-wind, and occasion that unusual degree of cold which mariners have so often complained of even under the line to the westward of America. To the same cause also must we attribute the thick fogs so common upon the southern parts of Chili and along the coasts of Peru, with the other peculiarities of that singular climate about Lima and the kingdom of Valles in South America; for the vapors which are exhaled in such great abundance in the warm regions on the sea-shore, are, at a little height above the earth, condensed by the cold winds which come from the mountains, and form these thick mists which are so often observed in this climate. The same effects are felt in some degree on the similar coast of Africa. But as the mountains of Africa are not so high as the Andes of America, nor approach so near the western coast, the effects are less sensible here than in America. The great height of the Andes above the mountains of the similarly situated country of Africa, is the only reason why the effects on that coast are not felt to an equal degree, although similar in kind.

A more singular deviation from the general trade-wind is observed to take place on the African and American coasts to the north of the line, than those we have taken notice of to the south of it. For it is observed, that from California to the bay of Panama, all along the coasts of New Spain, the winds blow almost constantly from the west or SW, nearly directly opposite to the trade-wind; and on the coast of Africa from C. Bayador to C. Verde, they blow chiefly from the NW, standing in upon the shore; from thence the wind bends gradually more and more from the north to the west, and so round to the SW, all along the coast of Guinea, as will be distinctly seen by the map. After what we have said of the winds on the southern parts of these regions, it will be unnecessary to spend much time in explaining the cause of these peculiarities, as it will evidently appear that they are nearly the same, the variations here observed being occasioned by the particular direction of the coast. Thus, along the coast of New Spain, the wind blows nearly the same direction in every place, as there are no remarkable bendings on the coast; being uniformly drawn towards the shore, by the great heat of the low part of the continent near the sea; which in these regions is always more heated than the water of the ocean, and occasions that inflection. But as the coast of Africa is more irregular, the winds also are found to be more different in their direction. To the north of C. Verde, as the coast stretches nearly south and north, the wind, being drawn towards it a little, blows from the NW. But beyond that, the coast bends more eastward to C. Palmas; from which it runs E or NE all along the coast of Guinea, the wind shifting gradually more and more to the west, still pointing in upon the coast. And as there is nothing to oppose the current of air, which comes from the south, along the coast of Angola, it stretches forward till it comes within the influence of the coast of Guinea, and is there drawn in towards the shore in a SW direction. But as it is only the lower regions of the coast of Guinea which are so much warmed, the high mountains within continuing cold; the northerly wind coming from these meeting and opposing the southerly winds in the higher regions of the air, by their mutual conflicts occasion those incessant rains and tremendous thunder so remarkable along the whole of this uncomfortable coast.

It has been often observed by mariners, that there is a track of sea to the west of Guinea from five to ten degrees of north latitude, in which the trade-wind blows with less steadiness than in any other part of that ocean, being almost constantly troubled with calms and tornadoes; the cause of which the reader will perceive by inspecting the map; as he will easily see that the winds are drawn from this quarter almost in every direction; so that there can be here no constant wind; but being exhausted of its air, it must become lighter than the circumjacent parts, and must then be sup- plied from either side; as chance or occasional circumstances may direct, which occasions those sudden hurries and tornadoes here observed.

Before we take our leave of this subject, it is necessary to observe, that in the Bay of Panama, the winds between September and March are easterly; but from March till September they blow chiefly from the SSW; that is, during the winter months, while the sun is far from them, the winds are off shore; and during the summer months, the lands being heated to a considerable degree, they are drawn towards the shore as usual. It is remarkable, however, that this is the only part on the west of a large continent where the wind shifts regularly at different seasons; which seems to be occasioned by the great height of the Isthmus of Darien, and the Terra Firma to the east of it, and the nearness of these to the sea, in comparison of the mountains near Benin on the similarly situated coast of Africa; which is greatly assisted by the deepness of the bay, which, by bending so much to the eastward from C. Lorenzo, is in a great measure screened from the force of the south winds, which allows the winter breeze to extend itself upon the bay with more facility. We ought here also to remark, that along the coast of Mexico, between C. Pelanco and Guatemala, there are land winds which blow in the months of May, June, and July, called by the Spaniards Popogaios. They greatly resemble the Summaventa winds in the Bay of Campeachy, as they blow both night and day a moderate breeze without intermission, sometimes three or four days or a week together. But as these blow from the land in summer only, whereas the Summaventa's blow only in winter, they must be occasioned by a different cause; which seems to be this: As the continent which divide the south sea from the Bay of Mexico and Gulf of Honduras, is but of very small breadth, and in many places very high ground, the heat which it receives from the sun in summer is not so great as on the similar coast of Africa; and as the trade-wind coming from the great Atlantic ocean sweeps along the eastern part of the American coast from C. St Augustine to the Bay of Honduras with very great violence at that season, the small heat of this narrow continent, is not sufficient to stop it entirely during that season; so that at some times it blows for a short time quite across it, and occasions those winds called Popogaios.

