in Natural History, a deep pit under ground, from whence various kinds of minerals are dug out; but the term is more particularly applied to those which yield metals. Where stones only are procured, the appellation of quarries is universally bestowed upon the places from which they are dug out, however deep they may be.

The internal parts of the earth, as far as they have been yet investigated, do not consist of one uniform substance, but of various strata or beds of substances, extremely different in their appearances, specific gravities, and chemical qualities, from one another. Neither are these strata similar to one another either in their nature or appearance in different countries; so that even in the short extent of half a mile, the strata will be found quite different from what they are in another place. As little are they the same either in depth or solidity. Innumerable cracks and fissures, by the miners called lodes, are found in every one of them; but these are so entirely different in size and shape, that it is impossible to form any inference from their size in one place to that in another. In these lodes or fissures the metallic ore is met with; and, considering the great uncertainty of the dimensions of the lodes, it is evident that the benefits of mining, which depends on that size, must in like manner be quite uncertain and precarious. Mr Price, in his treatise on the Cornish mines, observes, that "the comparative smallness of the largest fissures to the bulk of the whole earth is really wonderful. In the finest pottery we can make, by a microscopic view, we may discover numerous cracks and fissures, so small as to be impenetrable by any fluid, and impervious to the naked eye; as, by the laws of nature originally imposed by the Creator, it happens that matter cannot contract itself into solid large masses, without leaving fissures between them, and yet the very fissures are as necessary and useful as the strata through which they pass. They are the drains that carry off the redundant moisture from the earth; which, but for them, would be too full of fens and bogs for animals to live or plants to thrive on. In these fissures, the several ingredients which form lodes, by the continual palling of waters, and the menstrua of metals, are brought out of the adjacent strata, collected and conveniently lodged in a narrow channel, much to the advantage of those who search for and pursue them; for if metals and minerals were more dispersed, and scattered thinly in the body of the strata, the trouble of finding and getting at them would be endless, and the expense of procuring them exceed the value of the acquisition.

The insides of the fissures are commonly coated over with a hard, crystalline, earthy substance or rind, which very often, in the breaking of hard ore, comes off along with it, and is commonly called the caps or walls of the lode: but Mr Price is of opinion, that the proper walls of the lode are the sides of the fissure itself, and not the coat just mentioned, which is the natural plaster upon those walls, furnished perhaps by the contents of the fissures, or from oozings of the surrounding strata.

The breadth of a lode is easily known by the distance betwixt the two incrusting sides of the stones of ore; and if a lode yields any kind of ore, it is a better sign that the walls be regular and smooth, or at least that one of them be so, than otherwise; but there are not many of these fissures which have regular walls until they have been sunk down some fathoms.

Thus the inner part of the fissure in which the ore lies, is all the way bounded by two walls of stone, which are generally parallel to one another, and include the breadth of the vein or lode. Whatever angle of inclination some fissures make in the solid strata at their beginning, they generally continue to do the same all along. Some are very uncertain in their breadth, as they may be small at their upper part and wide underneath, and vice versa. Their regular breadth, as well as their depth, is subject to great variation; for though a fissure may be many fathoms wide in one particular place, yet a little further east or west it may not perhaps be one inch wide. This excessive variation happens generally in very compact strata, when the vein or fissure is squeezed, as it were, through hard rocks which seem to compress and straiten it. A true vein or fissure, however, is never entirely obliterated, but always shows a string of metallic ore or of a veiny substance; which often serves as a leader for the miners to follow until it sometimes leads them to a large and richly impregnated part. Their length is in a great measure unlimited, though not the space best fitted for yielding metal. The richest slate for copper, according to Mr Price, is from 40 to 80 fathoms deep; for tin, from 20 to 60; and though a great quantity of either may be raised at 80 or 100 fathoms, yet, "the quality (says our author) is often too much decayed and dry for metal."

