318 THE PRINCIPLES OF SCIENCE. [CHAP. 



determination of the bulk of a given weight of water at a 

 certain temperature is an operation involving many dif- 

 ficulties, and it cannot be performed in the present day 

 with a greater exactness than that of about one part in 

 5000, the results of careful observers being sometimes 

 found to differ as much as one part in looo. 1 



Weights, on the other hand, can be compared with 

 each other to at least one part in a million. Hence if 

 different specimens of the kilogram be prepared by direct 

 weighing against water, they will not agree closely with 

 each other ; the two principal standard kilograms agree 

 neither with each other, nor with their definition. Accord- 

 ing to Professor Miller the so-called Kilogramme des 

 Archives weighs 1543 2*34874 grains, while the kilogram 

 deposited at the Ministry of the Interior in Paris, as the 

 standard for commercial purposes, weighs 15432-344 grains. 

 Since a standard weight constructed of platinum, or plati- 

 num and iridium, can be preserved free from any appreci- 

 able alteration, and since it can be very accurately com- 

 pared with other weights, we shall ultimately attain the 

 greatest exactness in our measurements of mass, by assum- 

 ing some single kilogram as a provisional standard, leaving 

 the determination of its actual mass in units of space and 

 density for future investigation. This is what is practi- 

 cally done at the present day, and thus a unit of mass 

 takes the place of the unit of density, both in the French 

 and English systems. The English pound is defined by a 

 certain lump of platinum, preserved at Westminster, and 

 is an arbitrary mass, chosen merely that it may agree as 

 nearly as possible with old English pounds. The gallon, 

 the old English unit of cubic measurement, is defined by 

 the condition that it shall contain exactly ten pounds 

 weight of water at 62 Fahr. ; and although it is stated that 

 it has the capacity of about 277-274 cubic inches, this 

 ratio between the cubic and linear systems of measure- 

 ment is not legally enacted, but left open to investigation. 

 While the French metric system as originally designed 

 was theoretically perfect, it does not differ practically in 

 this point from the English system. 



Clerk Maxwell's Theory of Heat, p. 79. 



