356 Scientific Proceedings, Royal Dublin Society. 



engineering work and in all scientific work done ' with care,' and 

 could only be detected with those elaborate appliances and pre- 

 cautions, and the lavish expenditure of time and money necessary 

 for 'great care': conditions fulfilled only once or twice in a 

 century. 



Next as to weights. 



Professor Miller's comparison between the imperial lb. and the 

 kilogramme was made ' with great care.' According to him the 



lb. = 453-59265 grammes, 

 = 453-6 (1 - -000,016,2). 



Hence the error of taking 453*6 grammes as equal to the 

 imperial lb. amounts to exactly one-quarter of an imperial grain 

 in the kilogramme, or to a little more than one-tenth of a grain 

 in a pound. This is the weight of a little more than a third of a 

 cubic inch of air; and moreover, it does not come into account 

 except in the rare cases where it is absolute and not relative 

 weights that are in question. Accordingly, for all mercantile, and 

 almost all scientific work, it may legitimately be neglected, and the 

 convenient number 453*6 used for the number of grammes in the 

 imperial lb. 



Next as to standards of liquid measure. 



The standard gallon is defined by law as containing lOlbs. of 

 distilled water weighed against brass weights, in air, at the tem- 

 perature of 62° F., the barometer being at 30 inches (see the Acts 

 of 1824 and 1878). 



The Royal Commission appointed in 1816, on whose report 

 this definition was founded (of which Commission Kater was a 

 member), determined the weight of a cubic inch of water, weighed 

 as above, to be 252*458 grains ; whence it follows that the number 

 of cubic inches in a gallon is 277*2738. l 



1 This determination was made with, 'great care.' Another determination, -which 

 however does not appear to be entitled to more authority, has since been made 

 with 'great care,' and according to it the number of cubic inches in a gallon is 

 277'123. Hence between two determinations made with ' great care ' the difference is 

 more than five parts in 10,000. In fact, the determination is of such a kind that the 

 probable error must necessarily be large. Nor need this surprise us when we consider 

 the definition which the Acts of Parliament lay down. "Water, if exposed to air, absorbs 

 gas : this renders its density indefinite. The temperature to be determined, 62° F., is 

 one which cannot be determined with such accuracy as to avoid a sensible variation in 



