in agriculture: A machine for turning the plough up the soil by the action of cattle, contrived to save the time, labour, and expense, which, without this instrument, must have been employed in digging the ground, and fitting it for receiving all sorts of seed. See Agriculture, p. 83—95.

Amidst all the varieties which can occur in the manner of ploughing the ground, arising from difference of soil, local habits, and other causes, there is still a nameless in the task which gives a certain uniformity to the chief parts of the instrument, and should therefore furnish principles for its construction. There is not, perhaps, any invention of man that more highly merits our utmost endeavours to bring it to perfection; but it has been too much neglected by those persons who study machines, and has been considered as a rude tool, unworthy of their attention. Any thing appears to them insufficient for the clumsy task of turning up the ground; and they cannot imagine that there can be any nicety in a business which is successfully performed by the ignorant peasant. Others acknowledge the value of the machine, and the difficulty of the subject; but they think that difficulty insuperable, because the operation is so complicated, and the resistances to be overcome so uncertain, or so little understood, that we cannot discover any unequivocal principle, and must look for improvement only from experience or chance.

But these opinions are ill founded. The difficulty is indeed great, and it is neither from the ignorant farmer nor the rude artificer that we can expect improvement. It requires the serious consideration of the most accomplished mechanician; but from him we may expect improvement. We have many data: we know pretty distinctly what preparation will fit the ground for being the proper receptacle for the seed, and for supporting the nourishing plants; and though it is, perhaps, impossible to bring it into this state by the operation of any instrument of the plough kind, we know that some ploughs prodigiously excel others in reducing the stiff ground to that uniform crumbling state in which it can be left by the spade. The imperfections of their performance, or what yet remains to be done to bring the ground into this state, is distinctly understood. It seems, then, a determinate problem (to use the language of mathematicians), because the operation depends on the invariable laws of mechanical nature.

It will therefore be very proper, under this article, to ascertain, if possible, what a plough in general ought to perform to be, by describing distinctly its task. This will give ly point out a general form, the chief features of which must be found under every variety that can arise from particular circumstances.

The plough performs its task, not by digging, but by being pulled along. We do not aim at immediately reducing the ground to that friable and uniform state into which we can bring it by the spade; but we wish to bring it into such a state that the ordinary operations of the season will complete the task.

For this purpose, a slice or sod must be cut off from the firm land. This must be shovelled to one side, that the plough and the ploughman may proceed in their labour; and the sod must be turned over, so that the grass and stubble may be buried and rot, and that fresh soil may be brought to the surface; and all must be left in such a loose and open condition, that it may quickly crumble down by the influence of the weather, without baking into lumps, or retaining water. The first office is performed by the coulter, which makes a perpendicular cut in the ground. The point of the stock follows this, and its edge gets under the sod, and lifts it up. While lifting it up, it also heels it over, away from the firm land. The mould-board comes last, and pushes it aside, and gradually turns it over as far as is required.

The general form of the body of a plough is that of a wedge, or very blunt chisel, AFEDBC, (fig. 1.), having the lower corner D of its edge considerably more advanced than the upper corner B; the edge BD and the whole back AFDB is in the same perpendicular plane; the bottom FDB approaches to a triangular form, acute at D, and square at F; the surface BCED is of a complicated shape, generally hollow, because the angle ABC is always greater than FDE: this consequence will be easily seen by the mathematician. The back is usually called the land-side by the ploughmen, and the base FDE is called the sole, and FE the heel, and BCED the mould-board. Lastly, the angle AFE is generally square, or a right angle, so that the sole has level both as to length and breadth.

By comparing this form with attention, the reader will perceive that if this wedge is pulled or pushed along in the direction FD, keeping the edge BD always in the perpendicular cut which has been previously made by the coulter, the point D will both raise the earth and shove it to one side and twist it over; and, when the point has advanced from F to D, the sod, which formerly rested on the triangle DFE, will be forced up along the surface BCED, the line DF rising into the position DF, and the line EF into the position EF.

Had the bottom of this furrow been covered with a bit of cloth, this cloth would be lying on the mould-board, in the position DF: the slice, thus deranged from its former situation, will have a shape something like that represented in fig. 2.

