plate is laid the paper, first well moistened, to receive the impression; and over the paper two or three folds of flannel. Things being thus disposed, the arms of the cross are pulled, and by that means the plate with its furniture is passed through between the rollers, which pinching very strongly, yet equally, presses the moistened paper into the strokes of the engraving, whence it takes out the ink, and receives the required impression.

PRESSURE ENGINES for raising water by the pressure and descent of a column inclosed in a pipe, have been lately erected in different parts of this country. The principle now adverted to was adopted in some machinery executed in France about 1731 (see Belidor de Arch. Hydraul. lib. iv. ch. 1.), and was likewise adopted in Cornwall about fifty years ago. pressure engine of which we are about to give a particular deBut the scription is the invention of Mr. R. Trevithack, who probably was not aware that any thing at all similar had been attempted before. This engine, a section of which, on a scale of a quarter of an inch to a foot, is shewn in pl. XXIII. was erected about six years ago at the Druid Copper Mine, in the parish of Illogan, near Truro. AB represents a pipe six inches in diameter, through which water descends from the head to the place of its delivery to run off by an adit at s, through a fall of 34 fathoms in the whole; that is to say, in a close pipe down the slope of a hill 200 fathoms long, with 26 fathoms fall; then perpendicularly six fathoms, till it arrives at B, and thence through the engine from в to s two fathoms. At the turn в the water enters into a chamber c, the lower part of which terminates in two brass cylinders four inches in diameter; in which two plugs or pistons of lead, D and E, are capable of moving up and down by their piston rods, which pass through a close packing above, and are attached to the extremities of a chain leading over and properly attached to the wheel o, so that it cannot slip.

The leaden pieces D and E are cast in their places, and have no packing whatever. They move very easily; and if at any time they should become loose, they may be spread out by a few blows with a proper instrument, without taking them out of their place. On the sides of the two brass cylinders, in which D and E move, there are square holes communicating towards F and G, which is an horizontal trunk or square pipe, four inches wide and three inches deep. All the other pipes G, G, and R, are six inches in diameter, except the principal cylinder wherein the piston H moves; and this cylinder is ten inches in diameter, and admits a nine-feet stroke, though it is here delineated as if the stroke were only three feet.

The piston-rod works through a stuffing-box above, and is

VOL. II.

X

attached to MN, which is the pit-rod, or a perpendicular piece divided into two, so as to allow its alternate motion up and down and leave a space between, without touching the fixed apparatus, or great cylinder. The pit-rod is prolonged down into the mine, where it is employed to work the pumps, or if the engine were applied to mill-work, or any other use, this rod would form the communication of the first mover.

KL is a tumbler or tumbling-bob, capable of being moved on the gudgeons v, from its present position to another, in which the weight L shall hang over with the same inclination on the opposite side of the perpendicular, and consequently the end K will then be as much elevated as it is now depressed.

The pipe RS has its lower end immersed in a cistern, by which means it delivers its water without the possibility of the external air introducing itself; so that it constitutes a torricellian column or water barometer, and renders the whole column from A to s effectual: as we shall see in our view of the operation.

The operation. Let us suppose the lower bar KV of the tumbler to be horizontal, and the rod PO so situated, as that the plugs or leaden pistons D and E shall lie opposite to each other, and stop the water-ways G and F. In this state of the engine, though each of these pistons is pressed by a force equivalent to more than a thousand pounds, they will remain motionless, because these actions being contrary to each other, they are coustantly in equilibrio. The great piston H being here shewn as at the bottom of its cylinder, the tumbler is to be thrown by hand into the position here delineated. Its action upon or, and consequently upon the wheel o̟, draws up the plug D, and depresses E, so that the water-way G becomes open from AB, and that of F to the pipe R: the water consequently descends from A to c; thence to GGG, until it acts beneath the piston H. This pressure raises the piston, and if there be any water above the piston, it causes it to rise and pass through F into R. During the rise of the piston (which carries the pit-rod MN along with it), a sliding block of wood 1, fixed to this rod, is brought into contact with the tail K of the tumbler, and raises it to the horizontal position, beyond which it oversets by the acquired motion of the weight L.

The mere rise of the piston, if there were no additional motion in the tumbler, would only bring the two plugs D and E to the position of rest, namely to close G and F, and then the engine would stop; but the fall of the tumbler carries the plug D downwards quite clear of the hole F, and the other plug E upwards, quite clear of the hole G. These motions require no consumption of power, because the plugs are in equilibrio, as was just observed.

In this new situation the column AB no longer communicates with G, but acts through F upon the upper part of the piston H, and depresses it; while the contents of the great cylinder beneath that piston are driven out through GGG, and pass through the opening at E into R. It may be observed, that the column which acts against the piston is assisted by the pressure of the atmosphere, rendered active by the column of water hanging in R, to which that assisting pressure is equivalent, as has already been noticed.

When the piston has descended through a certain length, the slide or block at T, upon the pit-rod, applies against the tail K of the tumbler, which it depresses, and again oversets; producing once more the position of the plugs DE, here delineated, and the consequent ascent of the great piston н, as before described. The ascent produces its former effect on the tumbler and plugs; and in this manner it is evident that the alternations will go on without limit: or until the manager shall think fit to place the tumbler and plugs DE in the positions of rest; namely, so as to stop the passages F and G.

