REPORT ON ELECTRICITY, MAGNETISM, AND IIEAT. 
17 
in such cases the disturbing masses, which assume their different 
positions by being turned round a vertical axis, will be nearly 
in the same magnetical attitude in all their changes, and there¬ 
fore their effect will not much be altered. Thus Mr. Barlow’s 
correction will be nearly complete in those cases in which it is 
most important. Mr. Barlow informs me, that in the voyages 
recently made towards the north pole, advantage was taken of 
the near coincidence of the magnetic equator with the horizon; 
and the contrivance of a counteracting plate, which had pro¬ 
perly been rejected in other cases for the reasons just mentioned, 
was adopted with great success. 
I do not consider it to belong to my present purpose to notice 
those experimental inquiries concerning magnetism which have 
not yet been brought into manifest connexion with the theory. 
One of the most important branches of the subject, that which 
has to do with Terrestrial Magnetism, has already been the sub¬ 
ject of a Report presented to the Association by Prof. Christie, 
and since published*, and of a supplementary Report by Capt. 
Sabine in the present volume. I proceed therefore to another 
of the subjects which are included in my present task. 
Heat .—The doctrine of the Conduction and Radiation of Heat, 
mathematically treated, is a subject which has excited con¬ 
siderable notice of late years; and its history brings before us 
several important tpiestions of physics and mathematics. I will 
speak in order : 1st, of the Experimental Evidence of the Princi¬ 
ples of this doctrine; 2ndly, of certain Difficulties which affect 
the Fundamental Equations; 3rdly, of the Mathematical Pro¬ 
cesses by which these equations have been treated ; and 4thly, of 
the Application of the Mathematical results to several subjects 
of speculation. 
I. Experimental Thermotical Principles .—The first step 
in the application of mathematical principles to conducted and 
radiated heat was made in the Principia. “ It was in the des¬ 
tiny of that great work,” says Fourier, “ to exhibit, or at least to 
indicate, the causes of the principal phsenomenaof the universe.” 
Newton assumed, as a simple rule evidently agreeable to facts, 
that the rate at which a body parts with its heat is proportional 
to the excess of heat; and on this assumption he rested the ve¬ 
rification of his scale of temperatures. It is an easy deduction 
from this law, that if times of cooling be taken in arithmetical 
progression, the heat will decrease in geometrical progression. 
Kraft, and after him Richman, tried to verify this law by direct 
experiments on the cooling of vessels of warm water ; and from 
* Report of Third Meeting, p. 105. 
C 
1835 . 
