REPORT ON ELECTRICITY, MAGNETISM, AND HEAT. 27 
insensible. (Thus the diurnal and annual oscillations of tem¬ 
perature become insensible at a certain depth below the surface 
of the earth.) 
2nd. The oscillations of longest period are sensible to the 
furthest distance. (Thus the annual alternations are felt at a 
greater depth than the diurnal.) 
3rd. The diminution of the range of the oscillations is less 
rapid as the conducting power of the substance is greater. 
4th. The maximum temperature occurs at different epochs at 
different distances from the origin. (Thus the maximum of 
annual temperature is later as we go deeper.) 
The skill and resource shown by Fourier in this investigation, 
and the interesting and instructive nature of the results, make 
the series of his labours one of the most important portions of 
the physico-mathematical researches of the present century. 
His memoirs, as we have said, remained unpublished, except 
in extracts, till 1824 ; but they were consulted in the archives 
of the Institute by MM. Poisson and Cauchy. The former 
analyst turned his own eminent talents to this subject, and 
two memoirs of his upon it were read to the Institute, one 
in May 1815, and one in December 1821 ; and though not 
immediately published, were made known by abridgements and 
extracts in the Bulletin cles Sciences (May 1815), Annales de 
Chimie* (1821), and Journal de V Ecole Poly tec hnique\ (July 
1823). In the actual results of the calculation there was no 
difference between him and Fourier; and he confirmed, for in¬ 
stance, the curious laws which we have just noticed of the pro¬ 
pagation of heat in an infinite solidJ. One principal object of 
M. Poisson appears to have been to establish the fundamental 
equations by reasonings founded on his own views of molecular 
action. In these he agreed with Laplace, whose objections to 
Fourier’s reasoning we have already endeavoured to appreciate 
rightly. M. Poisson, indeed, carries much further than Laplace 
himself the Laplacian views of molecular action; and has at¬ 
tempted to show the entire insufficiency of Laplace’s theory of 
capillary action, because it does not consider the variation of 
density which must take place near the surface of a fluid, when 
it is considered as a collection of discrete particles affecting each 
other by their mutual attractions. But when it is recollected 
that M. Poisson obtains for the capillary attraction of a fluid 
mass the same expression which Laplace obtains; the same 
constant quantities, borrowed from observation, being involved 
according to each method, and the difference consisting only in 
* Ann. Chim., 19 (1821), p. 337. 
f Journal de 1'Ecole Poly technique, call. 19, p. 1. J Ibid., p. 75. 
