Experiments end Obfervntions. 23 
only unwarranted by actual experiments, but do neither feem 
analogous to the other operations of nature. The late ingenious 
Mr. Canton, Member of this Society, was the firft, who 
endeavoured to account for the daily variation of the magnetic 
needle by the heating and cooling of the magnetic bodies in 
different parts of the earth’s furface ; which was in confequence 
of his having firft oblerved, that the aCtion of the magnet on 
the needle was diminifhed by heating, and increafed by 
cooling 
Following Mr. Canton’s judicious method of deriving the 
explanation of natural appearances from properties actually 
proved by experiments, I think, that the increafe and diminu- 
tion of magnetic attraction by heating and cooling of the mag- 
net, as obferved by Mr. Canton, together with the refult of 
the experiments recited in this Paper, feem fully fufficient to 
account for the general variation of the needle. 
If we colleCt under one point of view all the caufes hitherto 
afcertained, which* can increafe or diminifh the attraction be- 
tween magnetic bodies, we fhall find, that the attraction 
between the magnet and iron, or between magnet and magnet, 
is increafed by cooling, by a regeneration of iron or phlogifti- 
cation of its calx, and within certain limitations by the aCtion 
of acids upon the iron ; that this attraction is diminifhed by 
heating, and by the decompofition of iron* and, laftly, that it 
is probably annihilated by a very great degree of heat. 
Thefe truths being premifed, it muft be confidered, firft, 
that, according to innumerable oblervations and daily expe- 
rience, the body of the earth contains almoft every where /er- 
ruginous bodies m various ftates and bulks ; iecondly, that the 
magnetic needle muft be attracted by all thofe bodies, and its 
o 
* Fhilofophical Tranfaftions, vol. LI. p. 398. 
fituation 
