'no 
Mr. cavendish’s Account of 
again by itfelf, which gives the quantity of refpirable air ufed. 
It is needlefs to determine the quantity of nitrous air by 
weight ; becaufe, as the quantity ufed is always fufficient to 
produce the full diminution, a fmall difference in the quantity 
makes no fenlible difference in the diminution*. In this man- 
ner of determining the quantities by weight, care fhould be 
taken to proportion the lengths of the copper wires in fuch 
manner that the lurface of the water in A and M fhall be on 
* Mr. de saussure alfo determines the quantity of air which he ufe 3 by 
weight ; but does it by weighing the veflels containing it in air. This method is 
liable to fome inaccuracy, as the air in tlve veflel is apt to be comprefled by put- 
ting in the hopper ; though, I believe, that, if care is taken to pufti in the 
hopper (lowly, the error arifing from thence is but fmall. It is alfo lefs expedi- 
tious than weighing them under water, as fome time is neceflarily loh in 
wiping the wet off the veflels ; but, on the other hand, it requires lei's apparatus, 
which makes it titter for a portable apparatus as Mr. de saussure’s was. If any 
gentleman is delirous of adapting this method of determining the quantities to the 
above defcribed manner of mixing the airs, nothing more is required than to 
have glafs hoppers fitted to the veflel A and to the bottle M. 
It is needlefs to mention, that in both thefe methods no fenfible error can arifc 
from any difference in the fpecific gravity of the air; for the thing found by 
weighing the veflel is the difference of weight of the included air and of an equal 
bulk of water, which, as no air is lefs than 500 times lighter than water, is very 
nearly equal to the weight of a quantity of water, equal in bulk to the included 
air. 
It mu ft be obferved, that a common balance is not convenient for weighing the 
veflels of air under water, without fome addition to it; for the lower the veflel 
of air finks under the water, the more the air is comprefled, which makes the 
veflel heavier, and thereby caufes that end of the beam to preponderate. This 
makes it neceflary either to have the index placed below the beam, as in many 
aflay balances ; or by fome other means to remove rile center of gravity of the 
beam fo much below the center of fufpenfion as to make the balance vibrate, not- 
withllanding the tendency which the compreflibifiry of the air in the veflels has to 
re vent it. 
'' tll«* 
