1865.] BRITISH ASSOCIATION. 325 



the earth does, may yet enjoy, as in fact it seems to enjoy, nearly 

 a similar climate, with snows alternately gathering on one or the 

 other of its poles, and spreading over large spaces around, but not, 

 apparently beyond the latitude of 50 deg. or 40 deg. ; the equa- 

 torial band of 50 deg. or 40 deg. north or south being always free 

 from snow-masses bright enough and large enough to catch the eye 

 of the observer. Mars may, therefore, be inhabited, and we may 

 see in the present state of this inquiry reason to pause before 

 refusing the probability of any life to Jupiter and even more dis- 

 tant planets. 



The history of suns and planets is in truth the history of the 

 effects of light and heat manifested in them, or emanating from 

 them. Nothing in the universe escapes their influence ; no part 

 of space is too distant to be penetrated by their energy; no kind 

 of matter is able to resist their transforming agency. Many if not 

 all the special forces which act in the particles of matter 

 are found to be reducible into the general form of heat ; as this is 

 convertible and practically is converted into proportionate measures 

 of special energy. Under this comprehensive idea of convertibility 

 of force, familiar to us now by the researches of Joule (h), the rea- 

 sonings of Grove (0 and Helmholtz, and the theorems of Rankine (&), 

 it has been attempted by Mayer, \Vaterston, and Thomson (l) 7 

 to assign a cause for the maintenance of the heat-giving power of 

 the sun in the appulse of showers of aerolites and small masses of 

 matter, and the extinction of their motion on the surface of the 

 luminary. By calculations of the same order, depending on the 

 rate of radiation of heat into space, the past antiquity of the earth 

 and the future duration of sunshine have been expressed in thou, 

 sands or millions of centuries (m). In like manner the physical 

 changes on the sun's disk, by which portions of his darkly heated 



(h) Phil. Mag. 1843; Reports of the British Association, 1845; Trans, 

 of the Royal Society, 1850. 



(i) Grove on the Correlation of Physical Forces. 1846. 



(k) Rankine, Trans, of the Royal Society of Edinburgh, 1850-51 ; 

 Phil. Trans. 1854. 



(Z) Communication to the Royal Society of Edinburgh, 1854. 



(in) Professor Thomson assigns to the sun's heat, supposing it to be 

 maintained by the appulse of masses of matter, a limit of 300,000 years; 

 and to the period of cooling of the earth from universal fusion to its 

 actual state, 98,000,000 years. These are the lowest estimates sanctioned 

 by any mathematician. 



