20 M. It. Schwendler on an Arrangement for the 



in which ¥{w) denotes an arbitrary function of w, represents 

 the complete integralof the preceding partial differential equation. 

 But for steam at maximum of density we suppose w, and ac- 

 cordingly also F(w), constant; then M = constant; whence fol- 

 lows that for steam at maximum of density ^M = ; or 



pdp — ypdp — 0; 

 consequently 



dp dp 



P P 



If this equation is compared with the logarithmic differential 

 of formula (36), we find 



whence follows 



dp 



, -..dp add 



7 



Consequently this appears., to me to be at the same time an 

 incontrovertible proof of the validity of Baron Wrede's formula, 

 and of the correctness of the principle laid down here. 



[As many of Joule's early papers obtained publicity in the Philo- 

 sophical Magazine, and as those of Mayer have also appeared in its 

 pages, I have thought that the above translation (which has recently 

 been made for me) of one of Colding's memoirs (of date 1850) on 

 the same subject might with propriety be inserted in the same 

 Journal. There are other papers by Colding, of at least equal im- 

 portance, which may perhaps also appear in English. — P. G. Tait.] 



II. Arrangement for the Discharge of long Overland Telegraph 

 Lines. By Louis Schwendler, Esq.* 



WHEN organizing more regular and instantaneous tele- 

 graphic communication between the Presidency towns of 

 India, and especially between Calcutta and Kurrachee, it was 

 observed tbat discharges occurred sufficiently strong to affect the 

 relay of the sending station, and giving rise to the so-called 

 "return beats." These discharges f, through the relay of the 



* From the Journal of the Asiatic Society of Bengal, vol. xl. part 2 

 (1871). Communicated by the Author. 



f It is well known that an overland telegraph-line acts as a Leyden jar 

 in the same manner as a submarine cable, having, however, only a much 

 smaller capacity, on account of the insulating layer (the air between the 

 telegraph-wire and surrounding conductors) being very thick. But though 

 the capacity may he small in comparison with that of any cable, it is 

 evident that a long, well-insulated overland line may show nevertheless 



