About 154 of such elements would be required to produce the same 

 electromotive force as a single cell of Daniell's ; if, in Daniell's bat- 

 tery, the whole chemical action were electrically efficient. A 

 battery of 1000 copper and bismuth elements, with the two sets 

 of junctions at 0° and 100" cent., employed to work a galvanic 

 engine, if the resistance in the whole circuit be equivalent to that 

 of a copper wire of about 100 feet long and about one-eighth of 

 an inch in diameter, and if the engine be allowed to move at such 

 a rate as by inductive reaction to diminish the strength of the 

 current to the half of what it is when the engine is at rest, 

 would produce mechanical effect at the rate of about one-fifth of a 

 horse-power. The electromotive force of a copper and bismuth ele- 

 ment, with its two junctions at 0° and 1°, being found by Pouillet to 

 be about jIq of the electromotive force when the junctions ate at 

 0° and 100°, must be about 163. The value of 0q for copper and 

 bismuth, according to these results (and to the value 160-16 of /a 

 at 0°), or the quantity of heat absorbed in a second of time by a cur- 

 rent of unit strength in passing from bismuth to copper, when the 

 temperature is kept at 0°, is j^, or very nearly equal to the 

 quantity required to raise the temperature of a grain of water from 

 0° to 1° cent. 



Monday^ bth January 1852. 



Right Reverend BISHOP TERROT, Vice-President, in 



the Chair. 



The following Communications were read : — 



1. On the Absolute Intensity of Interfering Light. By 

 Professor Stokes. Communicated by Professor Kel- 

 land. 



In this communication Professor Stokes described a method which 

 he had discovered, by which he could express, mathematically, the ab- 

 solute intensity of interfering light, as in the case of the images 

 found in the focus of a telescope pointed to a star, and having a 

 grating over the object-glass. The result was the same as that 

 previously aimed at by Professor Kelland, but the mode of getting 

 at it was shorter. 



