barrel prevents the box from immediately yielding. The rod therefore slips through the hole of the cross-bars AB. The plate E, therefore, detaches itself from the box. When the shoulder D presses on the bar AB, the box must yield, and be pushed down the barrels, and the water gets up through the perforation. When the piston-rod is drawn up again, the box does not move till the plate E lodges in the seat PP, and thus shuts the water-way; and then the piston lifts the water which is above it, and acts as the piston of a sucking-pump.

This is a very simple and effective construction, and makes a very tight valve. It has been much recommended by engineers of the first reputation, and is frequently used; and, from its simplicity, and the great solidity of which it is capable, it seems very fit for great works. But it is evident that the water-way is limited to less than one-half of the area of the workingbarrel. For if the perforation of the piston be one-half of the area, the diameter of the plate or ball EF must be greater; and therefore less than half the area will be left for the passage of the water by its sides.

15. We come now to consider briefly the forms which may be given to the valves of an hydraulic engine.

The requisites of a valve are, that it shall be tight, of sufficient strength to resist the great pressures to which it is exposed, that it afford a sufficient passage for the water, and that it do not allow much to go back while it is shutting.

The butterfly-valve represented in figures 29, &c. is free from most of the inconveniences, and seems the most perfect of the clack valves. Some engineers make their great valves of a pyramidal form, consisting of four clacks, whose hinges are in the circumference of the water-way, and which meet with their points in the middle, and are supported by four ribs which rise up from the sides, and unite in the middle. This is an excellent form, affording the most spacious water-way, and shutting very readily. It seems to be the best possible for a piston. The rod of the piston is branched out on four sides, and the branches go through the piston-box, and are fastened below with screws. These branches form the support for the four clacks. We have seen a valve of this form in a pump of six feet diameter, which discharged 20 hogsheads of water every stroke, and made 12 strokes in a minute, raising the water above 22 feet,

16. There is another form of valve, called the button or tail valve. It consists of a plate of metal AB (fig. 4. pl. XXIV.) turned conical, so as exactly to fit the conical cavity ab of its box. A tail CD projects from the under side, which passes through a cross-bar EF in the bottom of the box, and

has a little knob at the end, to hinder the valve from rising too high.

This valve, when nicely made, is unexceptionable. It has great strength, and is therefore proper for all severe strains, and it may be made perfectly tight by grinding. Accordingly it is used in all cases where this is of indispensable consequence. It is most durable, and the only kind that will do for passages where steam or hot water is to go through. Its only imperfection is a small water-way; which, from what has been said, cannot exceed, nor indeed equal, one-half of the area of the pipe.

If we endeavour to enlarge the water-way, by giving the cone very little taper, the valve frequently sticks so fast in the seat that no force can detach them.-And this sometimes happens during the working of the machine; and the jolts and blows given to the machine in taking it to pieces, in order to discover what has been the reason that it has discharged no water, frequently detaches the valve, and we find it quite loose, and cannot tell what has deranged the pump. When this is guarded against, and the diminution of the water-way is not of very great consequence, this is the best form of a valve.

17. Analogous to this is the simplest of all valves. It is nothing more than a sphere of metal, to which is fitted a seat with a small portion of a spherical cavity. Nothing can be more effectual than this valve; it always falls into its proper place, and in every position fits it exactly. Its only imperfection is the great diminution of the water-way. If the diameter of the sphere do not considerably exceed that of the hole, the touching parts have very little taper, and it is very apt to stick fast. It opposes much less resistance to the passage of the water than the flat under-surface of the button-valve. The spherical valve must not be made too light, otherwise it will be hurried up by the water, and much may go back while it is returning to its place.

Belidor describes with great minuteness (vol. ii. p. 221, &c.) a valve which unites every requisite. But it is of such nice and delicate construction, and its defects are so great when this exactness is not attained, or is impaired by use, that we think it hazardous to introduce it into a machine in a situation where an intelligent and accurate artist is not at hand. For this reason we have omitted the description, which cannot be given in few words, nor without many figures; and desire our curious readers to consult that author, or peruse Dr. Desaguliers's translation of this passage. Its principle is precisely the same with the following rude contrivance.

18. Suppose ABCD (fig. 2. plate XXIV.) to be a square wooden trunk. EF is a piece of oak board, exactly fitted to the trunk in an oblique position, and supported by an iron pin which goes through it at 1, one-third of its length from its lower extremity E. The two ends of this board are bevelled, so as to apply exactly to the sides of the trunk. It is evident, that if a stream of water come in the direction BA, its pressure on the part IF of this board will be greater than that upon EI. It will therefore force it up and rush through, making it stand almost parallel to the sides of the trunk. To prevent its rising so far, a pin must be put in its way. When this current of water changes its direction, the pressure on the upper side of the board being again greatest on the portion IF, it is forced back again to its former situation; and its two extremities resting on the opposite sides of the trunk, the passage is completely stopped. This board therefore performs the office of a valve; and this valve is the most perfect that can be, because it offers the freest passage to the water, and it allows very little to get back while it is shutting; for the part 1E brings up half as much water as IF allows to go down. It may be made extremely tight, by fixing two thin fillets H and G to the sides of the trunk, and covering those parts of the board with leather which apply to them; and in this state it perfectly resembles Belidor's fine valve.