In as much as the wedge raises the earth, the earth presses down the wedge; and as the wedge pushes the earth to the right hand, the earth presses the wedge to the left; and in this manner the plough is strongly pressed, both to the bottom of the furrow by its sole, and also to the firm land by its back or land-side. In short, it is strongly squeezed into the angle formed along the line FD (fig. 1.) by the perpendicular plane abDF and the horizontal plane FDE; and in this manner the furrow becomes a firm groove, directing the motion of the plough, and giving it a resisting support, by which it can perform all parts of its task. We beg our readers to keep this circumstance constantly in mind. It evidently suggests a fundamental maxim in the construction, namely, to make the land-side of the plough an exact mental plane, and to make the sole, if not plane, at least run straight from point to heel. Any projection would tear up the supporting planes, destroy the directing groove, and expend force in doing mischief.

This wedge is seldom made of one piece. To give it the necessary width for removing the earth would require a huge block of timber. It is therefore usually framed of several pieces, which we shall only mention in order to have the language of the art. Fig. 3., represents the land-side of a plough, such as are made by James Small at Rosebank, near Foord, Mid Lothian. The base of it, CM, is a piece of hard wood, pointed before at C to receive a hollow shoeing of iron CO, called the Stock, and tapering a little towards the hinder end, M, called the Heel. This piece is called the Head of the plough. Into its fore part, just behind the stock, is mortised a sloping post, AL, called the Sheath, the front of which is worked sharp, forming the edge of the wedge. Nearer the heel there is mortised another piece, PQ, sloping far back, called the Stilt, serving for a handle to the ploughman. The upper end of the sheath is mortised into the long Beam RH, which projects forward, almost horizontally, and is mortised behind into the stilt. To the fore end of the beam are the cattle attached. The whole of this side of the wedge is fashioned into one plain surface, and the intervals between the pieces are filled up with boards, and commonly covered with iron plates. The Coulter, WFE, is firmly fixed by its shank, W, into the beam, rakes forward at an angle of 45° with the horizon, and has its point E about six inches before the point of the stock. It is brought into the same vertical plane with the land-side of the plough, by giving it a knee outward immediately below the beam, and then kneeling it again downward. It is further supported on this side by an iron stay FH, which turns on a pin at F, passes through an eye-bolt I on the side of the beam, and has a nut screwed on it immediately above. When screwed to its proper slope, it is firmly wedged behind and before the shank.—Fig. 3. No. 2., represents the same plough viewed from above. ST is the right hand or small stilt fixed to the inside of the mould-board LV.

Fig. 4., represents the bottom of the wedge. CM is the head, covered at the point by the stock. Just behind the stock there is mortised into the side of the head a smaller piece DE, called the wrest, making an angle of 16° with the land-side of the head, and its outside edge is in the same straight line with the side of the stock. From the point to the heel of the head is about 33 inches, and the extreme breadth of the heel is about nine. The side of the wedge, called the furrow side, is formed by the mould-board, which is either made of a block or plank of wood, or of a thick iron plate.

The stock drawn in this figure is called a Spear Stock, and is chiefly used in coarse or stony ground, which requires great force to break it up. Another form of the stock is represented in the next figure 4., No. 2. This is called a Feather Stock, and has a cutting edge CF on its furrow side, extending back about ten inches, and to the right hand or furrow side about six. use of this is to cut the sod below, and detach it from the ground, as the coulter detaches it from the unploughed land. This is of great use when the ground is bound together by knotted roots, but it is evident that it cannot be used to advantage in very stony ground. In general, the feather stock is only fit for ground which has been under tolerable culture; but it greatly facilitates the labour of separating the sod. It may reasonably be asked, why the feather is not much broader, so as to cut the whole breadth of the furrow? This is sometimes done. But we must recollect that the sod is not only to be pushed aside, but also to be turned over. If it were completely detached by the feather, and chanced at any time to break off the back of the stock, it would only be pushed aside; but by leaving a little of the sod uncut, it is held fast below while it is shoved aside above, which cannot fail to twist it round. As the wrest advances, it easily destroys the remaining connection, which in general is very slight and crumbling.

