Or) 



A HISTORY OF 



a large body of air upon them ; they are lan- 

 guid, relaxed, and feeble, when the air is 

 light, and refuses to give our fibres their pro- 

 per 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 ri- 

 ses higher in cold weather than in warm ; 

 and is usually higher at morning and evening 

 than at mid-day : it generally descends low- 

 er after rain than it was before it. There 

 are also frequent changes in the air, without 

 any sensible alteration in the barometer. 



As the barometer is thus used in predict- 

 ing the changes of the weather, so it is also 

 serviceable in measuring the heights of moun- 

 tains, which mathematicians cannot so readi- 

 ly do : for as, the higher we ascend from the 

 surface of the earth, the air becomes lighter, 

 so the quicksilver in the barometer will de- 

 scend in proportion. It is found to sink at the 

 rate of the tenth part of an inch for every ninety 

 feet we ascend ; so that in going up a moun- 

 tain, if I find the quicksilver fallen an inch, I 

 conclude that I am got upon an ascent of 

 near nine hundred feet high. In this there 

 has been found some variation ; into a detail 

 of which, it is not the business of a natural 

 historian to enter. 



In order to determine the elasticity of air, 

 the wind-gun has been invented, which is an 

 instrument variously made ; but in all upon 

 the principle of compressing a large quanti- 

 ty of air into a tube, in which there is an ivo- 

 ry ball, and then giving the compressed elas- 

 tic air free power to act, and drive the ball 

 as directed. The ball thus driven, will 

 pierce a thick board ; and will be as fatal, at 

 small distances, as if driven with gunpowder. 

 I do not know whether ever th" force of this 

 instrument has been assisted by means of 

 heat ; certain I am, that this, which could be 

 very easily contrived by means of phospho- 

 rus, or any other hot substance applied to 

 the barrel, would give such a force as I doubt 

 whether gunpowder itself could produce. 



The air-pump is an instrument contrived 

 to exhaust the air from round a vessel adapt- 



ed to that purpose, called a receiver. This 

 method of exhausting, is contrived in the 

 simple instrument, by a piston, like that of a 

 syringe, going down into the vessel, and thus 

 pushing out its air ; which, by means of a 

 valve, is prevented from returning into the 

 vessel again. But this, like all other compli- 

 cated instruments, will be better understood 

 by a minute inspection, than an hour's de- 

 scription : it may suffice here to observe, that 

 by depriving animals, and other substances, 

 of all air, it shows us what the benefits and 

 effects of air are in sustaining life, or promo- 

 ting vegetation. 



The digester is an instrument of still more 

 extraordinary effects than any of the former; 

 and sufficiently discovers the amazing force 

 of air, when its elasticity is augmented by 

 fire. A common tea-kettle, if the spout were 

 closed up, and the lid put firmly down, would 

 serve to become a digester, if strong enough. 

 But the instrument used for this purpose is a 

 strong metal pot, with a lid to screw close 

 on, so that, when down, no air can get in or 

 return : into this pot meat and bones are put, 

 with a small quantity of water, and then the 

 lid screwed close ; a lighted lamp is put un- 

 derneath, and, what is very extraordinary, 

 (yet equally true,) in six or eight minutes the 

 whole mass, bones and all, are dissolved into 

 a jelly; so great is the force and elasticity of 

 the air contained within, struggling to escape, 

 and breaking in pieces all the substances 

 with which it is mixed. Care, however, must 

 be taken not to heat this instrument too vio- 

 lently ; for then the inclosed air would be- 

 come irresistible, and burst the whole, with 

 perhaps a fatal explosion. 



There are numberless other useful instru- 

 ments made to depend on the weight, the 

 elasticity, or the fluidity, of the air, which do 

 not come within the plan of the present 

 work; the design of which is not to give an 

 account of the inventions that have been 

 made for determining the nature and proper- 

 ties of air, but a mere narrative of its effects. 

 The description of the pump, the forcing- 

 pump, the fire-engine, the steam-engine, the 

 syphon, and many others, belong not to the 

 naturalist, but the experimental pnilosopher: 

 the one gives a history of Nature, as he finds 

 she presents herself to him ; and he draws 



