Diving ISLichint. i?3 



within, rife and dcfcend when he thinks proper, as will ap«< 

 pear from what follows : 



" Thofe who have made experiments with air will not 

 doubt that 50 cubic feet of it are fufficient to maintain the 

 life of a man for two hours ; and the poffibility of afc'ending 

 and defcending at pleafure may be thus explained : 



" As this machine contains fuch a volume of air that it 

 IS lighter than water, it will require a confiderable weight to 

 make it fink below the furface. That as little, however, as 

 poffible of the fpace deftined for containing air may be loft, 

 lead may be employed, in the infide of the machine at b 9 

 fufficient to make it fink fo far that a cubic foot of it only 

 fhall remain above the furface of the water. An additional 

 weight then of 100 pounds will not only deprefs it below the - 

 furface of the water, but make it fink even to the bottom. 



" But the fame thing may be effected without weight ?>■ 

 by leflening the volume of the contained air. This is done 

 by means of the pi [ton c, which fits clofely into the cylinder 

 tl, and which, by means of the rack e, tbe pinion f, the 

 wheel g, and the endlefs fcrew b, together with the winch i, 

 can be moved either upwards or downwards. 



" The machine, however, mull not only be ftrong an<f 

 durable, but be conftrucled according to the depth to which 

 it is deftined to defcend and the preffure on the pifton, that a 

 man at fuch a depth may have fufficient ftrength to deprefs- 

 it by means of the winch, and thereby to enlarge again the 

 volume of air. It will readily appear that, by railing the 

 pifton, tbe machine mull fink to the bottom; and that, by 

 depreffmg it, the machine muft again afcend ; as was proved 

 by a fmall model which I made for that pvrrpofe. 



" If a machine be conftrudtcd on a huge fcale, according 

 to the proportions exhibited in the annexed figure, it will 

 be found, by calculation, that, if we eft i mate the friction o£~ 

 the pifton, when made water-tight, at 200 pounds, and that 

 of the wheel-work at 300, though it may perhaps not be fo 

 much, the winch at the depth of 130 feet will not require a 



force 



