THE EARTH. 903 



thus confined and expandinjj, was snfficient lor the explo^i(ln of 

 a world. 



Many instruments have been formed to measure and deter- 

 mine these different properties of the air; and which serve 

 several useful purposes. The barometer serves to measure its 

 weight ; to tell us when it is heavier, and when lighter. It is 

 composed of a glass tube or pipe, of about thirty inches in 

 length, closed up at one end : this tube is then filled with quick- 

 silver ; tliis done, the maker clapping his finger upon the 

 open end, inverts the tube, and plunges tiie open end, fin. 

 ger and all, into a basin of quicksilver, and then takes his 

 finger away ; now the quicksilver in the tube will, by its own 

 weight, endeavour to descend into that in the basin ; but the 

 external air, pressing on the surface of the quicksilver in the 

 basin without, and no air being in the tube at top, the quicksil- 

 ver will continue in the tube being pressed up, as was said, by 

 the air, on the surface of the basin below. The height at which 

 it is known to stand in the tube, is usually about twenty-nine 

 inches when the air is heavy ; but not above twenty-six when 

 the air is very light. Thus, by this instrument, we can with 

 some exactness determine the weight of the air ; and, of conse- 

 quence, tell before-hand the changes of the weather. Before 

 fine dry weather, the air is charged with a variety of vapours, 

 which float in it unseen, and render it extremely heavy, so that 

 it presses up the quicksilver ; or in other words, the barometer 

 rises. In moist, rainy weather, the vapours are washed down 

 or there is not heat sufficient for them to rise, so that the air 

 is then sensibly lighter, and presses up the quicksilver with 

 Jess force ; or, in other words, the barometer is seen to fall. 

 Our constitutions seem also to correspond with the changes of the 

 weather-glass ; they are braced, strong, and vigorous, with a large 

 body of air upon them ; they are languid, relaxed, and feeble when 

 the air is light, and refuses to give our fibres their proper tone. 



But although the barometer thus measures the weight of the. 

 air with exactness enough for the general purposes of life, yet it 

 is often affected with a thousand irregularities that no exactness 

 in the instrument can remedy, nor no theory account for. When 

 high winds blow, the quicksilver generally is low : it rises higher 

 in cold weather than in warm ; and is usually higher at morning 

 and evening than at mid-day : it generally descends lower afte/ 