Mr Price informs us, that the fissures or veins of the Cornish mines extend from east to west; or, more properly, one end of the fissure points west and by south, or west and by north; while the other tends east and by south, or east and by north. Thus they frequently pass through a considerable tract of country with very few variations in their directions, unless they be interrupted by some intervening cause. But, besides this east and west direction, we are to consider what the miners call the underlying or head of the vein or lode; viz. the deflection or deviation of the fissure from its perpendicular line, as it is followed in depth like the slope of the roof of a house, or the declivity of the steep side of a hill. This slope is generally to the north or south; but varies much in different veins, or sometimes even in the same vein; for it will frequently slope or underlie a small space in different ways, as it may appear to be forced by hard strata on either side.—Some of the fissures do not vary much from a perpendicular, while some deviate more than a fathom; that is, for every fathom they descend in perpendicular height, they deviate likewise as much to the south or north. Others differ so much from the perpendicular, that they assume a position almost horizontal; whence they are also called horizontal or flat lodes, and sometimes flat plots. Another kind of these has an irregular position with regard to the rest; widening horizontally for a little way, and then descending perpendicularly almost like stairs, with only a small string or leader to follow after; and thus they alternately vary and yield ore in several flat or horizontal fissures. This, by the Cornish tinners, is called (but in Mr Price's opinion erroneously) a floor or squat; which, properly speaking, is a hole or chasm impregnated with metal, making no continued line of direction or regular walls. Neither does a floor of ore descend to any considerable depth; for underneath it there appears no sign of a vein or fissure, either leading directly down or any other way. This kind of vein is very rare in Britain. The fissures most common in Britain are the perpendicular and inclined, whether their direction be north or south, east or west.

The perpendicular and horizontal fissures (according to our author) probably remain little altered from their first position, when they were formed at the inundation of the strata immediately after the waters left the land. The perpendicular fissures are found more commonly situated in level ground, at a distance from hills, and from the sea shore; but with regard to the latter, we find that the upper and under masses of strata differ in their solidity and other properties. "Hence," (says our author) "it is very plain, that inclined fissures owe their deflection or underlie to some secondary cause, violence, or subsidence, of the earth: for though perpendicular fissures are seldom to be seen, yet such as are inclined at very considerable depths, become more and more perpendicular, as the more central strata by reason of the vast superincumbent weight, do not seem so likely to be driven out of their position as those which lie nearer the surface."

The fissures are often met with fractured as well as inclined; the reason of which, in Mr Price's opinion, has been a subsidence of the earth from some extraordinary cause. "The original position (says he) must have been horizontal, or parallel to the surface of the earth; but we often find these strata very sensibly declined from that first position; nay, sometimes quite reversed, and changed into perpendicular. When we see a wall lean, we immediately conclude that the foundation has given way, according to the angles which the walls make with the horizon; and when we find the like declination in strata, we may conclude, by parity of reason, that there has been a like failure of what supported them, in proportion to that declination; or that whatever made the strata to fall so much awry, must also cause everything included in those strata to fall proportionally. Wherever the greatest subsidence is to the north, the top of the lode or fissure will point to the north, and of consequence underlie to the south, and vice versa: the side or heave of the lode manifests the greater subsidence of the strata; but the same lode is frequently fractured and heaved in several places, all of which, by due observation, will show us they were occasioned by so many several shocks or subidences, and that the strata were not unfooted, shaken, or brought to fall only once or twice, but several times."

Mr Price, in the course of his work, observes, that, though the metallic veins generally run from east to west, they are frequently intersected by veins or lodes, as he calls them, of other matters, which run from north to south. Some of these cross veins contain lead or antimony, but never tin or copper. Sometimes one of these unmetallic veins intersects the true one at right angles, sometimes obliquely; and sometimes the mixture of both is so intimate, that the most expert miners are at a loss to discover the separated part of the true vein. When this last is intercepted at right angles, it is moved either north or south, a very little way, perhaps not more than one fathom; in which case, the miners having worked to a small distance in one of these directions, if they find themselves disappointed, turn to the other hand, and seldom fail of meeting with what they expected. Sometimes they are directed in their search by the pointing of a rib or string of the true vein; but when the interruption happens in an oblique direction, the difficulty of finding the vein again is much greater.

