44 Wm. A. Norton on the Variations 



In answer to this objection, I have to offer the following consid- 

 erations* 1. It is to be observed that the entire horizontal mag- 

 netic force of the vapor is added or abstracted with it, while the 

 horizontal force is otherwise affected only by the variations of 

 temperature. 2. The force of the vapor, which, from a situa- 

 tion above the needle falls below it, is changed from a diminish- 

 ing to an increasing action. This will be readily seen on refer- 

 ring to fig. 11 ; in which N represents the position of the needle, 



Fig. 11. 



and HNRa horizontal line at the height of the needle and sit- 

 uated in the plane of the magnetic meridian. A particle m, in 

 this plane will exert its force in the direction Ns, and therefore 

 tend to diminish the horizontal force (R being supposed to be 

 south of N), but when it falls to the position m' it acts in the di- 

 rection Nr, and therefore now tends to increase the horizontal 

 force. It is here taken for granted that a particle continues to 

 act magnetically after it has left the earth's surface. It is only 

 by a detailed discussion that we can determine whether this sup- 

 position be true or not. It is enough, for our present purpose, 

 that it is not at variance with the theory. 3. The depth to which 

 any considerable daily variations — or at least variations which in- 

 dicate a change in the absolute amount of heat — extend, does 

 not probably exceed nine inches. For, according to the observa- 

 tions of Cluetelet, Director of the Observatory at Brussels, made 

 from 1834 to 1839, the velocity of propagation of the diurnal 

 variations of temperature is less than 1$ inches per hour, or less 

 than 18 inches in twelve hours ; and, the variations of tempera- 

 ture will be very much less below the depth of nine inches than 

 above it. We may form some estimate of the difference from 

 the following statement of the annual variations of temperature 

 at various depths, given by duetelet. The depths are in metres, 

 and the degrees Centigrade. 