The breadth of the stock at the heel determines the width of the furrow. Nine inches will give enough of room for a horse or man to walk in. A greater breadth is of no use, and it expends force in pushing the earth aside. It is a mistake to suppose that a broad stock gives more room for the turned slice to stand on; for whatever is the breadth of the furrow, the successive slices will be left at their former distances, because each is shoved aside to the same distance. When the breadth of a slice exceeds its depth, and it is turned on its side, it will now stand on a narrow base, but higher than before, and therefore will stand looser, which the farmers desire. But in this case it generally falls on its back before it has been far enough removed, and is then pushed aside, and left with the grassy side down, which is not approved of. On the other hand, when the depth considerably exceeds the breadth, the sods, now turned on their sides, must be squeezed home to the ploughed land, which breaks them and tosses them up, making rough work. In wet clay soil, this is also apt to knead them together. On the whole, it is best to have the breadth and depth nearly equal. But all this is workmanship, and has no dependence on the width of the stock behind.

We have already said that the stock is generally level from right to left at the heel. This was not the case formerly, but the wrest was considerably raised behind. It resulted from this form, that the furrow was always shallower on the right side, or there was left a low ridge of unfurled earth between the furrows. This circumstance alone was a bad practice; for one great aim of ploughing is the renewal of the superficial soil. In this way of ribbing the furrows, the sod tumbling over as soon as it is pushed to the top of the rib on the right of the rut made by the plough; the finest parts of it fall undermold, and the rest crumbles above it, making the work appear neat; whereas it is extremely unequal, and what most needs the influence of the weather to crumble it down is sheltered from it. Add to these circumstances, that the hollow is a receptacle for water, with a surface which can retain it, having been consolidated by the pressure of the plough. For all these reasons, therefore, it seems advisable to form the furrow with a flat or level bottom, and therefore to keep the heel of the wrest as low as the heel of the head. For the same reason it is proper to hold the plough with the landside perpendicular, and not to heel it over to that side, as is frequently done, producing the same ribbed furrow as an ill-formed stock.

There is great variety of opinions about the length of the plough. If considered merely as a pointed instrument, or even as a cutting instrument acting obliquely on a given length of sod, there can be no doubt but that it will be more powerful as it is longer; that is, it will require less force to pull it through the ground. But it must also throw the earth aside, and if we double its length we cause it to act twice as much earth at once; for when the plough has entered as far as the heel, the whole furrow side is acting together in pushing the earth to the side. Now it is found, that the force necessary for pushing a mass of earth horizontally along the rough ground is nearly equal to its weight. It would seem, therefore, that nothing is to be gained by making the base of the plough of a great length, except a greater facility in making the first penetration, and this is chiefly performed by the coulter and stock; and a great length renders the plough heavy and cumbersome; and, by causing it to act long on the sod, tends to knead and cake it.

Nothing very precise can be offered on this subject. Some sensible advantage is derived by making the plough taper, especially forward, where it acts as a boring and cutting instrument; and for this purpose it is convenient to give the coulter a slope of 45 degrees. (This has also the advantage of throwing up the stones and roots, he coulter which it would otherwise drive before it through the firm ground.) And for the same reason the edge of the feather has a great slope, it being ten inches long and only six inches broad. But if we pursue this advantage too far we expose ourselves to another risk. It is sometimes necessary to heel over the plough to the right in order to get over some obstruction. In doing this, the coulter necessarily raised for a moment, and the slanting cut now made by the feather becomes the directing groove for the plough. When the feather has a very long slope, this groove has force enough to guide the whole plough; and it is almost impossible for the ploughman to prevent it from running out of the ground to the land-side (a). The feather, therefore, should not exceed 10 or 12 inches in length.

But to return to the length of the plough, from which this observation has diverted us a little, we must add, that a long plough has a great advantage in the steadiness of its motion, having a much more extensive support both on the land-side and below, and being therefore less affected by its inequalities. Accordingly they are now made considerably longer than formerly; and 33 inches has been allowed as a proportion to 9 inches of breadth, in conformity to the most approved ploughs now in use.