When two metallic veins in the neighbourhood of each other run in an oblique direction, and of consequence meet together, they commonly produce a body of ore at the place where they intersect; and if both are rich, the quantity will be considerable; but if one be poor and the other rich, then both are either enriched or impoverished by the meeting. After some time they separate again, and each will continue its former direction near to the other; but sometimes, though rarely, they continue united.

It is a sign of a poor vein when it separates or diverges into strings; but on the contrary, when several of them are found running into one, it is accounted a promising sign. Sometimes there are branches without the walls of the vein in the adjacent strata, which often come either obliquely or transversely into it. If these branches are impregnated with ore, or if they underlie farther than the true vein, that is, if they dip deeper into the ground, then they are said to overtake or come into the lode, and to enrich it; or if they do not, then they are said to go off from it, and to impoverish it. But neither these nor any other marks either of the richness or poverty of a mine are to be entirely depended upon; for many mines, which have a very bad appearance at first, do nevertheless turn out extremely well afterwards; while others, which in the beginning seemed very rich, turn gradually worse and worse: but in general, where a vein has a bad appearance at first, it will be imprudent to be at much expense with it.

Veins of metal, as has been already observed, are frequently, as it were, so compressed betwixt hard strata, that they are not an inch wide: nevertheless, if they have a string of good ore, it will generally be worth while to pursue them; and they frequently turn out well at last, after they have come into softer ground. In like manner, it is an encouragement to go on if the branches or leaders of ore enlarge either in width or depth as they are worked; but it is a bad sign if they continue horizontal without inclining downwards, though it is not proper always to discontinue the working. ing of a vein which has an unfavourable aspect at first. Veins of tin are worth working when only three inches wide, provided the ore be good; and copper ores when six inches wide will pay very well for the working. Some of the great mines, however, have very large veins, with a number of other small ones very near each other. There are also veins crossing one another sometimes met with, which are called contras, vulgarly counters. Sometimes two veins run down into the ground in such a manner that they meet in the direction of their depth; in which case the same observations apply to them which are applicable to those that meet in a horizontal direction. Sometimes a vein will suddenly disappear without giving any warning, by becoming narrower, or of worse quality; which by the miners is called a flort or leap, and is very common in the mines of Cornwall. In one day's time they may thus be disappointed in the working of a rich vein of tin, and have no further sign of anything to work upon. At the fractured extremity of their vein they perceive a body of clay or other matter; and the method of recovering their vein is to drive on the work in the direction of the former part, so that their new work shall make the same angle with the clay that the other part of the vein does. Sometimes they sink a shaft down from the surface; but it is generally a matter of difficulty to recover a vein when thus lost.

The method of discovering mines is a matter of so much difficulty, that it seems surprising how those who were totally unacquainted with the nature of metals first came to think of digging them out of the earth. According to Lucretius, the discovery was made by the conflagration of certain woods, which melted the veins of metal in the earth beneath them; but this seems rather to be improbable. Aristotle, however, is of the same opinion with Lucretius, and tells us, that some shepherds of Spain having set fire to the woods, the earth was thus heated to such a degree that the silver near the surface of it melted and flowed into a mass; and that in a short time the metallic mass was discovered by the rending of the earth in the time of an earthquake; and the same story is told by Strabo, who attributes the discovery of the mines of Andalusia to this accident. Cadmus is said by some to have been the first who discovered gold; while others attribute this to Thoas the Thracian, to Mercury the son of Jupiter, or to Pifus king of Italy; who having left his own country, went into Egypt, where he was elected king after the death of Mizraim the son of Ham; and, on account of his discovery, was called the Golden God. Others say, that Eacis or Cæacus the son of Jupiter, or Sol the son of Oceanus, was the first discoverer; but Æschylus attributes the discovery not only of gold, but of all other metals, to Prometheus. The brass and copper mines in Cyprus were first discovered by Cinyra the son of Agryops; and Hesiod attributes the discovery of the iron mines of Crete to the Cretan Dasyli Idaei. The extraction of lead or tin from its ore in the island of Cassiterides, according to several ancient authors, was discovered by Midacritus.—The Scripture, however, attributes the invention of brass and iron, or at least of the methods of working them, to Tubal Cain before the flood.

