having a brass plate fixed on one side, divided as usual, to inches, tenths, and twentieths. On this plate slides a nonius moveable by a small knob, which reads off, as in other barometers, to the 500th of an inch: to this nonius is attached a small portion of brass tube, which embraces the barometer tube; its lower edge being in observation made a tangent to the convex surface of the mercury, as in other well-constructed barometers, and the very narrow slit behind furnishes sufficient light for observation.

When Sir Henry proposes to make an observation, about five minutes before he arrives at the place he takes out the detached thermometer from its place in the end of the mahogany tube, holding it by the upper end at nearly arm's length from his body, and, if the sun shines, in the shade of his person it very soon takes the temperature of the air, and is not sensibly affected by the heat of the hand. The heat being observed and written down, the barometer is turned up, the brass tube half turned, and the instrument held between the finger and thumb of the left hand above the slide, so as to let it hang freely in a vertical position. Since few persons, if any, have sufficient steadiness of hand to prevent little vibrations in the mercury in this position; the hand should be either rested against any fixed body, or, if no such occurs, by kneeling on one knee; the cistern should be let down so as to touch the ground, the left hand holding the barometer in a vertical position, which a little practice will render easy. The index must then be moved by the knobs till its under surface is tangent to the mercury: and a few slight taps should be given to the tube, to ascertain that the mercury is fallen as low as it can. The height being then read off and registered, together with that of the attached thermometer, the brass tube is turned back so as to cover the slits, the instrument gently inverted; and the whole is finished in about two minutes.

Sir Henry does not detail the rules by which the altitude may be deduced from the observations, since these have been given by many authors. But he furnishes a table which indeed is engraven on the barometer, that will enable a traveller to compute altitudes with great facility; the results not deviating far from the truth. He remarks that observations with a single barometer may lead to tolerably accurate results; and states his method of proceeding; which, however, need not here be detailed. The weight of this improved barometer does not exceed a pound and a half. The maker is Mr. Jones, of Mount Street, Berkeley Square.

Several minutiæ in the mechanical construction of these instruments will be more obvious from a few minutes' inspection

than by any further details here. The rules for their use in the ascertaining of altitudes may be learnt by turning to the theoretic part of this work: book iv.

BEAM COMPASSES. See COMPASSES.

BEER-DRAWING MACHINES are contrivances by means of which the beer is drawn from three or four casks at once, from cocks standing in one frame, in the bar of a tavern, or any convenient place above a cellar. These machines are nothing else than an assemblage of small pumps, either sucking or forcing, whose pipes of communication are attached to the lower parts of the respective casks from which the liquor is drawn. The motion is given to the piston sometimes by levers, at others by cranks; most frequently, we believe, by means of a hammerformed lever moving in a vertical plane.

BELLOWS, an instrument constructed for the purpose of alternately drawing and expelling air. In the common culinary bellows the air rushes in at a hole or holes in the bottom, called feeders, over which is a flapping valve, and is expelled through a conical pipe called the nozzle, by means of a kind of mechanism which is too well known to need any description here.

It is not the impulsive force of the blast that is wanted in most cases, but merely the copious supply of air to produce the rapid combustion of inflammable matter; and the service would, in general, be better performed if this could be done with moderate velocities and an extended surface. What are called air-furnaces, where a considerable surface of inflammable matter is acted on at once by the current which the mere heat of the expended air has produced, are found more operative, in proportion to the air expended, than blast-furnaces animated by bellows. There is, indeed, a great impulsive force required in some cases; as, for blowing off the scoriæ from the surface of silver or copper in refining furnaces, or for keeping a clear passage for the air in great iron furnaces. But in general we cannot procure this abundant supply of air in any other way than by giving it a great velocity by means of a great pressure or impulse; the air is admitted into a very large cavity, and then forcibly expelled from it through a small orifice.

The method of producing a continual blast by a centrifugal force has been long known, being mentioned by Agricola de Re Metallica, lib. 6. p. 62. But the first bellows acting upon this principle, of which we recollect a distinct account amongst the moderns, is that invented by M. Teral, in 1729, and described in the Recueil des Machines approuvées par L'Academie Roy, des Sciences, tome 5. This machine is represented in fig. 7. pl. VIII. where AB is a cubical box, with a top rather

arched to this box is adapted a hollow pyramidal frustum c, at the extremity of which is the tube or nozzle D; the capacity of the pyramid not being separated from that of the box. This box contains an arbor or shaft carrying vanes, as GF, posited horizontally, and which are here placed, as it were, out of the box, that their shape and number may be seen. The ends of the arbor run in a proper collar on each side of the box, and one end, as F, passes through the side of the box, and carries a pulley over this pulley passes a cord or band, which also runs round part of a wheel H1, situated at some distance from the bellows, and which is turned by the handle M. Thus it will be manifest, that as this handle turns the wheel HI, it will, by means of the band, turn the pulley F and the arbor and vanes, with a velocity which will be to that of the wheel as the radius of the wheel to that of the pulley. Hence the greater the diameter of the wheel, and the less that of the pulley, the more rapidly will the exterior air (which enters by small holes hh, into the top of the box) be driven by the vanes, and compressed into the truncated pyramid c, and thence expelled at D, in a continued blast; which will likewise be the more violent the greater the action at the handle м. This machine, being very simple, is easily constructed, and at a small expence.