We come now to treat of the mould-board. This is board

(a) This is often felt with the excellent plough described by Mr Arbuthnot of Surry, in the Transactions of the Society for the Encouragement of Arts, &c., London. Plough is the most delicate part of the plough, and is to be seen in the greatest variety in the works of different artists, each of whom has a nostrum of great value in his own opinion. It is here indeed that the chief resistances are exerted and must be overcome; and a judicious form of this part of the plough may diminish them considerably, while it performs the work in the best manner. Without pretending to say that the different resistances are susceptible of an accurate determination, we can still draw sufficient information from palpable rules of mechanics to direct us to what would be nearly the best possible form for a mould-board. The task to be performed is to raise, push aside, and turn over to a certain degree, a slice already cut off from the firm ground. As we cannot provide for every inequality of the cohesion or tenacity of the earth, our safest way is to consider it as uniform: the weight of it is always so. As we cannot provide for every proportion between the tenacity and the weight, we must take an average or medium proportion which is not far from that of equality. Conceiving the slice at first as only tenacious, and without weight, it is an easy problem to determine the form which shall give it the intended twist and removal with the smallest force. In like manner we can proceed with a slice that has weight without tenacity. It is equally easy to combine both in any proportion; and it is easiest of all to make this combination on the supposition of equality of weight and cohesion. Supposing the slice like a brick, we know that it requires the greatest force to begin to raise it on one edge, and that the strain becomes less as it rises, till its centre of gravity is perpendicularly above the supporting angle. It requires no force to raise it further; for on pushing it beyond this position, it would fall over of itself, unless withheld by the tenacity of what is not yet raised. But on considering the form or plan of the stock, we find that while the weight of the sod resists most strongly, there is less of it in this situation actually rising, and this nearly in the same proportion with the labour of raising it; and we see that after the sod has attained that position in which it is ready to fall over, it has reached the wider part of the wreft, and is now pushed aside, which requires nearly the same force as to raise it; and this continues to the end of the operation.

When we take all these circumstances into consideration, it appears probable, that the compound resistance does not change much from first to last. If this be really the case, it is an undoubted maxim that the whole operation should proceed equably; if it does not, there must be some part of the sod that makes a resistance greater than the medium; and as the resistances in all this class of motions increase nearly as the squares of the velocities with which they are overcome, it is demonstrable that we shall lose power if we render them unequal.

Hence we deduce this maxim, That as the plough advances through equal spaces, the twist and the lateral sliding of the sod should increase by equal degrees. And this determines a priori the form of the mould-board. This principle occurred to Mr James Small a ploughmaker in Berwickshire, and he published a treatise on the subject in 1784. He has given several methods for constructing mould-boards, which he supposes are in conformity to his principle; but being merely a country artist, and unacquainted with science, his rules do not produce mouldboards having this property of equable operation, although they do not deviate far from it. His book is a very useful and instructive performance, and level to the capacity of those for whom it is intended; and we have here availed ourselves of the author's information on many points.

The high character which Small's ploughs have maintained for 25 years is a strong argument for the truth of the maxim. We shall therefore give such instructions as will enable any intelligent workman to construct such a mould-board without any risk of failure; and if future theory or experience should discover any error in the principles from which this maxim is deduced, by showing that either the weight, the tenacity, or the lateral resistance, is exerted according to a different law from what has been assumed, the directions to be given are of such a nature that they adapt themselves with precision to these changes of principle, and will still produce a perfect and efficacious plough. Our readers will readily acknowledge that this is gaining a great point; because at present the instrument is constructed very much at random, and by a guess of the eye.

Let us now return to the wedge formerly made use of for illustrating the action of the plough. Suppose it placed in a furrow already ploughed, and that the space before the line FE (fig. 1.), which is square from the line of motion FD, is covered with a piece of cloth or carpet, and that the point of the wedge enters upon it at E, and advances to D. It will evidently raise the cloth, which will now cover the side of the wedge, forming the triangle FDE. The line FD is what formerly lay in the angle along the line FD, and FE formerly lay on FE. It is this line FE therefore that we are to raise, shove aside, and twist round, by equal degrees, while the plough advances through equal spaces.

Now, if the length DF of the plough-wedge, reckoned from the point of the stock to the heel, be 33 inches, and the breadth FE behind be 9 inches, the angle DEF or DE will be nearly 74°. The construction of the furrow side of the plough is therefore reduced to this very simple problem, "To make the angle DEF turn equably round the axis DE, while the angular point E advances equably from D to E."