In more modern times, we know that mines have been frequently discovered by accident; as in sea cliffs, among broken craggy rocks, by the washing of the tide or floods, also by eruptions and torrents of water issuing out of hills and mountains, and sometimes by the wearing of high roads. Mr Price mentions another way by which mines have been discovered, viz. by fiery corrugations; which, he says, he has heard from persons whose veracity he is unwilling to question. "The tinners (says he) generally compare these effluvia to blazing stars or other whimsical likenesses, as their fears or hopes suggest; and search with uncommon eagerness the ground over which these jack-a-lanterns have appeared and pointed out. We have heard but little of these phenomena for many years; whether it be, that the present age is less credulous than the foregoing, or that the ground, being more perforated by innumerable new pits sunk every year, some of which, by the stannary laws, are prevented from being filled up, has given these vapours a more gradual vent, it is not necessary to inquire, as the fact itself is not generally believed."

Mines, however, are now most commonly discovered by investigating the nature of such veins, ores, and stones as may seem most likely to turn to account; but there is a particular sagacity, or habit of judging from particular signs, which can be acquired only by long practice. Mines, especially those of copper, may also be discovered by the harsh and disagreeable taste of the waters which issue from them; though it is probable that this only happens when the ore lies above the level of the water which breaks out; for it does not seem likely that the taste of the ore could affect, unless we were to suppose a pond or lake of water standing above it. The presence of copper in any water is easily discovered by immersing in it a bit of polished iron, which will thus instantly be turned of a copper colour, by reason of the precipitation of the metal upon it. A candle, or piece of tallow put into water of this kind, will in a short time be tinged of a green colour.

Another and still more remarkable method of discovering mines is said to be by the virgula divinatoria, or "divining rod;" which, however incredible the stories related concerning it may be, is still relied on by some, and among others by Mr Price. It is not known who was the inventor of this method; but Agricola supposes that it took its rise from the magicians, who pretended to discover mines by enchantment. No mention is made of it, however, before the 11th century, since which time it has been in frequent use; and the Corporeal Philosophy has even been called in to account for it. But before we pretend to account for phenomena so very extraordinary as those reported of the virgula divinatoria, it is necessary, in the first place, to determine whether or not they exist. Mr Price, as has been already hinted, believes in it, though he owns that by reason of his constitution of mind and body, he is almost incapable of co-operating with its influence. The following account, however, he gives from Mr William Cookworthy of Plymouth, a gentleman of known veracity and great chemical abilities.

He had the first information concerning this rod from one Captain Ribeira, who deserted from the Spanish service in Queen Anne's reign, and became captain-commandant in the garrison of Plymouth; in which town he satisfied several intelligent persons of the virtues of the rod, by many experiments on pieces of metal hid in the earth, and by an actual discovery of a copper mine near Oakhampton, which was wrought for some years. This captain very readily showed the method of using the rod in general, but would not by any means discover the secret of distinguishing the different metals by it: though, by a constant attention to his practice, Mr Cookworthy discovered it. Captain Ribeira was of opinion, that the only proper rods for this purpose were those cut from the nut or fruit trees; and that the virtue was confined to certain persons, and those, comparatively speaking, but few: but Mr Price says, that the virtue resides in all persons and in all rods under certain circumstances.

"The rod (says he) is attracted by all the metals, by coals, limestone, and springs of water, in the following order: 1. Gold; 2. Copper; 3. Iron; 4. Silver; 5. Tin; 6. Lead; 7. Coals; 8. Limestone and springs of water. One method of determining the different attractions of the rod is this: Stand, holding the rod with one foot advanced; put a guinea under that foot, and an halfpenny under the other, and the rod will be drawn down; shift the pieces of money, and the rod will be drawn towards the face, or backwards to the gold, which proves the gold to have the stronger attraction.

"The rods formerly used were shoots of one year's growth that grew forked; but it is found, that two separate shoots tied together with packthread or other vegetable substance answer rather better than such as are naturally forked, as the shoots of the latter are seldom of an equal size. They are to be tied together by the greater ends, the small ones being held in the hands. Hazel rods cut in the winter, such as are used for fishing rods, and kept till they are dry, do best; though, where these are not at hand, apple-tree suckers, rods from peach trees, currants, or the oak, though green, will answer tolerably well."

