On the Temperature of the Freezing-point of Mercury. 475 



theless changed in both directions. When, however, we reflect on 

 this circumstance, we are led to see that since we have two sets of 

 disturbances taking place simultaneously, so we must also have two 

 sets of earth-currents. Now one of these disturbances, which we may 

 perhaps call the soft-iron one, reproduces those small and rapid 

 changes which take place in the primary force, while on the other 

 hand the hard-iron disturbance averages these small changes and 

 presents us with a disturbance- wave of long period. Precisely, then, 

 as in the magnetic curves we have waves of short period superim- 

 posed upon waves of long period, so will it be in the earth-current 

 curves. Those currents due to the soft-iron disturbances will be 

 superimposed upon those due to the hard-iron ones, with this 

 difference, that we are not entitled to assume that the proportion in 

 intensity between the two simultaneous earth-currents must be pre- 

 cisely that which exists between the rates of change of the two cor- 

 responding simultaneous disturbances. It will be apparent that this 

 feature of duality ought also to be presented by the aurora ; and here 

 it is well known that we have at least two phenomena, one of a more 

 fitful and the other of a more permanent character, namely, the 

 streamers and the auroral arch. We may suppose the first of these 

 phenomena to correspond to the soft-iron, and the second to the hard- 

 iron disturbances. Indeed it is questionable whether the different 

 varieties of aurorse are confined to these two ; for General Sabine 

 has informed me that he himself, along with the late Sir Edward 

 Parry, observed at Lerwick in the Shetland Isles in 1818, at the 

 same instant, two auroral arches crossing one another at an angle. 

 But, be this as it may, when we reflect that there are many kinds of 

 particles in our earth, some of which may be affected more rapidly 

 than others by a primary magnetic force, we shall cease to wonder 

 that the phenomena presented are of a complicated description. 



All these considerations have induced me to think that it is lost 

 labour to attempt a quantitative comparison when our observation 

 of the magnetic disturbances and their corresponding earth-currents 

 is confined to one locality ; and it will be seen from this paper, that 

 while endeavouring to uphold the hypothesis of induced action, I 

 have done so by a comparison of a general and qualitative rather than 

 by one of a quantitative nature. 



"On the Change in the Elastic Force of a Constant Volume of dry 

 Atmospheric Air, between 32° F. and 212° F., and on the Tempe- 

 rature of the Freezing-point of Mercury." By Balfour Stewart, 

 M.A., F.R.S. 



The author gave a detailed description of his apparatus, and of 

 the method employed in drying the air. 



The result of his experiments gave as the coefficient of increase of 

 elasticity of air of constant volume for 1°F. 0-002040, this being 

 slightly different from that given by Begnault, which is 0-002036. 



He also finds that the temperature of the freezing-point of mercury 

 is constant, and that its value on Fahrenheit's scale, as given by an 

 air-thermometer, is — 3 ?°- 93, while as recorded by a standard mercu- 

 rial thermometer it is — 38°-00. It is probable that this difference 

 is owing to an anomalous contraction of mercury before it freezes, 

 similar to the corresponding expansion of water ; but this effect in the 



