310 Daniell on a new Hygrometer. 



sible to exist in the space of a cubic foot, its weight would be 

 found upon the same line as its pressure, 4.910grs. But its 

 bulk is expanded by the existing temperature of the air ; there- 

 fore we must seek in the fourth column for the degree of ex- 

 pansion, at 55° 1.114, and at 70° 1.145, and apply the cor- 

 rection thus ; — 



Bulk at 70O. Bulk at 550. Grs. Grs. 



1.145 : 1.114 : : 4.910 : 4.777 

 the weight required. 



Now the state of the atmosphere, assumed above, would con- 

 stitute fine weather, and one of two things, or a modification of 

 both, must happen before any precipitation of water could take 

 place; either the temperature of the air must fall below 55°, or 

 the quantity of vapour must increase to 7.776grs. in the cubic 

 foot, the maximum quantity that could exist at 70", or the point 

 of condensation might become intermediate, by a corresponding- 

 rise and fall of the two. 



In the first case, the precipitation would probably be only 

 slight and transitory, such as mist, fog, or small rain. In the 

 second case, it would assume the form of hard rain and storms; 

 while in the third, some conjecture might be formed of its 

 probable duration and quantity, according as one or other of its 

 causes prevailed. 



But the hygrometer can be made to measure, not only the 

 quantity and force of vapour existing at any time in the air, 

 but also, it may be applied to indicate the force and quantity of 

 evaporation. Mr. Dalton, in the course of that important train 

 of investigation to which I have before had occasion to refer, 

 ascertained that the quantity of water evaporated in a given 

 time, bore an exact proportion to the force of vapour at the 

 same temperature. The atmosphere obstructs its diflfusion, 

 which would otherwise be almost instantaneous, as in vacuo, 

 but this obstruction is overcome in proportion to the force of 

 the vapour. The obstruction, however, does not arise from the 

 weight of the atmosphere, for that would prevent any vapour 

 from arising under 212°; but, as Mr. Dalton observe, is caused 

 by the vis inerticB of the particles of air, and is similar to that 

 which a stream of water meets with in descending amongst 

 pebbles. In ascertaining this point at ordinary atmospheric 



