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ment, behind a fcreen near the middle of the day in 
fummer ; then, if the fcreen be fo moved, that the 
fun may fhine only on the eaftern magnet, the needle 
will fenfibly vary in its direction, and move towards 
the weft ; and if the eaftern magnet be fhaded, while 
the fun fhines on the weftern, the needle will move 
the contrary way. By this theory, the diurnal varia- 
tion in the fummer ought to exceed that in the win- 
ter ; and I accordingly find by obfervation, that the 
diurnal variation in the months of June and July, is 
almoft double that of December and January. 
The irregular diurnal variation muft arife from fome 
other caufe than that of heat communicated by the 
fun ; and here I muft have recourfe to fubterranean heat, 
which is generated without any regularity as to time, 
and which will, when it happens in the north, affedt 
the attractive power of the magnetic parts of the earth 
on the north end of the needle. The reverend Dr. 
Hales has a good obfervation on this heat, in the 
Appendix to the fecond volume of his Statical Eftays, 
which I fhall here tranfcribe. “ That the warmth of 
“ the earth, at fome depth under-ground, has an 
u influence in promoting a thaw, as well as the 
<c change of the weather from a freezing to a thaw- 
“ ing ftate, is manifeft from this obfervation ; viz. 
“ Nov. 29, 1731, a little fnow having fallen in the 
“ night, it was, by eleven the next morning, moftly 
“ melted away on the furface of the earth, except in 
“ feveral places in Bufiy-Park , where there were 
“ drains dug, and covered with earth, where the 
“ fnow continued to lie, whether thofe drains were 
“ full of water, or dry; as alfo where elm-pipes lay 
“ under-ground; a plain proof that thefe drains inter- 
8 “ cepted 