The length of the stroke may be varied by altering the positions of the pieces T and 1, which will shorten the stroke the nearer they are together; as in that case they will sooner alternate upon the tail K.

As the sudden stoppage of the descent of the column AB, at the instant when the two plugs were both in the water-way, might jar and shake the apparatus, those plugs are made half an inch shorter than the depth of the side holes; so that in that case the water can escape directly through both the small cylinders to R. This gives a moment of time for the generation of the contrary motion in the piston and the water in GGG, and greatly deadens the concussion which might else be produced.

Some former attempts to make pressure engines upon the principle of the steam-engine have failed; because water, not being elastic, could not be made to carry the piston onwards a little, so as completely to shut one set of valves and open another. In the present judicious construction, the tumbler performs the office of the expansive force of steam at the end of the stroke.

Mr. Boswell suggests, as a considerable improvement, that the action of this engine should be made elastic by the addition of an air-chamber, on the same principle as that used in fireengines; this, he thinks, might be best effected by making the piston hollow, with a small orifice in the bottom, and of a larger size, to serve for this purpose, as the spring of the air would then act both on the upward and downward pressure of the water. (Nich. Jour. N. S. vols. i. ii.)

PULLEY, one of the simple machines, or as they are commonly called, mechanical powers; its theory is laid down in arts. 148-151, 267, &c. of our first volume. The present article is introduced for the purpose of mentioning some ingenious practical combinations of pulleys, in addition to those exhibited in pl. VI. vol. i.

The usual methods of arranging pulleys in their blocks may be reduced to two. The first consists in placing them one by the side of another upon the same pin: the other, in placing them directly under one another upon separate pins. Each of these methods however is liable to inconvenience; and Mr. Smeaton, to avoid the impediments to which these combinations are subject, proposes to combine these two methods in one.

2

A very considerable improvement in the construction of pulleys has been made by Mr. James White, who obtained a patent for his invention, of which he gives the following description. Fig. 6. pl. XIX. shews the machine, consisting of two pulleys, and R, one fixed and the other moveable. Each of these has six concentric grooves capable of having a line put round them, and thus acting like as many different pulleys, having diameters equal to those of the grooves. Supposing then each of the grooves to be a distinct pulley, and that all their diameters were equal, it is evident that if the weight 144 were to be raised by pulling at s till the pulleys touch each other, the first pulley must receive the length of line as many times as there are parts of the line hanging between it and the lower pulley. In the present case there are 12 lines b, d, f,&c. hanging between the two pulleys, formed by its revolution about the six upper and lower grooves. Hence, as much line must pass over the uppermost pulley as is equal to twelve times the distance of the two. But, from an inspection of the figure, it is plain that the second pulley cannot receive the full quantity of line by as much as is equal to the distance betwixt it and the first. In like manner, the third pulley receives less than the first by as much as is the distance between the first and third; and so on to the last, which receives only one-twelfth of the whole. For this receives its share of line n from a fixed point in the upper frame, which gives it nothing; while all the others in the same frame receive the line partly by turning to meet it, and partly by the line coming to meet them.

Supposing now these pulleys to be equal in size, and to move freely as the line determines them, it appears evident, from the nature of the system, that the number of their revolutions, and consequently their velocities, must be in proportion to the number of suspending parts that are between the fixed point above mentioned, and each pulley respectively. Thus the outermost

pulley would go twelve times round in the time that the pulley under which the part n of the line, if equal to it, would revolve only once; and the intermediate times and velocities would be a series of arithmetical proportionals, of which, if the first number were 1, the last would always be equal to the whole number of terms. Since then the revolutions of equal and distinct pulleys are measured by their velocities, and that it is possible to find any proportion of velocity on a single body running on a centre, viz. by finding proportionate distances from that centre; it follows, that if the diameters of certain grooves in the same substance be exactly adapted to the above series (the Fine itself being supposed inelastic, and of no magnitude), the necessity of using several pulleys in each frame will be obviated, and with that some of the inconveniences to which the use of the pulley is liable.

In the figure referred to, the coils of rope by which the weight is supported are represented by the lines a, b, c, &c.: a is the line of traction, commonly called the fall, which passes over and under the proper grooves, until it is fastened to the upper frame just above n. In practice, however, the grooves are not arithmetical proportionals, nor can they be so; for the diameter of the rope employed must in all cases be deducted from each term; without which the smaller grooves, to which the said diameter bears a larger proportion than to the larger ones, will tend to rise and fall faster than they, and thus introduce worse defects than those which they were intended to obviate.

The principal advantage of this kind of pulley is, that it destroys lateral friction, and that kind of shaking motion which is so inconvenient in the common pulley. And lest (says Mr. White) this circumstance should give the idea of weakness, I would observe, that to have pins for the pulleys to run on is not the only nor perhaps the best method; but that I sometimes use centres fixed to the pulleys, and revolving on a very short bearing in the side of the frame, by which strength is increased, and friction very much diminished; for to the last moment the motion of the pulley is perfectly circular: and this very circumstance is the cause of its not wearing out in the centre as soon as it would, assisted by the ever-increasing irregularities of a gullied bearing. These pulleys, when well executed, apply to jacks and other machines of that nature with peculiar advantage, both as to the time of going and their own durability; and it is possible to produce a system of pulleys of this kind of six or eight parts only, and adapted to the pocket, which, by means of a skain of sewing silk, or a clue of common thread, will raise upwards of a hundred weight.