This will be done by means of the following very simple tool or instrument. Let IHFK (fig. 5.) be a piece of hard wood, such as oak, a foot long, three inches broad, and an inch thick. Plant on this another piece of BHFC of the same breadth, four inches long, and half an inch thick. This will leave beyond it a flat 8 inches long. We shall call this the stock of the instrument. Let ABC be a piece of clean oak, half an inch thick, 20 inches long, and three inches broad at the end BC. Let this be fashioned like the stile of a sun-dial, having its angle ABC 74°. Let it have a part BCE square, to the extent of four inches from C, and the rest EA worked into the form of a straight slender rod. Let EFG be a semicircle of clean plane-wood or metal four inches radius: fasten this by small screws to the square part of the stile CE, so that its centre may be at C. Let this semicircle be divided into 180 degrees, and numbered from G along the arch GFE, so that 0° may be at G, and 180° at E. Let this stile and semicircle turn round the line BC by means of small hinges. This instrument may be called the mould-board gage, or protractor. When the stile is folded down on the stock... stock BIK, the point G will be at F; and when it is raised up to any angle, the degrees will be pointed out on the semicircle by the straight edge CF.

Nothing can be more obvious than the manner of employing this instrument once we have determined the most proper position for the sod when the work is completed. Now it seems to be the opinion of the most intelligent farmers, that the best position of the sod is that represented in fig. 6.

Fig. 6 represents a section of the ground and the working parts of the plough, as viewed by a person standing straight before it. ABDC is the unploughed ground, and WB the coulter, kneed in Small's manner. FGKB is the section of the plough (or rather of the whole space through which the plough has passed, for no part of the plough has this section). HOFE is the section of a slice, pushed aside and turned over, so as to lean on the next. HE is that side of the slice which formerly lay on KB. EF is the side cut off by the coulter; and FO is the upper or grassy side. The lower corners are supposed to be a little bruised inwards, as must generally happen.

The sod is pushed 9 inches to the right-hand, and it leans with its grassy side on the preceding furrow, in an angle of about 50 degrees. In this position the grass is turned down so as to rot; and there is a hollow left below to allow the rain water to run freely off, and to receive the earth as it crumbles down by the weather; and if the harrow is dragged across these ridges, it distributes along the surface the mould which was formerly at the bottom. The sod has got a twist of 130 degrees; but it is evident, that after it has been turned 90 degrees, or even a little before this, it is ready to fall over of itself. It is sufficient therefore that it be turned 90 degrees when the heel of the wreft has reached it, and the remainder of the twist is given to it by the wing or flap of the mould-board. This, then, dictates to us the manner of applying the instrument.

Divide the edge DE (fig. 7) of the wreft, or of a lath nailed on it, into 90 equal parts, and continue the divisions backwards to G in the same line to 130. Number the divisions backwards from the point of the fock; then place the protractor on the edge of the wreft, with the point B of fig. 5, at the 90th division (fig. 7); that is, just at the heel, with the stock under the wreft, and the file raised to 90°, and press it home to the joint, so that the stock may be square to the edge, and then the file will be in the position fitting that part of the mould-board. In like manner slide the stock forward to the 80th division, and lower the file to 80°, and it will have the position which suits that part of the mould-board. In the same way slide it forward to 70, 60, 50, &c., and lower the file to 70°, 60°, 50°, &c., and we shall have the position for these several parts of the mould-board; and thus it may be formed to the very point of the fock, because the straight edge of the wreft may be continued so far. A block of wood may be hewed to fit these several positions of the protractor file; and this, when placed with its straight edge on the outer line of the wreft, and cut away behind in the land-side plane, will be the exact shape of the plough-wedge. It would rise up indeed into a tall piece of singular shape, gradually tapering down to the point of the fock; but when cut off parallel to the ground, at the height of about 12 inches, it will form the mould-board, the front or edge of the sheath, and the whole back of the fock except the feather, which is an extraneous piece. The wing or flap of the mould-board is formed in the same manner, by sliding the stock of the protractor to 100, 110, 120, 130, and opening the file to 100°, 110°, 120°, 130°. This will extend the top of the mould-board to about 22 or 23 inches; but the lower part of the wing must be cut away, because it would push the sod too far aside after it has got the proper twist. The form of this part should be such as would exactly apply itself to a plank set at the head of the wreft, parallel to the land-side of the head, and leaning outward 40 degrees. This will be very nearly the case if it be made a sweep similar to the edge of the sheath. Fig. 8 is a resemblance of the surface of the mould-board; AD being the edge of the sheath, E the heel of the wreft, and EBC the wing or flap. When cut through in a perpendicular direction, the section is hollow; if cut horizontally it is convex; and if in the direction CE, making an angle of 74° with ED, it is straight. If the protractor be set on it at D, and gradually slid backwards, the mould-board will gradually open the file, and the file will skim its whole surface without any vacancy between them.

