IMPERIAL STANDARD TROY POUND WEIGHT. 
495 
We thus obtain for the absolute weight of S. p , if U had its specific gravity, 
= 7’9 . . . . 5759*4397 grains troy. 
= 8*0 5759*4507 
= 8-1 5759*4614 
= 8-2 5759-4719 
= 8-3 5759-4821 
= 8-4 5759-4921 
= 8-5 5759-5018 
There remains consequently (if even we suppose U of brass) an uncertainty about its 
absolute weight = 5759-5018 — 5759-4397 = 0*0621 gr., or about 0‘06 gr. 
In fact, though we have five different pounds in excellent preservation, and com- 
pared with the lost standard with the greatest care and the best instruments, and 
though the number of these comparisons surpasses 600, there remains an uncertainty 
of 0-08 gr., or at least 0-06 gr., as to its real weight ; and this solely on account of its 
specific gravity and expansion not being known. It is to be hoped that no pound 
will in future ever be declared a legal standard, unless these elements (the knowledge 
of which is indispensable even for a single comparison with a good balance) are pre- 
viously determined with the greatest possible precision. A standard pound is in- 
tended for the purpose of obtaining from it accurate copies ; and it therefore involves 
a contradiction if those elements are not well ascertained. 
N.B. The formula, for the “ Barometer reduction” in page 467, should have the 
sign — prefixed to it. 
