For the Coining of Medals the progress is the same, in effect, with that of money: the principal difference consists in this, that money, having but a small relievo, receives its impression at a single stroke of the engine; whereas, for medals, the height of their relievo makes it necessary that the stroke be repeated several times: to this end the piece is taken out from between the dyes, heated, and returned again; which process, in medallions, and large medals, is sometimes repeated fifteen or twenty times, before the full impression be given; care being taken, every time the planchet is removed, to take off the superfluous metal stretched beyond the circumference, with a file.

An improvement has been lately suggested in the coiningpress, by a Mr. Huigenan, we believe, who has introduced the principle of the heart-wheel both in this contrivance and in his universal lever. The method Mr. H. recommends may be understood by referring to fig. 4. pl. XIV. CB is part of a table or plane on which is fixed the box containing the dye F of the image side of the coin, and CA is a lever to which is attached the dye E of the reverse side in a case retained by screws; and this is so posited, that by turning CA on the centre c the parts E and F may be brought the one immediately above the other. G is an elliptical or heart-wheel turning upon a fixed centre by the handle or winch H, and, acting upon the friction-wheel D, gradually forces down the end A of the lever, and carries with it the dye E, causing it to press very hard upon the metal placed on the lower dye F, at the time the extremity of the elliptical wheel is in contact with the upper part of the wheel D. Then the motion of the winch proceeding, the spring s raises up the lever CA, and thus leaves room to remove the metal: place another at F, and repeat the operation. The whole, it is obvious, may be carried on with considerable expedition; but whether the method is on the whole preferable to that before described is what we do not here attempt to decide.

In the machinery invented for coining by Boulton and Watt, and lately introduced in the Mint, the screw presses for cutting out the circular pieces of metal are worked with great facility, and both the edges and the faces of the money are coined at the same time, with such superior excellence and cheapness of workmanship as will prevent clandestine imitation. By means of this machinery four boys can strike 30,000 pieces of money in an hour: the machine has this farther advantage, that it serves as a register, and keeps an unerring account of the number of pieces struck.

COMPASSES (BEAM), a kind of compasses used to draw large arcs, and to take large extents, &c. These compasses consist of a straight beam or bar, of 18 inches, 2 feet or more

in length, carrying two brass cursors; one of these being fixed at one end, the other sliding along the beam, with a screw to fasten it on occasionally. To the cursors may be screwed points of any kind, as of steel, brass, pencils, &c. The fixed cursor has sometimes an adjusting or micrometer screw applied to it, for the more nice obtaining of extents.

The beam is divided commonly into inches, tenths, and half tenths but Mr. Walton, an ingenious mechanic, in the proof department of the Royal Arsenal, Woolwich, has improved this instrument and much extended its utility, by applying a nonius to its scale, which renders it fit to take distances to hundredth parts of an inch. Part of a beam with the additions of Mr. Walton are shewn in fig. 1. pl. XVIII. where IK represents more than 4 inches in length of a beam, which is made of ebony, the divisions being marked upon brass laid into the ebony, ABCD and EFGH are two brass cases which nearly fit the beam, and may slide to and fro upon it: these brass cases carry the cursors and points L and M, which are fastened into sockets by means of screws at N and o. The case ABCD has two screws bc and a, both of which are turned by means of forked turnscrews the first of these screws, bc, serves to move the brass case backwards and forwards on the beam, in order to adjust the point L so as to correspond with the commencement of the divisions on the beam; and when that is done the screw a, by pressing a spring, makes the whole fast to the beam. The other brass case EFGH carries the cursor and point м, as well as the moveable nonius ei: this nonius is at the estremity of a piece efghki, which is moved to and fro upon the case EFGH by means of the screw opqs, which is turned by the milled head rst: the shoulders at p and q prevent the screw from moving either backward or forward with respect to the line FH, while the threads of the screw between o and p, by taking upon the moveable piece efghki, cause the nonius to move along the edge of the graduated scale of the beam: turning the head of the screw in the direction rst moves the nonius in the direction from K towards 1 on the beam and turning that head in the direction tsr advances the nonius according to the increasing measure upon the scale from 1 towards K. The screw d with its milled head P, by pressing upon a spring, will at any time make the case EFGH fast to the beam, and thus prevent, when necessary, any change of distance between L and M. Fig. 2. is a transverse section of the brass case EFGH it serves to shew the form 1, 2, 3, 4, 5, of the beam, bevelled off to an edge at 4; also the bevel of the nonius at e; the dovetail at q, against which one shoulder of the micrometer screw presses; and the piece fo, into which the three screws l, m, n (fig. 1.) enter.

Other parts of the construction will be sufficiently obvious from these figures.

CONDENSER, a pneumatic engine or syringe, by which an extraordinary quantity of air may be crowded or pushed into a given space; .so that frequently ten times as much air as an equal space would contain out of the engine may be thrown in by means of it, and its egress prevented by valves properly disposed.

The condenser is made either of metal or of glass, and either in a cylindrical or globular form; and the air is forced into it by an injecting syringe. The receiver or vessel containing the condensed air should be made very strong, to bear the force of the air's elasticity thus increased: for which reason it is commonly made of brass. When glass is used it will not sustain so great a condensation of air; but the experiment will, notwithstanding, be rendered more entertaining, as the effect of the condensed air upon any subject put within the receiver may be viewed through the glass.

