THE MASS OF A CUBIC INCH OF DISTILLED WATER. 
339 
In such weighings in water minute bubbles of air are found to be carried down 
by the gravimeter or by the wire by which it is suspended, but by repeated immer¬ 
sions, and examination with a small telescope and a glow-lamp, it is possible 
to avoid appreciable error in this direction. When not in use the gravimeters were 
kept in air of similar temperature to that at which the weighings were made. 
It is, of course, difficult, during such weighings, to find the precise temperature 
of a comparatively large volume of water, and as any uncertainty of 0°'2 Fahr. in 
the reading of the thermometer during the weighing of the cylinder C in water 
would amount to nearly three grains, it was desirable not only to use sensititive 
thermometers, but to place their bulbs at proper depths, and to read them quickly 
by means of cathetometers during the periods of weighing. 
All weighings in air were reduced to “ normal air/’ or air which at the tempe¬ 
rature of 62° Fahr., the barometer being at 30 inches at 32° Fahr., reduced to 
latitude 45° and at sea level, contains four volumes of carbonic anhydride in every 
10,000 volumes of air, and also contains two-thirds of the amount of aqueous vapour 
contained in saturated air. This is the average condition of the air at Westminster 
(latitude 51° 29' 53", at 16 feet above sea level), where the weighings were made. 
A litre of such air, if dry, but containing the above proportion of carbonic anhydride, 
would weigh 1'293934 grammes ; and, if dry, but containing no carbonic anhydride, 
would weigh 1'293519 grammes. 
2x2 
