PNEUMATICS.
353
by 500, and we obtain the impulse in pounds. Mr. Rouse ofLeicestershire made many experiments, which are mentionedwith great approbation by Mr. Smeaton. His great sagacity andexperience in the erection of wind-mills oblige us to pay a con-siderable deference to his judgement. These experiments con-firm our opinion, that the impulses increase faster than the sur-faces. The following table was calculated from Mr. Rouse’s ob-servations, and may be considered as pretty accurate.
Velocity
Impulse on a
Foot in Pounds.
10
0.129
20
0.915
30
2-059
40
3.660
50
5.718 .
60
8.234
70
11.207
80
14.638
90
18.526
100
22.872
110
27.675
120
32.926
130
38.654
140
44.830
150
51.462
The square of the velocity in feet, being multiplied by 16, theproduct will be the impulse or resistance on a square foot in grainsaccording to Mr. Rouse’s numbers. The greatest deviation fromthe theory occurs in the oblique impulses. Mr. Robins comparedthe resistance of a wedge, whose angle was 90 °, with the resistance
of its base; and instead of finding it less in the proportion of 2to 1, as determined by the theory, he found it greater in the pro-portion of 55 to 68 nearly ; and when he formed the body into apyramid, of which the sides had the same surface and the same in-clination as the sides of the wedge, the resistance of the base andface were now as 55 to 39 nearly; so that here the same surfacewith the same inclination had its resistance reduced from 68 to 39by being put into this form. Similar deviations occur in the ex-periments of Chev. Borda; and it may be collected from both,that the resistances diminish more nearly in proportion of the sinesof incidence than in the proportion of the squares of those sines.The irregularity in the resistance of curved surfaces is as great as inplane surfaces. In general, the theory gives the oblique impulseson plane surfaces much too small, and the impulses on curvedsurfaces too great. The resistance of a sphere does not exceed thefourth part ot the resistance of its great circle, instead of being itshalf; but the anomaly is such as to leave hardly any room for cal-culation. It would be very desirable to have the experiments onthis subject repeated in a greater variety of cases, and on largersurfaces, so that the errors of the experiments may be of less con-sequence.
Of the Effects of the Pressure, and Elasticity ofthe Air.
The strong cohesion which takes place between polished sur-faces which are wetted orsnieared with grease is remarkable. That thiscohesion is owing to the atmospheric pressure, is evident from theease with which the plates may be separated in an exhausted re-ceiver. To this cause is also ascribed the very strong adhesion ofsnails, periwinkles, limpets, and other univalve shells, to the rocks.Phe animal forms the rim of its shell, so as to fit the shape of therock to which it intends to cling. It then fills its shell w ith water.In this condition we must act with a force equal to fifteen poundsfor every square inch of touching surface before we can detach it.Phis may be illustrated by tilling a drinking glass to the brim with^ater; and havmg covered it with a piece of thin wet leather, turnon a table, and then try to pull it straight up ; it will require aconsiderable force. But if we expose a snail adhering to a stone111 the exhausted receiver, we shall see it drop off by its ownJ^eight. In the same manner do the remora, the polypus, thelamprey, and many other animals, adhere with such firmness.
oys often amuse themselves by pulling out large stones from thePavement by means of a circle of stiff wetted leather fastened to aVOL. IV.—no. ISO.
string. It is owing to the same cause that the bivalve shell-fisheskeep themselves so firmly shut. We think the muscular force ofan oyster prodigious, because it requires such force to open it;but if we grind off a bit of the convex shell, so as to make a holein it, though without hurting the fish in the smallest degree, it willopen with great ease, as it does also in vacuo. In this way, thepressure, of the air contributes much to the cohesion of bodies,where we do not suspect ils influence. The tenacity of our mor-tars and cements would frequently be ineffectual without this as-sistance. It is owing to the pressure of the atmosphere that a caskwill not run by the cock unless a hole be opened in some otherpart of the cask. If the cask is not quite full, some liquor, indeed,will run out, but it will stop as soon as the diminished elasticity ofthe ait above the liquor is in equilibrio (together with the liquor)with the atmospheric pressure. In like manner, a tea-pot musthave a small hole in ils lid to ensure its pouring out the lea. If,indeed, the hole in the cask is of large dimensions, it will run with-out any other hole, .because air will get in at the upper side of thehole, while the liquor runs out by the lower part of it. On thesame principle depends the performance of an instrument used byspirit-dealers and excisemen for taking out a sample of their spirits.It consists of a long tin plate-tube AB, fig. 14, Plate CXLIII.open a-top at A, and ending in a small hole at B. The end B isdipped into the spirits, which rises into the tube; then the thumbis clapped on the mouth A, and the whole is lifted out of the cask.The spirit remains in it till the thumb be taken off, when it is al-lowed to run into a glass for examination. It is chiefly owin'* tothe pressure of the air that frosts immediately occasion a scantinessof water in our fountains and wells. 'Phis is erroneously accounted.for, by supposing that the water freezes in the bowels of the earth..But the most intense frost of a Siberian winter would not freezethe ground two feet deep; yet a very moderate frost will consoli-date the whole surface of a country, and make it impervious to theair; especially if the frost lias been preceded by rain, which hassoaked the surface. When this happens, the water which was fil-tering through the ground is all arrested, and kept suspended in-its capillary tubes by the pressure of the air, in the very samemanner as the spirits are kept suspended in the instrument just de-scribed by the thumb’s shutting the hole A. A thaw melts thesuperficial ice, arid allows the water to rim in the same manner asthe spirits run when the thumb is removed. Atmospheric, orcommon air, is necessary for supporting the lives of most animals.If a small animal, such as a mouse or bird, be put under the re-ceiver of an air-pump, and the air be exhausted, the animal willquickly be thrown into convulsions and fall down dead ; if the airbe immediately readmitted, the animal will sometimes revive,,especially if the rarefaction has been briskly made, and has notbeen very great. We do not know that any breathing animal canbear the air to he reduced to one-fourth of its ordinary density,nor even one-third ; nor have we good evidence that an animalwill ever recover if the rarefaction is pushed very far, although,continued for a very short time. But the mere presence of the airis by no means sufficient for preserving the life of the animal ; forit is found, that an animal shut up in a vessel of air cannot live init for any length of time. If a man be shut up in a box contain-ing a wine-hogshead of air, he cannot live in it much above an.hour, and long before this he will fiqd his breathing very unsatis-factory and uneasy. A gallon of air will support him a minute.A box EE, fig. 15, may be made,- having a pipe AB inserted intoits top, and fitted with a very light valve at B, opening upwards.This pipe sends off a lateral branch a I) d C, which enters the box .at the bottom, and is also fitted with a light valve at C openingupwards. If a person breathe through the pipe, keeping his nos-trils shut, it is evident that the air winch he expires will not enter ■the box by the hole B, nor return through the pipe CDrf; andby this contrivance he will gradually employ the whole air of thebox. With this apparatus experiments can be made without any.risk or inconveniency, and the quantity of air necessary for agiven time of easy breathing may be accurately ascertained. .Since heated air expands, it must vise, and the pressure of the sur-rounding air will force a current into the rarefied part; hence wemay understand the nature of land and sea breezes, the ascent ofaiv and smoke up chimneys, and the draught of air into warmrooms. Hence we have some data for constructing such stovesand fire-places as may be. suited for particular purposes, AH that4 X- »