Intelligence and Miscellaneous Articles, 157 



It is noteworthy that in 1872 a somewhat similar depression oc- 

 curred on January 1 7-18. The recorded minima were as follows : — 

 Kew, 28-90 ; Falmouth, 28*75 ; Valencia, 28-50 ; Aberdeen, 28-09 ; 

 Glasgow, 28-08; Stonyhurst, 28-07; Armagh, 28-06. In 1871 a 

 similar depression occurred on January 15-16, the minima being as 

 follows :— Kew, 28*878 ; Falmouth, 28-769; Stonyhurst, 28*592; 

 Aberdeen, 28-411; Glasgow, 28-313 ; Valencia, 28-198; Armagh, 

 28-176. In 1870 a depression occurred on January 14, minima as 

 follows :— Kew, 29*40 ; Falmouth, 29-30 ; Valencia, 29*15 ; Aber- 

 deen, 29-00 ; Glasgow, 28-85 ; Stonyhurst, 28-80 ; Armagh, 28*75. 

 In 1869 a depression occurred on January 14-15, minima as fol- 

 lows : — Kew, 29*55 ; Aberdeen, 29*50 ; Glasgow, 29*25 ; Falmouth, 

 29*15; Armagh, 29*05; Stonyhurst, 29*05 ; Valencia, 28*85. 

 These depressions taking place so nearly at the same epoch in five 

 consecutive years, appear to indicate that about this time in January 

 there is a tendency to a great reduction of pressure over Western 

 Europe. 



The above may be interesting at the present time, especially as 

 the last great depression is so recent. 



January 24, 1873. "W. B. BlRT. 



ON THE THERMAL EFFECTS OF MAGNETIZATION. BY J. MOUTIER. 



The experiments of MM. Jamin and Koger have shown that 

 the intermittent passage of a current in the wire of an electromag- 

 net produces heat : heat is developed at the interruption of the 

 circuit ; it is due to the vanishing of the temporary magnetism of 

 the electromagnet. M. Cazin has lately announced*, after new 

 experiments, that the heat thus produced is proportional to the 

 square of the intensity of the magnetism and to the polar distance. I 

 have sought to account for this simple law by theoretical consider- 

 ations. 



M. Clausiust has demonstrated the following theorem relative 

 to the stationary motion of a system of points — that is to say, a 

 motion in which the position and the velocity of each point do not 

 continually change in one and the same direction, but remain com- 

 prised within certain limits : — The mean vis viva of the system is 

 equal to its virial. The virial, which in questions of mechanics plays 

 a part analogous to that of the potential, is, as is known, half the 

 sum of the products obtained by multiplying the distance between 

 any two points of the system by the force which acts between those 

 two points. 



This theorem conducts to peculiar consequences in the case of 

 magnetization. Let us consider a lengthened bar of soft iron, and 

 suppose magnetism developed by placing the bar in the centre of 

 a coil which is traversed by a current. 



* Comptes JRendus de V 'Acad, des Sciences, vol. Ixxv. p. 1265. 



t Comptes Rendus, vol. lsx. p. 1314 ; Phil. Mag. S. 4. vol. xl. p. 122. 