This form is given to the mould-board on the authority of the supposition that the sum of the resistances arising from weight and tenacity remains pretty constant in its whole length. This cannot be affirmed with confidence in any case, and is by no means true in all. In stiff clay foils the effects of tenacity prevail, and in light or crumbling soils the weight is the chief resistance. The advantage of this mode of construction is, that it can be adapted to any soil. If the difficulty of cutting and raising the sod is much greater than that of shoving it aside and turning it over, we have only to make the rise and twist more gentle towards the point of the fock, and more rapid as we advance; and it is easy to do this according to any law of acceleration that we please. Thus, instead of dividing the edge of the wreft DE (fig. 9) continued to G into 130 parts, draw a line GG perpendicular to it, and draw some curve line DG convex toward DG, and divide this into equal parts in the points 10, 20, 30, 40, &c.; and then draw perpendiculars to the wreft edge, cutting it in 10, 20, 30, 40, &c., and apply the protractor to these points. It is evident that the divisions of the wreft line are bigger at D, and grow gradually less towards G; and therefore, because each has 10° more twist than the preceding, the twist will be more rapid as it approaches the end of the mould-board. This curve may be chosen so as to produce any law of acceleration. On the contrary, we produce a retarded or diminished twist by making the curve concave towards DG, as represented by the dotted curve.

The mathematical reader will observe, that this construction aims at regulating the twist round the line of the wreft ED. This does not produce precisely the same regulation round the line FD, which is the line of the plough's motion, and of the sod's position before it is ploughed over. The difference, however, is not worth attending to in a matter so little susceptible of precision. But the twist round the line FD may be regulated according to any law by this instrument with equal facility. Instead of placing the stock of the protractor square with the edge of the wreft, it may be placed square with the land-side of the plough. To do this, draw a line BL (fig. 5, n° 2.) across the stock from the point B, making the angle LBC 16°; and put a brass pin at L, making a hole in the style that it may not be prevented from folding down. Then in using the instrument let the points B and L rest against the edge of the wrest, and proceed as directed.

A still greater variety of forms, and accommodation to particular views, with the same general dependence on principle, will be procured by giving the rod BA a motion round B in the plane of the file, so as to form a file of a variable angle.

A tool may even be constructed in which the rod BA might be a cutting knife: and the whole may be led along by a screw, while this knife turns round according to any law, and would gradually pare away the mould-board to the proper form.

Thus have we reduced the fashioning the operative part of the plough to a rule which is certain. We do not mean by this, that a mould-board made according to the maxim now given will make the best possible plough; but we have given a rule by which this part of the plough can be made unequivocally of a certain quality by every workman, whatever this quality may be, and this without being obliged to copy. No description of any curve mould-board to be met with in books has this advantage; and we say that this rule is capable of any systematic variation, either with respect to the width of furrow, or the quantity or variation of its twist. We have therefore put it in the power of any intelligent person to make such gradual and progressive changes as may serve to bring this most useful of all instruments to perfection. The angle of the head and wrest, and the curve for dividing the wrest line, can always be expressed in writing, and the improvements communicated to the public at large.

After this description of the working parts of a plough, and directions for giving it the most effective form, it will not be improper to consider a little its mode of action, with the view of attaining a more distinct conception of what is done by the ploughman and the cattle, and to direct him in his procedure.

Returning again to the wedge (fig. 1.), we see that it is pressed down at the point D, and as far back along the mould-board as its surface continues to look upward, that is, all the way to the heel of the wrest. Behind this, the perpendicular sections of the mould-board overhang, and look downward; and here, while pressing down the sod, the plough is pressed upwards. These two pressures tend to twist the plough round a transverse line somewhere between the heel and the point. The plough therefore tends to rise at the heel, and to run its point deeper into the ground. Upon the whole, the pressure downwards is much greater than the upward pressure. It is exerted over a much greater space, and is greater in most parts of that space. Behind, very little downward pressure is necessary, the sod being ready to fall down of itself, and only requiring a gentle touch to lay it in a proper position.