Our author next proceeds to describe the manner of holding the rod; of which he gives a figure, as he says it is difficult to be described. The small ends being crooked, are to be held in the hands in a position flat or parallel to the horizon, and the upper part in an elevation not perpendicular to it, but at an angle of about 70 degrees. "The rod (says he) being properly held by those with whom it will answer, when the toe of the right foot is within the semidiameter of the piece of metal or other subject of the rod, it will be repelled towards the face, and continue to be so while the foot is kept from touching or being directly over the subject; in which case it will be sensibly and strongly attracted, and be drawn quite down. The rod should be firmly and steadily grasped; for if, when it has begun to be attracted, there be the least imaginable jerk or opposition to its attraction, it will not move any more till the hands are opened, and a fresh grasp taken. The stronger the grasp the livelier the rod moves, provided the grasp be steady and of an equal strength. This observation is very necessary, as the operation of the rod in many hands is defeated purely by a jerk or counteraction; and it is from thence concluded, that there is no real efficacy in the rod, or that the person who holds it wants the virtue; whereas, by a proper attention to this circumstance in using it, five persons in six have the virtue, as it is called; that is, the nut or fruit-bearing rod will answer in their hands. If a rod, or the least piece of one of the nut-bearing or fruit kind, be put under the arm, it will totally destroy the operation of the virgula divinatoria, in regard to all the subjects of it, except water, in those hands in which the rod naturally operates. If the least animal thread, as silk, or worsted, or hair, be tied round or fixed on the top of the rod, it will in like manner hinder its operation; but the same rod placed under the arm, or the same animal substances tied round or fixed on the top of the rod, will make it work in those hands, in which without these additions it is not attracted."

Such are the accounts of this extraordinary rod, to which it is probable that few will attend; and we believe the instances of mines have been discovered by it are but very rare. Another and very ancient mode of discovering mines, less uncertain than the divining rod, but extremely difficult and precarious, is that called flooding; that is, tracing them by loose stones, fragments, or bodies, which may have been separated or carried off to a considerable distance from the vein, and are found by chance in running waters, on the superficies of the ground, or a little under.—"When the miners (says Mr Price) meet with a loose single stone of tin ore, either in a valley or in ploughing or hedging, though at 100 fathoms distance from the vein it came from, those who are accustomed to this work will not fail to find it out. They consider, that a metallic stone must originally have appertained to some vein, from which it was severed and cast at a distance by some violent means. The deluge, they suppose, moved most of the loose earthy coat of the globe, and in many places washed it off from the upper towards the lower grounds, with such a force, that most of the backs or sides of veins which protruded themselves above the soil were hurried downwards with the common mass: whence the skill in this part of their business lies much in directing their measures according to the situation of the surface." Afterwards, however, our author complains that this art of flooding, as he calls it, is in a great measure lost.

The following account of a method of finding silver mines by Alonso Barba seems to be similar to that of flooding just now mentioned. "The veins of metal (says he) are sometimes found by great stones above ground; and if the veins be covered, they hunt them out after this manner; viz. taking in their hands a sort of mattock, which has a steel point at one end to dig with, and a blunt head at the other wherewith to break stones, they go to the hollows of the mountains, where the current of rain water descends, or to some other part of the skirts of the mountains, and there observe what stones they meet with, breaking in pieces those that seem to have any metal in them; whereof they find many times both middling sort of stones and small ones also of metal. Then they consider the situation of that place, and whence these stones can tumble, which of necessity must be from higher ground, and follow the track of them up the hill as long as they can find any of them;" &c.

"Another way (says Mr Price) of discovering lodes is by working drifts across the country, as we call it, that is, from north to south, and vice versa. I tried the experiment in an adventure under my management, where I drove all open at grats about two feet in the shelf, very much like a level to convey water upon a mill wheel; by so doing I was sure of cutting all lodes in my way: and I did accordingly discover five courses, one of which has produced above 180 tons of copper ore, but the others were never wrought upon. This method of discovering lodes is equally cheap and certain; for 100 fathoms in shallow ground may be driven at 50s. expence."

