1890.] True Weight of a Cubic Inch of Distilled Water. 231 



Methods and Apparatus Employed. 



The weight of a given volume of water is best determined by 

 ascertaining the weight of water displaced by a body or gravimeter, 

 whose weights in air and in vacua, and external linear dimensions, 

 may be precisely determined. For the purpose of the present experi- 

 ments three such gi-avimeters were used : 



C. A platinised hollow bronze circular cylinder, 9 inches in 

 diameter and height. 



Q. A quartz cylinder, 3 inches in diameter and height. 



S. A hollow 6-inch brass sphere. 



The dimensions of C, Q, and S were measured by two comparators, 

 designed for these measurements, geometric lines being traced on 

 C and S for this purpose ; to To7Vo~oth P ar ^ f an inch. The actual 

 rate of expansion of each gravimeter by heat was not separately 

 determined, as the probable errors which arise in ascertaining the 

 rates of expansion of bodies of the particular sizes and forms of C, 

 Q, and S, would be larger than the probable errors which arise in 

 applying the rates of expansion obtained from experiments made by 

 the Fizeau optical method on smaller cubes of similar materials. 



Water. For the rate of the expansion of water the mean corrected 

 observations of Despretz, Kopp, and Pierre, as taken by W. H. Miller 

 (1856), and Foerster (1864), have been followed. If those of Hagen 

 and Mathiessen had been included, the weight of the cubic inch would 

 have been affected by + O0009 grain. For the normal temperature 

 to which we wish to reduce the cubic inch (2 = 62 F.), the maximum 

 density of water to its density at t = 62 F. is 



at 4 C. = 1-000000 

 / 62 o F . = 0'9y8881' 



The water was in each case twice distilled ; no chlorine, carbonic- 

 acid, lead, or lime, being traced, in any quantity to affect the weigh- 

 ings. No correction for the absorption of air was applied, as the 

 distilled water was so far deprived of air, by boiling, and under an 

 air-pump. 



Thermometers. Six standard thermometers, verified both before 

 and after the experiments were used, viz., Centigrade 4517, 4518 

 (Tonnelot) ; Fahrenheit 430 (Kew Committee), 12765 (Negretti and 

 Zambra), and 20065 (Hicks) ; the verifications being based on the 

 two thermometers 4517 and 4518, the values of which had been 

 expressed by Dr. Rene Benoit and Dr. Fernet in relation to the 

 hydrogen thermometers to + O'OOl C., each thermometer being 

 corrected for exterior pressure, and its readings reduced to the 

 horizontal position. The experiments were made as nearly at 62 as 



