in the Magnetic Forces of the Earth. 45 



Depth. — Metres. 



019 

 0-45 

 0-75 

 1-00 

 1-95 

 3-90 

 7-80 



Annual Variations. 



13° 28 

 . 12 -44 



11 -35 



. 10 -58 



7 -59 

 . 4 -49 



1 -13 



They become entirely imperceptible at the depth of about 24 m. 

 If we suppose the daily variations of temperature to decrease 

 from the surface downwards at the same proportionate rate, it 

 will become less than 1° F. at the depth of 9 in., (taking the varia- 

 tion at the surface at 12° F., which is about the average for the 

 year. ) 



4. Although the amount of vapor deposited during the night, 

 may perhaps not exceed the T ^ ¥ of an inch, on the average, the 

 loss of its entire amount of sensible heat may still be more than 

 an equivalent for the daily variations of temperature of a depth 

 of six or nine inches of soil. 



In view of these statements it will not be deemed idle to en- 

 quire whether the alternate deposition and rise of vapor may not 

 afford an adequate explanation of the secondary variations of the 

 horizontal force. But before entering upon this enquiry, let us 

 go back, and following the indications of the observations en- 

 deavor to ascertain whether there is any known phenomenon 

 that satisfies the prominent conditions which they furnish. As 

 preparatory to this it is important to state the laws according to 

 which heat is radiated from the earth's surface into space, and 

 propagated from particle to particle below the surface. These 

 are as follows, 



1- The loss of heat, from nocturnal radiation, in a calm clear 

 n ight, is uniform at all temperatures. This law, we are told (see 

 the No. of this Journal for November, 1848, p. 420), has recently 

 been announced by Wilson, and has since been confirmed by the 

 Observations of Melloni. It is, moreover, in approximate accord- 

 ance with the general theory of radiation. The formula which 

 this theory, in conjunction with experiment, has furnished, for 

 the velocity or rate of cooling of a body by radiation is 



V = ma r - ma 



(i.) 



Ml which denotes the absolute temperature of the enclosure, or 

 external medium, toward which the body radiates, and t the 

 excess of the temperature of the body over that of the enclosure. 

 If and t are expressed in Centigrade degrees, then o = l-0077.m, 

 for a vitreous surface, is 2-037 ; and it is about the same for the 

 s °il } which has about the same radiating power as glass. Ac- 

 cording to Pouillet the temperature of space is about - 142° C. 





