1880.] 



J. F. Teiinant — On Standnrd Wciglrts. 



43 



As the Parliamentary Standard o£ England P S. has its true weight 

 in vacuo,* the first impression would be, that the Commercial Standard in 

 ordinary air should weigh the same as P S. in vacuo : but this has not been 

 the practical solution. When the Houses of Parliament were destroyed in 

 1834, the English standards were destroyed in them, and the new Standard 

 was meant to be a restoration of the old one. Now the old Standard 

 was a brass Troy Pound made in 1758, of which there were a variety of 

 copies more or less accurate. On the evidence from these, and some other 

 sources, was determined the difference between the lost pound and a piece 

 of platinum, both taken in vacuo. Then (the Government of the day 

 having determined that the new Standard should represent the Avoirdupois, 

 and not the Troy Pound as before), a second piece of Platinum P S. was 

 made which should weigh very nearly 7000 such grains as those of which 

 the destroyed Pound (U) contained 5,760, both being taken in vacuo, and it 

 is believed that the result was accurate to a very small fraction of a grain, 

 thanks to the great labours of Professor Miller. In reverting to the 

 Commercial Pound, that would be 7,000 grains of which U weighed 5,760, 

 both taken in air, and then, as the density of the new commercial Pound 

 was very close to that of U, all sensible uncertainty arising from the de- 

 struction of U and the impossibility of knowing its exact density would 

 vanish. 



Professor Miller found the Platinum Pound P S. to be 7000-00093 

 grains of U both weighed in vacuo, and by Act of Parliament, this was 

 declared to be the true standard of weight, and that one grain should be a 

 seven-thousandth part of it. The Commercial Pound W was an imaginary 

 Pound, supposed to be made of brass of a density of 8'15034, which was 

 what Professor Miller estimated as the density of the lost Pound U. 

 Though the standard in vacuo was changed, as above, by a minute quan- 

 tity, it would have been wrong to change the weight of W in air. In 

 order then that its weight in vacuo should become that of the Pound P S., 

 it became necessary to suppose that this weight in vacuof, and consequently 

 its density, were changed, and to ascribe to it a new density of 8'1430. 



The present definition of the English Commercial Pound then is — 



* I have followed the wording of my predecessors, hut I should prefer to call the 

 " iveight in vacuo" the Mass," and restrict the term " ^mV/Zii!" to the ajiparent force 

 exercised. If this distinction were made, the questions involved would be much clearer. 

 The Parliamentary Standard has been treated as one of Mass ; hence two of the gilt 

 secondary standards, each of the same Mass as P. S., wiU not have ordinarily the same 

 weight, unless they havo the same specific gravity. 



t The weight in vacuo was 7000 grains of U, and in consequence of the Act of 

 Parliament it became necessaiy that it should bo the same as that of P S. or 7OUO-O0093 

 grains of U. 



