applicable to the Gold Bullion Assay. 99 



temperature in our apartments is the rule rather than the 

 exception, the balance-beam is to be regarded in the light of 

 a body constantly undergoing fluctuations in its size. We 

 take the best precautions practicable under the circum- 

 stances, and we find that we are still unable wholly to 

 counteract the influence of this cause upon the equilibrium 

 of the beam ; the effect of heated walls, stoves, &c, are 

 among the causes which bring about unequal heating, and 

 therefore unequal expansion of the arms of the balance ; and 

 it is only in a chamber sunk in the earth to the depth of 

 approximate constant temperature that we can hope to 

 escape from them, and it need hardly be added that such 

 subterranean arrangements are incompatible with the con- 

 ditions under which the ordinary assay of bullion is 

 conducted. 



But if a balance-beam adjusted at 60° Fahr. were 

 slowly and equally raised in temperature to 90° Fahr., 

 would all its parts remain so far just in their proportions 

 as to retain accurately for the instrument, at this higher 

 temperature, its original equilibrium and sensitiveness ? 

 It cannot be said that this would follow. The following 

 extract from Professor W. H. Miller's elaborate memoir 

 on the construction of the new imperial pound, in the Trans- 

 actions of the Royal Society of London, ]856, is conclusive 

 on this point : — 



" In the course of making the preliminary observations 

 some peculiarities of the instrument were discovered, which, 

 though they probably exist in other balances, do not appear 

 to have been hitherto noticed. One of these is that the 

 expansion of one arm by heat, the left in the present case, 

 is a little greater than that of the other arm. Hence, when 

 the weights in the two pans are nearly equal and of equal 

 volume, the reading of the scale in the position of equi- 

 librium diminishes as the temperature of the beam increases. 

 Another is, that the sensibility of the balance, as measured 

 by the number of parts of the scale, equivalent to a given 

 weight, was found to diminish with an increase of tempera- 

 ture. The cause of this is obvious. The beam being of 

 bronze and the knife edges of steel, the balance-beam 

 becomes an over-compensated pendulum, and an increase of 

 temperature increases the distance between the middle knife- 

 edge and the centre of gravity of the beam and weights, 

 supposing the latter concentrated in the extreme knife- 

 edges. Possibly, also, the flexure of the beam may increase 



H 2 



