VOL. LI.] PHILOSOPHICAL TRANSACTIONS. 423 



in the magnetic meridian ; but if either of the magnets was removed, the needle 

 was attracted by the other, so as to stand at 45 degrees. The magnets were 

 both covered with brass weights of t6 oz. each. Into the eastern weight he 

 poured about 2 oz. of boiling water ; and the needle in one minute moved half 

 a degree, and continued moving westward for about 7 minutes, when it arrived 

 at 2%. It was then stationary for some time ; but, in 24 minutes from the be- 

 ginning, it came back to 2%, and in 50 minutes to 2*^-^. He then filled the 

 western weight with boiling water, and in one minute the needle came back to 

 1%; in 6 minutes more it stood half a degree eastward; and after that, in 

 about 40 minutes, it returned to the magnetic north, or its first situation. 



It is evident that the magnetic parts of the earth in the north on the east 

 side, and the magnetic parts of the earth in the north on the west side of the 

 magnetic meridian, equally attract the north end of the needle. If then the 

 eastern magnetic parts are heated faster by the sun in the morning than the 

 western, the needle will move westward, and the absolute variation will increase; 

 when the attracting parts of the earth on each side the magnetic meridian have 

 their heat increasing equally the needle will be stationary, and the absolute va- 

 riation will then be greatest ; but when the western magnetic parts are either 

 heating faster, or cooling slower than the eastern, the needle will move east- 

 ward, or the absolute variation will decrease ; and when the eastern and western 

 magnetic parts are cooling equally fast the needle will again be stationary, and 

 the absolute variation will then be least. This may be still further illustrated 

 bv placing the compass and two magnets, as in the last experiment, behind a 

 screen near the middle of the day in summer ; then, if the screen be so moved 

 that the sun may shine only on the eastern magnet, the needle will sensibly vary 

 in its direction, and move towards the west ; and if the eastern magnet be 

 shaded while the sun shines on the western, the needle will move the contrary 

 way. By this theory, the diurnal variation in the summer ought to exceed that 

 in the winter; and accordingly it is found by observation that the diurnal va- 

 riation in the months of June and July is almost double that of December and 

 January. 



The irregular diurnal variation must arise from some other cause than that of 

 heat communicated by the sun ; and here Mr. C. has recourse to subterranean 

 heat, which is generated without any regularity as to time, and which will, 

 when it happens in the north, affect the attractive power of the magnetic parts 

 of the earth on the north end of the needle. Dr. Hales has a good observa- 

 tion on this heat, in the Appendix to the second volume of his Statical Essays, 

 viz. " that the warmth of the earth, at some depth under ground, has an in- 

 fluence in promoting a thaw as well as the change of the weather from a freez- 

 ing to a thawing state, is manifest from t'" observation; viz. Nov. 29, 1731, 



