8 The Eev. T. R. Robinson on the Relation between the Temperature of 



lated for a decimal progression of P, and the intermediate values got by in- 

 terpolation. 



As these temperatures are reckoned from 32\ but the reading sets out 

 from the actual temperature of the atmosphere, a correction is applied by adding 

 to it the required amount. 



Wlien the heat was expected to be powerful, an additional resistance, 

 sometimes as much as 1500", was included in the circuit, and gradually lessened 

 till the full current passed. If this precaution be not attended to, the momen- 

 tum which the counterpoise, &c., acquires in its rapid descent, is sufficient to 

 produce a permanent elongation of the wire in its softened state, so that the 

 index will not return to zero. 



The battery used was at first on Daniell's principle, the acting surface of 

 each metal being fifty-four square inches. With small wires this acts very Avell ; 

 but I found that, when the resistance of the circuit is little, the sulphate of 

 copper is expended more rapidly than it can be supplied. In this respect, as 

 also in giving a more powerful current, the chloride of copper is a better 

 charge. Aftei'wards I used Grove's cells, each having 19.3 inches of platinum, 

 and found them much more convenient. When the negative charge is 2 

 nitric acid, 2 water, and 1 sulphuric acid, they exceed in power twice as 

 many of the others, and for a long time there is no extrication of nitric oxide. 

 All inconvenience from this may be avoided by arranging them outside the 

 window, and bringing the conductors through its wood-work. 



When the circuit is completed through the pjTometer, its index moves 

 very rapidly at first ; several seconds, however, elapse before the wire becomes 

 luminous, and 30'- or 3.5'- are necessary for its attaining its full heat : when it 

 becomes stationary, a few light taps are given to the stand, to loosen any 

 friction of the pivots. The ignition never extends to the extremities of the 

 wire, especially the lower one ; and the upper part of the wire is evidently the 

 hottest, both for the cause already noticed, and the ascending current of heated 

 air. It is a curious circumstance, that, when the temperature is above zero, 

 the wire, which is then dazzling white, seems enlarged to three or four times 

 its real diameter, an effect of irradiation which disappears when it is viewed 

 through a darkly coloured glass. By reducing the current, and with it the 

 intensity of the ignition, the dark portion at the bottom of the wire extends, 