In like manner the plough is pressed backward by the resistance made to the coulter and stock, and part of the resistance made to the sloping side of the mould-board: and it is pressed to the left by the other part of the pressure on the stock and mould-board.

All these pressures must be balanced by the joint action of the cattle, the resistance of the bottom, and the resistance of the firm ground on the left hand or land-side.

It is the action of the cattle, exerted on that point to which they are attached, which produces all these pressures. It is demonstrated by the principles of mechanics, that this force must not only be equal to the mean or compound force of these resisting pressures, but must also be in the opposite direction.

It is further demonstrated, that if a body be dragged through any resisting substance by a force acting on any point G, and in any direction whatever GH, and really moves uniformly in that direction, the force exerted exactly balances the resistances which it excites, both as to quantity and direction: And if the body advances without turning round the point by which it is dragged, the resistances on one side of this point are in equilibrium with those on the opposite side.

And lastly, it is demonstrated, that when this equilibrium is obtained, it is indifferent to what point in the line GH the force is applied. Therefore, in fig. 3, n° 1., the force acting in the direction HO may either be applied to the point of the beam H, or to the point N of the coulter, or to the point O of the stock.

When therefore a plough advances steadily, requiring no effort of the ploughman to direct it, if the line of draught OM (fig. 10.) be produced backwards to the point G of the mould-board, that point is the place round which all the resistances balance each other. This point may be called the centre of resistance and the centre of action.

It would be of importance to determine this point by principle; but this can hardly be done with precision even in a plough of a known form; and it is impossible to do it in general for all ploughs, because it is different in each. It even varies in any plough by every variation of the proportion between the weight and the cohesion of the sod. We see how it can be found experimentally in any given uniform sod, viz. by producing backwards the line of draught. Then, if the draught-rope, instead of being fixed to the muzzle of the beam, were fixed to this point, and if it were pulled in the same direction, the plough would continue to perform its work without any assistance from the ploughman, while the sod continued uniform. But the smallest inequality of sod would derange the plough so as to make it go entirely out of its path. Should the resistances between G and D prevail, the plough would go deeper, which would increase the resistances on that side where they already exceed, and the plough would run still deeper. Should the resistances behind G prevail, the heel would be pressed down, and the point would rise, which would still farther destroy the equilibrium, and, producing a greater deviation from the right path, would quickly throw the plough out of the ground.

For these reasons we must not think of attaching the draught to the centre of resistance; but must contrive a point of draught such as shall restore the plough to its proper position, when it has been driven out of it by any obstruction.

The muzzle or end of the beam is a point which will completely suit our purpose. For suppose that the re- It is not without its use to know the absolute force necessary for tilling the ground. This has been frequently measured with a spring steel-yard. One of Small's ploughs, worked by two horses, and employed in breaking up stiff land which had been ploughed before winter, and much consolidated by the rains, required a force of 360 lbs. avoirdupois; and we may state this as the ordinary rate of such work; but moderately firm sod, under good culture, requires at a medium 320 lbs.

As we wish to embrace every opportunity of rendering this work useful to the public, we shall conclude this article with an account of a plough which has just now been recommended to public notice by the Scots Highland Society as extremely proper for a hilly country. The inventor, the Rev. Alexander Campbell minister at Kilcalmonell in Argyleshire, was honoured with the Society's gold medal, value L. 25.

A, the fock (fig. 11); the land-side of which sup. The Ar applies the place of the coulter, and the sole of it serves as a plough. For a feather; it is 18 inches long, and is made of a plate of iron 12 inches broad when finished, and some what under half an inch thick.—B, the head; to be made of iron in a triangular form, 4 inches broad by 2 inches at the thickest part. There are 5 inches of the head fixed in the fock.—C, the beam, 4 inches thick by 5 inches deep, gradually tapered thinner; the length 6 feet.—E, the sheath, must be of the same thickness with the beam above and the head below, and is five inches broad. An iron screw-bolt connects the beam and head behind the sheath.—F, the handles are to be made that the slope of the mould-board, which is fixed to one of them, may be the longer and more gradual. They are 5 feet 8 inches long, and 2 feet 4 inches narrower at the ends.—G, the mould-board, consists of 7 rounded sticks 2 inches in diameter; the covert of them is in the plane of the sole, the rest in succession close to each other above it. This makes the mould-board 14 inches broad. To prevent any earth from getting over the mould-board, a thin slat 4 or 5 inches broad is fixed above it. The mould-board, land-side, and sole of the plough, are clad with iron.—The length is 20 inches; this added to 18 inches, the length of the fock, makes the length from point to heel 3 feet 2 inches.—The muzzle or bridle OP is also of a more convenient and better construction than those commonly in use. By means of the screw-pins at L and M different degrees of land may be given to the plough; the iron rod LH being thereby moved sidewise in the socket LN, and up and down by OP. The rod is 30 inches long, one, broad, and half an inch thick. It is hooked into a screw-bolt at H. Two inches of the rod project at N; in the form of an eye, before the muzzle, to receive the hook of the cross-tree.