Another bellows, furnishing a uniform blast, is described in the article PNEUMATICS, Encyclopædia Britannica, as below: one cylinder is made to deliver its air into another cylinder, which has a piston exactly fitted to its bore, and loaded with a sufficient weight. The blowing cylinder ABCD (fig. 3. pl. VIII.) has its piston P worked by a rod NP, connected by double chains with the arched head of the working beam No, moving round a gudgeon at R. The other end o of this beam is connected by the rod OP with the crank PO of a wheel-machine; or it may be connected with the piston of a steam-engine, &c. &c. The blowing cylinder has a valve or valves E in its bottom, opening inwards. There proceeds from it a large pipe CF, which enters the regulating cylinder GHKI, and has a valve at top, to prevent the air from getting back into the blowing cylinder. It is evident that the air forced into this cylinder must raise its piston L, and that it must afterwards descend, while the other piston is rising. It must descend uniformly, and make a perfectly equable blast.

Observe, that if the piston L be at the bottom when the machine begins to work, it will be at the bottom at the end of every stroke, if the tuyere T emits as much air as the cylinder ABCD furnishes; nay, it will lie a while at the bottom; for, while it was rising, air was issuing through T. This would make an interrupted blast. To prevent this, the orifice T must be

lessened; but then there will be a surplus of air at the end of each stroke, and the piston L will rise continually, and at last get to the top, and allow air to escape. It is just possible to adjust circumstances, so that neither shall happen. This is done easier by putting a stop in the way of the piston, and putting a valve on the piston, or on the conducting pipe кST, loaded with a weight a little superior to the intended elasticity of the air in the cylinder. Therefore, when the piston is prevented by the stop from rising, the shifting valve, as it is called, is forced open, the superfluous air escapes, and the blast preserves its uniformity.

The Hydraulic Forge Bellows, of Mr. J. C. Hornblower, is a very ingenious contrivance, and is, therefore, described here. This invention is shewn in plate V.

A the plunger, or working part of the bellows, 18 inches square within, which receives the air by a valve in the hinder part opening inwards, which at the stroke by the rockstaff E throws it down the tube indicated by the dotted lines, which has a valve opening into the reservoir D, whence it is led to the tuyere by the pipe P. Length of the plunger 20 inches, stroke nine inches. Diameter of P three inches; of the nozzle 0.6.

The whole is placed in a pit or cistern, having water sufficient to rise to the lower end of the tube where the valve hangs; this tube is the only communication between the upper part and the reservoir D: when as much water is poured in round the working part, over the wash-boards, as will rise within five inches of the upper edge of them, the bellows is ready for use. The little frame-work serves to keep it from rising, and affords a convenient support for the balance and the rockstaff. The area of the pit or cistern ought to be at least twice as much as that of the plunger A.

Mr. Hornblower mentions a very striking difference between the effect of this bellows and a common leathered 30-inch bellows in the same shop. The leathered bellows throws considerably more air to the fire, and its nozzle compared with this is as 75 to 60 in diameter, but it does not produce so great an effect in bringing on the heat, and the noise of this is so great as almost to drown that of the common one. The only difference in other respects is, that in the hydraulic bellows the pipe goes under ground for about eight feet, and the conducting pipe of the other comes down about the same distance from the shop above. (Nicholson's Jour. N. S. vol. I.)

When bellows are made more than usually large, for extensive furnaces, they have been frequently worked by water-wheels. But iron furnaces have, of late, been constructed of such magmilude that no leather bellows could be made sufficientlycapa

cious; and hence large forcing pumps have often been substituted for them. One of the blowing engines used at the Carron iron works, and constructed by the celebrated Smeaton, is described, with an illustrative plate, in the second volume of the PANTOLOGIA, to which the reader must be referred.

BELLOWS, or blowing engine by water. A machine of this kind, in which the stream of air is supplied by the flowing of water, has been long employed at the iron works of Poullaouen in France. The shower of water in its descent through the vertical pipe of the machine, carries down a mass of air along with it (upon the principle of the lateral adhesion of fluids) in the same manner as a shower of rain on the flat surface of the sea, produces that temporary blast of wind which the seamen term a squall. Its effects in producing a blast of air are inferior to that of the steam engine; but in situations which afford a plentiful supply and a sufficient fall of water, it may frequently be employed with advantage.

BLOCK MACHINERY. The machinery for manufacturing ship's blocks in the royal dock-yard at Portsmouth, invented by Mr. Brunel, is greatly and deservedly celebrated. A concise account of it is, therefore, here given.

The machines devoted to this purpose have been separated into four classes. 1. The sawing machine for converting the large timber into proper dimensions for the small machines to operate upon. 2. Those machines which are employed in forming the sheaves. 3. Those which form the iron pins for the blocks. 4. Those by which the shells of the blocks are manufactured. They are all worked by means of two steam engines, each of thirty-horse power. Either of these can be applied indifferently to work the chain pumps, or for turning the wood-mill; and their power is transmitted by a train of wheel-work, to a horizontal shaft, extending along the centre of the middle building, very near its roof. Upon this are a number of wheels and drums, which, by endless ropes and straps, communicates motion to the several subordinate machines.

The order of the processes is this. The elm trees are first cut into short lengths, proper to form the various sizes of blocks, by two large sawing machines, one a reciprocating, the other a circular saw. These lengths of the trees are next cut into squares, and ripped or split up into proper sizes by four sawing benches with circular saws, and one very large reciprocating saw, which is employed for cutting up the pieces for very large blocks.

The scantlings, thus prepared for the blocks, are perforated in three boring machines, with a hole through each to contain the centre pin for the sheaves of the block, and as many other