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Show 168 PRESSl11l.E OF AIR. the surfaces of all things, the bricklayer would ~eed no mortar, the joiner no nail and no glue; the tailor too, would have no use for thread, and the seam. ., of shoes would never give way. A world of that kind would be a very stable and lasting world, and the words " wear and tear" might be left out of the vocabulary. But there wot~ld be to? much o~ stability; and there would be little motwn, or change, and no life. Thus the extreme pureness of the atmosph~re, and the property that it. has of insinua~ing itself m~o the very smallest ope~ungs, and pressmg equally ll1 all directions makes tt the grand pathway on land; for whatever 'is moved on land is literally moved in the air; and not only that, but, as the air i.s pressed together by its own weight, and thu~ heavwst nearest the earth, so that even the heaviest substances are pressed a little more upward than they are pressed downward by ~he air, their. real 'Yeights are diminished by the wetght o~ a quantity of a1r equ~l to their bulk. At the same tlme, they are held m their upright position by the pres~ure of th~ air all around them ; and that pressure 1s so considerable as to amount to about thirteen tuns on the body of a man. That weight is, however, so nicely balanced, so perfectly the same at all points ?f .the same elevation from the ground, and the a1~ IS so perfectly springy or elastic,-forms so. dehg.htfully soft .a cushion around all nature, that 1ts reststance to orrhnary motions is not felt, a?d it does not ruffi~ the powdery plumage on the wm$ of. the most delicate moth. Walkin<r we do not feel1t at all; and even when we run with all our speed, it is nothing but a light zephyr in our face, which fans and cools us, and rea.ly assists in speeding us on. And it is worthy of remark how the natural co':"erings of many animal~ are " fine~ away" at. thetr extremities, till they ghde aln:tost u;tto the thu~ness of the air itself. Take an entire han of any ammal, FUR AND FEA TRERS. 169 more especially tho~e that steal UJ?On their prey un. awares, ~n~ you wtll find the pomt .so exquisitely fine that 1t 1s absolutely next to nothmg. Painters are ~ware of that.property, and so J?ake thPir finest pencils of the hau of the sc:. 'Jle, which admits of being made into a little brush that will hold a charge of colour, and yet all the points of the hairs united together make one point, as fine as that of the finest needle-indeed far finer. The same quality may 1 be observed, in greater perfection, if possible, in the fur of the bat, or the fringes of the owl's feathers· and the little feathers upon the night moths are th~ most wonderful of any. Creatures that are furnished in that manner act in concert with the air, as it were, while they are mov~ ing through it; and thus, though the bat be the most fluttering thing that flies, and the owls and the moths be generally far more clumsy than the dayhawks and the butterflies, yet they make their way through the ~ir ~ith much less noise. Many plants too have theu yteldmg borders ; and the wind murmurs in the groves when their leaves are on and ~oes not. howl as it .does among the leafless sprays ~n the wmter; and 1t never roars on fertile plains as 1t does among naked rocks. But the air is not merely the pathway of nature it is the carrier, and it is as sensitive in its own ~04 tions as it is yiel~iug to those of every thing else. !he least alterah~n. of teml?erature, or pressure, mstantly puts ~he air mto motwn. If any thing advances, the air moves off before it to make room· 3:nd if any thing !ecedes, the air follows at the sam~ time to support It. If any thing is heated above the average, the air ascends with the excess of heat · ~nd if anr thing is cooled, the air condenses and close~ m upon It, not only as a protection a<rainst greater cold, but to impart positive heat. N~ matter how great or ho~ small the object is, or how long or how short the d1stance, the air is sensible to the very p |