1874.] Contributions to Terrestrial Magnetism. 461 



where u and u' are tlie specific volumes o ice and water, and p, p r , v, 

 and v' the pressures and specific volumes of steam over ice and water 

 respectively. 



At the triple point v = v' and p -p' ; and M. Moutier further assumes 



that = , and therefore obtains by substitution 

 dt dt 



and as ^ is positive, being derived from formulae which have reference 

 dt 



to the maximum tension of the vapour, and u' u is negative, it follows 

 that Q, or the latent heat of water, is negative, a result which shows that 

 some of the premises must be false. 



The erroneous assumption, however, is not the possibility of the 

 existence of the triple point, but is contained in the equation 



dp_dp' t 

 ~dt~~dt' 



for Professor James Thomson has recently shown (Proc. Royal Society, 

 Dec. 11, 1873) that M. Regnault's experiments, on the whole, favour the 

 conclusion, which he draws from theoretical considerations, that 

 dp = d 

 dt dt 



and if this equation be true, 



Q = AT (v-te) M3-(t;-tt') 1 



= AT { 0*13 v- 1- 



( J dt 



whence, as at 0., v =210-66, while u and u' differ little from 0-001, 

 it is evident that for a temperature so near zero as that, of the triple 

 point, the expression within the brackets must be positive, and Q is, as 

 it should be, positive also. 



XXI. " Contributions to Terrestrial Magnetism." No. XIV. By 

 General Sir EDWARD SABTNE, R.A., K.C.B., F.R.S. Re- 

 ceived June 18, 1874. 



(Abstract.) 



This paper is presented by the author as No. XIV. of his " Contri- 

 butions to Terrestrial Magnetism," completing the magnetic survey of 

 the northern hemisphere (of which No. XIII. comprised the higher lati- 

 tudes). It consists of a very brief explanatory introduction, followed by 

 Tables, in which (as in No. XIII.) the three magnetic elements are 

 arranged in zones of latitude. These Tables, which form the body of the 

 work, are accompanied by three maps, presenting the results g 

 in isogonic, isoclinal, and isodyaamic lines, 