In that kind of ground called by our author feasible, and which he explains by the phrase tender land- ing, he tells us, that "a very effectual, proving, and consequential way is, by driving an adit from the lowest ground, either north or south; whereby there is a certainty to cut all lodes at 20, 30, or 40 fathoms deep, if the level admits of it. In driving adits or levels across, north or south, to unwater mines already found, there are many fresh veins discovered, which frequently prove better than those they were driving to."

After the mine is found, the next thing to be considered is, whether it may be dug to advantage. In order to determine this, we are duly to weigh the nature of the place, and its situation, as to wood, water, carriage, healthiness, and the like; and compare the result with the richness of the ore, the charge of digging, stamping, washing, and melting.

Particularly the form and situation of the spot should be well considered. A mine must either happen, 1. In a mountain; 2. In a hill; 3. In a valley; or, 4. In a flat. But mountains and hills are dug with much greater ease and convenience, chiefly because the drains and burrows, that is, the adits or avenues, may be here readily cut, both to drain the water and to form gangways for bringing out the lead, &c. In all the four cases, we are to look out for the veins which the rains or other accidental thing may have laid bare; and if such a vein be found, it may often be proper to open the mine at that place, especially if the vein prove tolerably large and rich; otherwise the most commodious place for situation is to be chosen for the purpose, viz. neither on a flat, nor on the tops of mountains, but on the sides. The best situation for a mine is a mountainous, woody, wholesome spot; of a safe easy ascent, and bordering on a navigable river.

The places abounding with mines are generally healthy; as standing high, and everywhere exposed to the air; yet some places where mines are found prove poisonous, and can upon no account be dug, though ever so rich; the way of examining a suspected place of this kind, is to make experiments upon brutes, by exposing them to the effluvia or exhalations, to find the effects.

Devonshire and Cornwall, where there are a great many mines of copper and tin, is a very mountainous country, which gives an opportunity in many places to make adits or subterraneous drains to some valley at a distance, by which to carry off the water from the mine, which otherwise would drown them out from getting the ore. These adits are sometimes carried a mile or two, and dug at a vast expense, as from 200l. to 400l. especially where the ground is rocky; and yet they find this cheaper than to draw up the water out of the mine quite to the top, when the water runs in plenty, and the mine is deep. Sometimes, indeed, they cannot find a level near enough to which an adit may be carried from the very bottom of the mine; yet they find it worth while to make an adit at half the height to which the water is to be raised, thereby saving half the expense.

Mr Coffar, considering that sometimes from small streams, and sometimes from little springs or collections of rain water, one might have a good deal of water above ground, though not a sufficient quantity to turn an overhot wheel, thought that if a sufficient fall might be had, this collection of water might be made useful in raising the water in a mine to the adit, where it may be carried off.

But now the most general method of draining mines is by the steam engine. See Steam-Engine.

the military art, denotes a subterraneous canal or passage, dug under the wall or rampart of a fortification, intended to be blown up by gunpowder.

The alley or passage of a mine is commonly about four feet square; at the end of this is the chamber of the mine, which is a cavity of about five feet in width and in length, and about six feet in height; and here the gunpowder is stored. The faucille of the mine is the train, for which there is always a little aperture left.

Two ounces of powder have been found, by experiment, capable of raising two cubic feet of earth; consequently 200 ounces, that is, 12 pounds 8 ounces, will raise 200 cubic feet, which is only 16 feet short of a cubic toise, because 200 ounces, joined together, have proportionally a greater force than two ounces, as being an united force.

