262

AIR-PUMPS, BY NEWMAN, WATKINS AND HILL, KNIGHT, Ac.

[Class X.

to the regular delivery of the coins being little more thanmomentary. 1

It will be understood that, for the reason above assigned, ;this account of its operation is merely conjectural. SirJohu Ilerschel remarks that it would be easy to devisea machine for a similar purpose, founded on the principles,first, of preventing the descent of either scale, unless theexcess of weight in it surpassed the legal “remedy/’ andallowance for wear and original error, which might bedone, by giving each end of the beam a support frombelow' equal to the remedy, but rising only on its descent;and, secondly, by presenting in the closest proximity tothe under-side of the beam, but not in actual contact, theextremity of a conducting rod of copper, completing agalvanic circuit through the beam and its central steelkuife-edge resting on a steel plane, and thus animatingone or other of two electro-magnets, which by its attrac-tion on a soft iron bar should shift either the point ofdelivering or the point of reception of the descendingcoins, on their egress from the scale. The circuit beingcompleted, the right-hand conductor would thus deviatethe coins into the receptacle for heavy, and the conductoron the left into that for light coins, while a state of rest ofthe beam, corresponding to any excess within the remedy ,either wav, v’ould cause no deviation, but allow the coinsto fall straight into the middle receptacle. This descrip-tion might enable any mechanist to construct such amachine, probably at small cost, and without infringingany patent right.

It is highly satisfactory to find an instrument of suchhigh importance as the balance so well represented in theExhibition ; and when it is considered, in order to havea balance as perfect as possible, how very many circum-stances are to be attended to, it must be deemed highlyhonourable to the exhibitors of good balances to have pro-duced instruments standing the test of the rigorousexamination to which they have been subjected. Weobserve, however, in the Exhibition no self-weighingbalances for small weights (up to 1,000 grains), on theprinciple of Marriott’s spring-balance for large ones, oron that of the simple extension of a long spiral spring,which is a very convenient form of instrument whengreat accuracy is of less consequence than expedition.

Air-Pumps.

Most of the important facts which we know relative tothe properties of air may be said to have been elicited bythe employment of the air-pump. It is satisfactory tofind that the Exhibition contains instruments of a newand improved construction; but it is matter of regret thatso small a number of manufacturers have contributed.

The air-pumps most commonly used are made eitherwith brass stop-cocks, or with valves of oiled silk orleather. The former, when properly constructed, andnew, generally act well, by exhausting the air thoroughly ;but after having been in use for some time, they becomeless accurate than those furnished with valves, after anequal amount of wear. But the valves themselves arealso imperfect, owing to the pressure of the external airon that within the piston preventing the latter from risingwhen the air is almost exhausted. Attempts to overcomethis difficulty have been made in the pumps exhibited,some of which are very superior in their action, and haveprobably never been surpassed.

The exhibitor of the best air-pump is Newman (No.674, p. 468*). This has a ground glass plate, to avoidinjury from sulphuric acid. It has two pumps with metalvalves: on one of these are two barrels, open at the topto the atmosphere, as in the common table air-pump; thisarrangement exhausts the receiver quickly, but on accountof the nature of the valves, not beyond 0*4 inch or 0*5inch of mercurial pressure. The other pump has a singlebarrel, with an oil cistern at the upper part, the air beinglifted through a valve at the bottom of this cistern. Ifanything re-enters the barrel it can only be oil, which isbrought out with the air at the next up-stroke of thepiston. The piston has a metal valve; but the openingof this valve is not necessary to the continuation of theexhaustion, as the piston at its lowest point passes belowthe aperture leading to the receiver. This construction i

of air-pump exhausts more thoroughly than any yetknown.

In the experiments which were tried, the reading ofthe barometer at the time being 30*08 inches, the gaugeof the air-pump stood at 30*06 inches. A Council Medalwas awarded to Mr. Newman for this air-pump.

Watkins and Hill (No. 650, p. 466*) exhibit a newdouble-barrelled air-pump, on a plan suggested by Mr.Grove. It has oiled-silk valves, and is so constructed asto leave the least possible residue of air iu the barrel,after each stroke of the piston. The piston is solid,'with-out a valve, and the shape of its lower part is an obtusecone: part of this cone rises at the top of eacli strokeabove the aperture leading to the receiver, and the airwhich has entered the barrel is, by the down stroke, forcedthrough a valve at the apex of the hollow cone terminatingthe lower end of the barrel, to which the lower end of thepiston fits very accurately. The piston-rods pass throughair-tight leather collars in the tops of the barrels. Thispump exhausted the air till the elastic force Mas only0*05 inch of mercury. A Prize Medal was voted toMessrs. Watkins and Hill.

Kniciit and Sons (No. 453, p. 462) exhibit an air-pump on Siemen’s patent. It consists of two cylinders,of different diameters, the smaller one placed below thelarger, and separated from it by a plate forming thebottom of the upper and the top of the lower cylinder. Apiston-rod, common to both cylinders, passes through astuffing-box in the plate, attached to which are two valvedpistons, working iu their respective cylinders. The ad-vantage of this construction is, that the pressure of theexternal air on the oiled-silk valve of the larger cylinderis taken off by the vacuum formed in the smaller one, andin consequence no greater resistance is offered by thevalve than that arising from its adhesion and tension.The exhaustion of this pump is very rapid, and in thetrial amounted to 0*24 inch of mercury.

Varley and Sons (No. 257, p. 436) exhibit an air-pumpupon a new construction. It is worked by a continuousrotatory motion of the handle, slide-valves being used toopen and close the communication. On the piston arriv-ing at one end to expel air from the barrel, it is followedby rarefied air from the receiver; the slide-valve closesupon the receiver, and connects the two sides of thepiston; the residual air expands into the larger space, be-comes equally rarefied, and the subsequent motion of thevalves separates these spaces, and connects the receiverwith the closed end; the piston then returns to exhaustair into this end of the barrel and to expel it from theother, and thus continuous exhaustion is kept up; for,how rare soever the air becomes, it keeps flowing afterthe piston continually. The barrel is twice filled forevery entire revolution of the handle. This pump has asingle barrel with double action : it exhausts quickly, andthe exhaustion was found to be 0*05 inch for a moment,but could not be maintained.

Varley and Sons also exhibit a second air-pump, smallerthan the former: it has a double-acting barrel. Thepiston is worked by means of a crank and continuous cir-cular motion of the handle.

IIeywood (No. 404a, p. 453). A rotary table air-pump, with self-opening valve worked by a crank motion.It acts with singular smoothness and ease.

Gogerty (No. 407, p. 454; exhibits an air pump of thecommon table form. It gave an exhaustion of 0*3 inchof mercury, which is considerable for this kind of air-pump.

Ladd (No. 291a, p. 440) exhibits an air-pump, whichis single-barrelled and of a cheap construction, withoutany other claim to notice.

Yeates (No. 332, p. 446) exhibits a double-acting air-pump of a cheap construction, which appears to be goodfor its price. Its valves are of oiled silk. The com-munication is in the middle of the barrel, a valve beingplaced there and at each end. Honourable Mention wasawarded to Mr. Yeates.

Bryan (No. 408, p. 454) lias exhibited a double-actionair-pump, constructed without valves, and having a rota-tory motion. The pump consists of a barrel, to whichtwo smaller ones are attached on either side. In the