Besides these more general winds, there are likewise some particular winds which are only felt in particular places at certain times, whose effects are so singular as to merit attention; some of which we shall here take notice of. In the Gulf of Persia, particularly at Ormuz, during the months of June and July, there sometimes blows from the west, for a day or two together, a hot suffocating fiery wind, which scorches up and destroys any animal that may be exposed to it; for which reason, almost every body leave their habitations at Ormuz during these two months, and retire to the mountainous near Schiras in Persia, where they enjoy a more comfortable climate. To explain the cause of which, it is necessary to observe, that along all the coasts of Asia, to the north of the Indian ocean, the diurnal sea and land breezes take place, as in every part of the torrid zone; by means of which, the monsoons are not felt close in upon the shores. But as the monsoon continues to blow regularly at a small distance from shore, so in all probability it continues its course without interruption at a small distance above the surface of the earth. Now when the monsoon is in its greatest vigour, its influence will sometimes descend even as low as the surface of the earth, and, interrupting the course of the breezes, hurry along with it these warm vapours, which ought to have ascended upwards, and produced the salutary sea-breeze; and as the earth is thus deprived of the refreshing influence and moisture of the sea-breeze, the air, by the strong reverberation of the sun-beams from such dry and sandy countries as Arabia, must soon be heated to an amazing degree, and produce these hot and suffocating winds. It is also remarkable, that these hot winds are more often experienced near headlands, where the sea-breezes are weakest, which seems to confirm this hypothesis. Winds similar to these in kind, though not in degree, are felt upon the coast of Coromandel during the months of June, July, and August, while the west monsoon reigns; and on the Malabar coast they are likewise felt in the months of December and January, while the east monsoon reigns; but these are much less powerful than either of the others. As these hot winds always come from the land, they are known upon these coasts by the name of Terrenos.

It has likewise been observed, that on the coast of Africa to the north of C. Verde, during the months of December, January, and February, there sometimes blows, for a day or two together, an easterly wind, so very intensely cold as to be almost as destructive as the warm winds at Ormuz. We have already in some measure explained the cause of this phenomenon. During these months, when the sun is far from them, his influence is less felt than at other seasons, and the northerly wind upon the coast is of course weakened, inasmuch that the cold produced by the mountains in the heart of the country being now in its greatest degree of force, buries its usual confinement for a time, spreading to the west with great violence, and producing those uncommon effects already mentioned. Those who sail on these coasts, distinguish this particular wind by the name of Hermatan.

These are the principal winds, whether constant or periodical, that take place within the tropics; and thus simple are their causes.

The succession of sea and land breezes renders the Torrid Zone not only habitable but comfortable. Besides, as these currents of cold air, rushing from each side of the globe, and carrying along with them vast quantities of aqueous vapours which they collect from the surface of the earth in their course, meet and oppose one another at that part of the atmosphere where the influence of the sun is greatest at the time, the water is there forced from the clouds in such prodigious quantities, as to produce a diversity of seasons in the Torrid Zone, something similar to what is experienced in more temperate climates; with this difference however, that whereas, in temperate climates, the warmest and most comfortable season is when the sun approaches nearest perpendicular to them, in these warmer climates the heavy rains which fall upon them at that season moderates the heat, and prevents the sun from having such an effect as at other times; so that their coldest and most inclement weather, which they call winter, is at that season, when, without this cause, they would be exposed to the sun's most powerful influence.

We shall only take notice of one other instance of the happy effects produced on our globe, by the laws of nature with respect to winds. We have seen, that in the great Atlantic and Pacific oceans, the trade-wind blows constantly from the easterly points throughout the whole year, so that

Ships sail from east to west within the tropics with the utmost facility; but it is absolutely impossible in these seas to sail from west to east, as the wind would be constantly against them, so that ships bound for any port to the eastward in these regions, must stand to the north or south till they are beyond the limits of the trade-winds, where they meet with variable breezes, by the help of which they fail to the eastward. But if the same constant trade-wind had taken place in the northern part of the Indian Ocean, it would have been impossible to have sailed to the eastward at all; because the continent of Asia would have prevented the ships from sailing far enough north to find the variable winds. But here, as in almost every case in which the operations of nature are concerned, we find, that what produces the difficulty, at the same time furnishes a remedy: for that very continent which would have stood in our way going northward, draws the wind towards itself at one season, which makes that course of navigation unnecessary, the shifting of the monsoons supplying a nearer and more commodious course. Thus we see, that wherever the sea is open to the south or north, near the tropic, so as that ships are at freedom to reach the variable winds, the trade-wind constantly blows in one direction; but wherever there is any extent of continent within the verge of the Torrid Zone, so as that they could not be at liberty to reach the variable winds, there the course of the trade-wind is altered, being drawn towards it in summer, and from it in winter, forming that shifting wind called monsoons. From which we may naturally infer, that as there are no monsoons in the Pacific or Atlantic, or in the western part of the Indian ocean, to the south of the line, there are no extensive continents near the tropics in either of these places.

POE

PNEUMATOCELE. See Medicine and Surgery.