All the turnings a miner uses to carry on his mines, and through which he conducts the faucille, should be well filled with earth and dung; and the masonry in proportion to the earth to be blown up, as 3 to 2. The entrance of the chamber of the mine ought to be firmly shut with thick planks, in the form of a St Andrew's cross, so that the enclosure be secure, and the void spaces shut up with dung or tempered earth. If a gallery be made below or on the side of the chamber, it must absolutely be filled up with the strongest masonry, half as long again as the height of the earth; for this gallery will not only burst, but likewise obstruct the effect of the mine. The powder should always be kept in sacks, which are opened when the mine is charged, and some of the powder scattered about: the greater the quantity of earth to be raised is, the greater is the effect of the mine, supposing it to have the due proportion of powder. Powder has the same effect upon masonry as upon earth, that is, it will proportionally raise either with the same velocity.

The branches which are carried into the solidity of walls do not exceed three feet in depth, and two feet six inches in width nearly: this sort of mine is most excellent to blow up the strongest walls.

The weight of a cubic foot of powder should be 80lb.; 1 foot 1 inch cube will weigh 100lb. and 1 foot 2 inches and 1/4 150lb.; and 200lb. of powder will be 1 foot 5 inches cube; however, there is a diversity in this, according to the quantity of saltpetre in the gunpowder.

If, when the mines are made, water be found at the bottom of the chamber, planks are laid there, on which the powder is placed either in packs or barrels of 100 lbs each. The faucille must have a clear passage to the powder, and be laid in an aglet or wooden trough, through all the branches. When the powder is placed in the chamber, the planks are laid to cover it, and others again across these; then one is placed over the top of the chamber, which is shaped for that purpose; between that and those which cover the powder, props are placed, which shore it up; some inclining towards the outside; others to the inside of the wall; all the void spaces being filled with earth, dung, brick, and rough stones. Afterwards, planks are placed at the entrance of the chamber, with one across the top, whereon they buttress three strong props, whose other ends are likewise propped against another plank situated on the side of the earth in the branch; which props being well fixed between the planks with wedges, the branch should then be filled up to its entrance, with the aforementioned materials. The faucilles which pass through the side branches must be exactly the same length with that in the middle, to which they join: the part which reaches beyond the entrance of the mine is that which conveys the fire to the other three; the faucilles being of equal length, will spring together.

From a great number of experiments, it appears,

1. That the force of a mine is always towards the weakest side; so that the disposition of the chamber of a mine does not at all contribute to determine this effect.

2. That the quantity of powder must be greater or less, in proportion to the greater or less weight of the bodies to be raised, and to their greater or less cohesion; so that you are to allow for each cubic fathom

| Of loose earth, | 9 or 10 lb. | |----------------|-------------| | Firm earth and strong sand, | 11 or 12 | | Flat clayey earth, | 15 or 16 | | New masonry, not strongly bound, | 15 or 20 | | Old masonry, well bound, | 25 or 30 |

3. That the aperture, entonnoir of a mine, if rightly charged, is a cone, the diameter of whose base is double the height taken from the centre of the mine.

4. That when the mine has been overcharged, its entonnoir is nearly cylindrical, the diameter of the upper extreme not much exceeding that of the chamber.

5. That besides the flock of the powder against the bodies it takes up, it likewise crushes all the earth that borders upon it, both underneath and sidewise.

To charge a mine so as to have the most advantageous effect, the weight of the matter to be carried must be known; that is, the solidity of a right cone, whose base is double the height of the earth over the centre of the mine: thus, having found the solidity of the cone in cubic fathoms, multiply the number of fathoms by the number of pounds of powder necessary for raising the matter it contains; and if the cone contains matters of different weights, take a mean weight between them all, always having a regard to their degree of cohesion.

As to the disposition of mines, there is but one general rule, which is, That the tide towards which one would determine the effect be the weakest; but this varies according to occasions and circumstances.

The calculation of mines is generally built upon this hypothesis, That the entonnoir of a mine is the frustum of an inverted cone, whose altitude is equal to the radius of the excavation of the mine, and the diameter of the whole lesser base is equal to the line of least resistance; and though these suppositions are not quite exact, yet the calculations of mines deduced from them have proved successful in practice; for which reason this calculation should be followed till a better and more simple be found out.

M. de Valliere found that the entonnoir of a mine was a paraboloid, which is a solid generated by the rotation of a semiparabola about its axis; but the difference between these two is very insignificant in practice, that of the frustum of a cone may be used.