56 Dr. Marshall Hall’s Description of a Thermometer 
the magnet, and also on its ength. Biot has shown that ina 
steel wire 24 inches long, and properly magnetizef, the pole 
is an inch and a half trom its extremity, and that this distance 
diminishes with every diminution in the length of the mag- 
net *. The centre of parallel forces or the pole of a magnet 
is similar to the centre of gravity of a body. In the one case 
the effect is the same as if all the matter of which the body 
is composed were concentrated in the centre of gravity, in the 
other the effect is the same as if the difference between the 
sum of all the attractive and repulsive forces were concen- 
trated in the pole. Now, in the case of the mutual attrac- 
tion of bodies, our measurements are always taken between 
the centres of gravity; in the case of magnetic attractions the 
distances of the magnets are, in fact, the distances between 
the poles. 
Let the distance of the 
poles of the magnets when 1 rr 
in apparent contact be call- a Ate ba 
ed2, as in fig. ( L.),and then (2.) * c ~ 
separated bY, an ae Lot —— 
of 1, as in fig. (2.), anc gi) 35 a —— 
by intervals of A as in (3+) nnssltetl 
fig. (3.) 
Then the distances between the centres of force in these 
three positions are 2, 3, 4. Hence if the law of the znverse 
squares of the distances, investigated by Coulomb, be the 
real law of action, the attractive forces will be inversely as 
2°, 3°, 4°, that is, as 4, 3, 7,3; but 4 is nearly the half of one 
fourth, and ;4, nearly the half of 4, as Mr. Fox found by 
actual experiment. ‘These experiments then, instead of lead- 
ing to a new law of action, afford a beautiful illustration of 
that law which universally prevails whenever we have matter 
acting on matter by attractive or repulsive forces. 
XIII. Description of a Thermometer for determining minute 
Differences of Temperature. By Marsuary Hatt, M.D., 
ERS. Se. ¢ 
| pursuit of the theory of the inverse ratio of the respira- 
tion and of the irritability in the animal kingdom, an- 
nounced in a late volume of the Philosophical ‘Transactions, 
I have found it absolutely necessary to determine the minute 
* Biot, Traité de Physique, tom. ili. p. 90, 
+ Communicated by tke Author. 
+ An abstract of Dr. Marshall Hall’s paper on this subject will be found in 
Phil. Mag. and Annals, N.S, vol. xi, p. 453.—Epir. : 