The advantages of this plough are said to be: It is not so liable to be interrupted or turned out of its course by stones, roots, &c., as other ploughs are; nor does it dip so deep as to be liable to be broken by large stones or flags. The motion of the muzzle is also thought an improvement. Another advantage it has over other ploughs is, its not being so liable to be choked up by stubble, &c. This we understand to be its chief excellency, and an object much desired in the construction of the plough. Upon the whole, we are informed that this plough is lighter, less expensive, and less liable to go out of trim than the ordinary plough, and that with it two horses can plough land which require four with any other plough.

Such are said to be the advantages of this construction; but we cannot help expressing our apprehension that the uniting the coulter and feather at the point of the lock will expose the plough to great risks of being put out of order. When the upright edge strikes a stone obliquely, especially on the landside, it must be violently twisted round the point of the head; and, having but a moderate thickness at this part, may be broken or permanently twisted. The plough will then be continually running out of its direction; and we apprehend that this defect cannot be amended without taking off the lock and putting it in the fire. When a coulter is bent by the same cause, the ploughman can either rectify it by altering the wedging, or he can straighten it in the field; and it must be observed, that the plough opposes much less resistance to the derangement of this sort of coulter than of the common one. In the common coulter the strain does not so much tend to twist the plough round the line of its motion, as to press it wholly to landward. The resistance to this is great; but a very moderate force will twist it round its line of motion. In either case, if the blow be given in that point of the coulter where the draught line crosses it, there will be no twist of the whole plough, but the point of the plough will be forced horizontally to or from the land. When the blow is out of this line, the strain tends to twist the beam or the plough. Experience will determine which of the two is the most hazardous. These ploughs are made by Thomas Lindsay, Abbeyhill, Edinburgh, and models are to be seen in the hall of the Highland Society.

Plough-drill. See Drill sowing, and Agriculture, p. 318; and Plate VII. and 2d Plate-VII.

In the Gentleman's Magazine for July 1793, p. 602, Mr Wickins of Pondhead Lodge, New Forest, gives an account of a simplified drill-plough invented by himself. Its importance is increased, he thinks, by the cheapness and easy construction of it, because it can be used upon a small scale by a single man, and upon a larger scale by two men, or a man and boy; so that the inconvenience suffered by horses trampling the ground, &c., is hereby avoided. To the drill for sowing is occasionally annexed a blade for hoeing between the rows: "the good effects of which (says Mr Wickins) are no less obvious from its nurturing the growth of the corn, and producing collateral shoots from the application of fresh soil; but also from its affording the means of extirpating the weeds which are so obnoxious to it."

He informs us likewise, that his single-hand-drill hath been seen and approved by the Bath Society; and they have in consequence been pleased to vote him an honorary and corresponding member. Since that time, however, he says, he has very materially improved and simplified it. Mr Wickins's description of his invention is far from being accurate; and, the drawing, of which there is an engraving in the same magazine, was taken when his machine was in its infant and less improved state. He promises, however, further information in the Gentleman's Magazine, and he offers more particulars to such agricultural people as shall desire it. We are far from decidedly thinking that this plough-drill is a real improvement, or that it ever will come to be really and generally useful. We have seen so many of these ploughs and such like improvements make a great noise for a while, and then fall into neglect, without having ever come into use, as makes us shy in forming opinions respecting the utility of those instruments which are so often and so boldly obtruded on the world as the ne plus ultra of improvements in their several spheres. We think it our duty, however, to give every attempt at improvement, especially in the useful arts, all the justice in our power; and, on this account, it has always been our custom to lay before our readers such claims to it as have occurred in the course of our work, whether those claims appeared to ourselves to be just or not.