CONDENSER of Forces, a name given by M. Prony to a contrivance for obtaining the greatest possible effect from a first mover, the energy of which is subject to augmentation or diminution within certain limits; and in general to vary at pleasure the resistance to which the effort of the first mover forms an equilibrium in any machine whatever, without changing any part of their construction.

The general problem in mechanics, of which this condenser is intended as a practical solution, is enunciated by M. Prony in these terms:

"Any machine being constructed, to find, without making any change in the construction, a means of transmitting to it the action of the first mover, by fulfilling the following conditions, viz.

"1. That it may be possible at pleasure, and with great speed and facility, to vary the resistance (against which the efforts of the first mover must continually make an equilibrium) in limits of any required extent.

"2. That the resistance, being once regulated, shall be rigorously constant until the moment when it is thought proper

to increase or diminish the same.

"3. That in the most sudden variations of which the effort of the first mover may be capable, the variation in velocity of the machine shall never undergo a solution of continuity."

M. Prony applies his solution of this problem to the dynamic effect of wind: it will be easy to make the same general when the other first movers are used.

The section and plan of the machine are exhibited in pl. XIV. oo represents the vertical arbor to which windmill sails are adapted; eeee is an assemblage of carpentry, of which one of the radii, oe, bears a curved piece bd, of iron or steel: vertical axes of rotation aaa, being placed round the axis oo, also divide the circumference in which they are found into equal parts.

Each of these axes carries a curve, af, of iron, steel, or copper; so situated, that when the wind acts upon the sails the curve bd presses against one of the curves af, and causes the vertical axis to which this last curve is fixed to make a portion of a revolution.

The curves bd and af must be so disposed, that when bd ceases to press on one of the curves af, it shall at the same instant begin to act upon the following curve: the number of axes which are proved with these curves must be determined by the particular circumstances of each case; and it is also practicable to substitute, instead of bd, a portion of a toothed wheel having its centre at the axis oo, and to place portions of pinions instead of the curves af; but the dispositions represented in the figure are preferable.

Each of these axes aaaa (which are all fitted up alike, though, for the sake of clearness, only one of them has its apparatus represented in the drawing), carries upon it a drum or pulley ttrr, on which is wound a cord that passes over a pulley p, and serves to support a weight o by means of the lever FG, upon which this weight may be slided and fastened at different distances from the point of motion G.

The same axes aa pass through the pinions qq, to which they are not fixed; but these pinions carry clicks or ratchets, which bear against the teeth rr; so that, when the weight o tends to rise, the ratchet gives way, and no other effect is produced on the pinion qq, either by the motion of the axis or of the drum ttrr, excepting that which causes the ascent of the weight qq. But the instant that the curve or tooth bd ceases to bear against one of the curves af, after having caused the corresponding weight o to rise, that weight o tends to redescend, and then the toothed wheel rr acts against the ratchet, so that o cannot descend without turning the pinion qq along with the drum ttrr.

The pinion qq takes in the wheel ab, from the motion of which the useful effect of the machine immediately results; so that the effect of the descent of one of the weights o is to solicit the wheel AB to motion, or to continue the motion in concurrence with all the other weights Q, which descend at the

same time. This wheel AB carries beneath it oblique or bevelled teeth GD, which take in a like wheel CE, and cause the buckets at s to rise.

From the preceding description it is seen that the machine, being supposed to start from a state of repose, the wind will at first raise a number of weights 9, sufficient to put the machine into motion, and will continue to raise new weights while those before raised are fallen; so that the motion once impressed will be continued.

Among the numerous advantages of this new mechanism we may remark the following:

I. No violent shock can take place in any part of the mechanism.

2. The useful effect being proportioned to the number of weights 9, which descend at the same time, this effect will increase in proportion as the wind becomes stronger, and causes the sails to turn with more velocity.

3. The weights 9 being moveable along the levers FG, it will always be very easy to place them in such a manner as to obtain that ratio of the effort of the first mover to the resistance which will produce the maximum of effect.

4. From this property it results that advantage may be taken of the weakest breezes of wind, and to obtain a certain product in circumstances under which all other windmills are in a state of absolute inactivity. This advantage is of great importance, particularly with regard to agriculture: the windmills employed for watering lands are sometimes inactive for several days, and this inconvenience is more particularly felt in times of drought. A machine capable of moving with the slightest breeze must therefore offer the most valuable advantages.

CRAB or GIN, an engine used for mounting large guns on their carriages, &c. It is composed of three long and stout legs meeting together at their tops; these legs are round poles of about 12 or 13 feet long, whose diameters at the lower ends are about four inches, five just below the roller, besides the cheeks that are added to them in that place, and about 3 inches above.

Two of these poles can be fixed at a certain distance from each other, by means of two iron bars placed horizontally, one being about four feet long, the other about seven; and a roller is made to run upon pivots turning on, or in, these two poles: this roller is commonly 7 inches in diameter, and six feet long. A portion of 20 inches is left square at each end, and holes made in each to receive the handspikes by which the men turn the roller: but the middle part is made cylindrical, to wind the cable upon. The transverse iron bars are fixed with one end