19. This construction of the valve suggests, by the way, a form of an occasional pump, which may be quickly set up by any common carpenter, and will be very effectual in small heights. Let abcde (fig. 2.) be a square box made to slide along this wooden trunk without shake, having two of its sides projecting upwards, terminating like the gable ends of a house. A piece of wood e is mortised into these two sides, and to this the piston-rod is fixed. This box being furnished with a valve similar to the one below, will perform the office of a piston. If this pump be immersed so deep in the water that the piston shall also be under water, we scruple not to say that its performance will be equal to any. The piston may be made abundantly tight, by covering its outside neatly with soft leather. And as no pipe can be bored with greater accuracy than a very ordinary workman can make a square trunk, we think this pump will not be very deficient even for a considerable suc

tion.

Thus much will, we hope, suffice for the descriptive part of these useful machines: as to the theory of the motion of water in pumps, at the same time that it is extremely intricate, it presents but few results that are of any practical utility. The curious student may be referred to the Maschinenlehre of Langsdorf, the Hydrodynamique of Bossut, the Hydraulique

of Buat, Hachette's Traité Elémentaire des Machines, the Architecture Hydraulique of Prony, and the article Pump in the Encyclopædia Britannica. The last two pieces have furnished us with the most valuable parts of the present article. Some remarks on the variable motion of the piston-rod may be seen under the title PARALLEL motion in this volume.

PYROMETER, a machine contrived to measure the expansion of metals, and other bodies, occasioned by heat.

Muschenbroeck was the original inventor of the Pyrometer: the nature and construction of his instrument may be understood from the following account. If we suppose a small bar of metal, 12 or 15 inches in length, made fast at one of its extremities, it is obvious that if it be dilated by heat it will become lengthened, and its other extremity will be pushed forwards. If this extremity then be fixed to the end of a lever, the other end of which is furnished with a pinion adapted to a wheel, and if this wheel move a second pinion, the latter a third, and so on, it will be evident that by multiplying wheels and pinions in this manner, the last will have a very sensible motion; so that the moveable extremity of the small bar cannot pass over the hundredth or thousandth part of a line, without a point of the circumference of the last wheel passing over several inches. If this circumference then have teeth fitted into a pinion, to which an index is attached, this index will make several revolutions, when the dilatation of the bar amounts only to a quantity altogether insensible. The portions of this revolution may be measured on a dial-plate, divided into equal parts; and by means of the ratio which the wheels bear to the pinions, the absolute quantity which a certain degree of heat may have expanded the small bar can be ascertained: of, conversely, by the dilatation of the small bar the degree of heat which has been applied to it may be determined.

Such is the construction of Muschenbroeck's pyrometer. It is necessary to observe that a small cup is adapted to the ma chine, in order to receive the liquid or fused matters, subjected to experiment, and in which the bar to be tried is immersed.

When it is required to measure, by this instrument, a considerable degree of heat, such as that of boiling oil or fused metal, fill the cup with the matter to be tried, and immerse the bar of iron into it. The dilatation of the bar, indicated by the index, will point out the degree of heat it has assumed, and which must necessarily be equal to that of the matter into which it is immersed.

This machine evidently serves to determine the ratio of the dilatation of metals, &c.: for by substituting in the room of the pyrometric bar other metallic bars of the saine length, and then

exposing them to an equal degree of heat, the ratios of their dilatation will be shewn by the motion of the index.

Muschenbroeck has given a table of the expansion of the different metals, in the same degree of heat. Having prepared cylindric rods of iron, steel, copper, brass, tin, and lead, he exposed them first to a pyrometer with one flame in the middle; then with two flames; and successively to one with three, four, and five flames. But previous to this trial, he took care to cool them equally, by exposing them some time upon the same stone, when it began to freeze, and Fahrenheit's thermometer was at thirty-two degrees. The effects of these experiments are digested in the following table, where the degrees of expansion are marked in parts equal to the part of an inch.

It is to be observed of tin, that it will easily melt, when heated by two flames placed together. Lead commonly melts with three flames, placed together, especially if they burn long.

From these experiments, so far as they are correct, it appears, at first view, that iron is the least rarefied of any of these metals, whether it be heated by one or more flames; aud therefore is most proper for making machines or instruments which we would have free from any alterations by heat or cold, as the rods of pendulums, for clocks, &c. So likewise the measures of yards or feet should, if of metal, be made of iron, that their length may be as nearly as possible the same, summer and winter. The expansion of lead and tin, by only one flame, is