180

Illustrations

[Book II.

of the hand is held over the aperture of an exhausted receiver, the weightor pressure of the air on the back being no longer balanced by its actionon the palm, the hand is irresistibly held to the vessel. A criminal ormaniac, whose hands and feet are thus treated, would be as effectuallysecured as by fetters of iron. Few things are better calculated to excite'wonder, and even horror, in the savage mind, than a part of the bodybeing thus rendered helpless, as if spell-bound by some invisible agent.A few years ago, an experiment was made with the chief of a delegationof Pottawatamies to the seat of government, at which the writer waspresent. Although the Interpreter previously endeavored to make themunderstand the intended Operation, it Mall readily be supposed that suchan attempt raust necessarily have been fruitless. When the receiver wasexhausted, he was amazed to find his hand immoveable,' and that, likeJeroboam ’s, “ he could not pull it in again to him.” In his endeavors tofree it, he rapidly uttered the characteristic interjections, ugh ! ugh ! andat last shrieked, as if in despair of being delivered from the power of thewhite enchanter; when his attending warriors flourished their tomahaw'ksand rushed to his rescue, as if roused by the war-whoop.

It is not, however, necessary to have recourse to the air-pump, forproofs of atmospheric pressure. Numerous operations daily occur incommon life which equally establish it. When a person washes hishands, if he lock them together so as to bring the palms close to eachother, and then attempt to raise the central parts so as to form a cavitybetween them, at the same time keeping the extremities of the palms inclose contact, he will feel the atmospheric pressure very sensibly : if ,theexperiment be made under water, the effect will be mo re obvious still.Analogous to this is the attempt to open the common household bellowswhen the valve and nozzle are closed. The boards are then forced openwith difficulty, in consequence of the pressure of the air on their exteriornot being balanced by its admission within. If the materials and jointswere made air-tight, and the orifices perfectly closed, the strongest manthat ever lived could not force them open. This experiment, we believe,was familiär to the ancients ; for we are indebted to them for both theblacksmith and domestic bellows. Another experiment of theirs, of asimilar kind, was with the syringe. When the small orifice is closed withthe finger, the piston is pulled up with difficulty, on account of the airpressing on its surface ; and the moment we let go of the handle, itinstantly drives it back, in whatever position the implement is held.The ordinary syringe seldom exceeds three-fourths or one inch in diame-ter, and any person can thus draw out the piston ; but one of six or seveninches diameter would require a giant’s strength; and one of a foot orfifteen inches would resist the efforts of two or three horses.

Numerous illustrations of atmospheric pressure may be derived fromthe animal kingdom. The boa constrictor when it swallows its preyaffords one. As soon as this serpent has killed a goat or a deer, he coversits surface with saliva : this appears necessary to lay the long hair of theseanimals close, in order to prevent air from passing between the body ofthe victim and the interior of the devourer’s throat. After taking thehead into his mouth, by a wonderful muscular energy he alternately dilatesand contracts the posterior portions of his body, until the pressure of theatmosphere forces into his flexible skin an animal whose bulk greatlyexceeds that of his own. But if air were to pass between the body of thevictim and the dilatable gullet of the boa, while the latter was making avacuum to receive it, the pressure of the atmosphere would be neutralizedas effectually as if a gash were made through his skin in front of the victim.