TRANSACTIONS OF TIIE SECTIONS. 
61 
primeval sun which have lain dormant as potential energy beneath seas and moun¬ 
tains for countless ages. 
We must look then to the sun as the source from which the mechanical energy 
of all the motions and heat of living creatures, and all the motion, heat, and light 
derived from fires and artificial flames is supplied. The natural motions of air and 
water derive their energy partly, no doubt, from the sun's heat, but partly also from 
the earth’s rotary motion and'the relative motions and mutual farces between the 
earth, moon, and sun. If we except the heat, derivable from the combustion of 
native sulphur, and of meteoric iron, every kind of motion (heat and light included) 
that takes place naturally, or that can be called into existence through man’s 
directing powers on this earth, derives its mechanical energy either from tile sun’s 
heat or from motions and forces among bodies of the solar system. 
In a speculation recently communicated to the Roj'hI Society of Edinburgh, the 
author has shown that the sun’s heat is probably due to friction iu the atmosphere 
between bis surface and a vortex of vapours, fed externally by the evaporation of 
small planets, in a region of very high temperature round the sun, which they reach 
by gradual spiral paths, and falling in torrents of meteoric rain, down from the 
luminous atmosphere of intense resistance, to the sun’s surface. 
A continuation of the inquiry raises the question, from what source do the 
planets, large and small, derive the mechanical energy of their motions? This is a 
question, to the answering of which mechanical reasoning may legitimately be 
applied: for we know that from age to age the potential energy of the mutual 
gravitation of those bodies is gradually expended, hall in augmenting their motions, 
ami hulf in generating heat; and we may trace this kind of action either backwards 
or forwards; backwards for a million of million of year's with as little presumption 
as forwards for a siugle day. If we trace them forwards, we find that the end of 
this world as a habitation for man, or for any living creature or plant at present 
existing in it, is mechanically inevitable; and if wc trace them backwards according 
to Ihe laws of matter and motion, certainly fulfilled in all the actions of nature 
which we have been allowed to observe, we find that a time must have been when 
the earth, with no sun to illuminate it, the other bodies known to us as planets, 
and the countless smaller planetary masses at present seen as the zodiacal light, 
must have been indefinitely remote from one another and from all other solids in 
space. AH such conclusion's are subject to limitations, as we do not know at what 
moment a creation of matter or energv may have given a beginning, beyond which 
mechanical speculations cannot lead us. If in purely mechanical science we are 
ever liable to forget this limitation, we ought to be reminded of it by considering 
that purely mechanical reasoning shows a time when the earth must lave ee “ 
tenantless ■ and teaches uh that our own bodies, as well as all living plants and 
animals, and all fossil organic remains, arc organized forms of matter to which 
science can point no antecedent except the Will of a Creator, a truth amply con¬ 
firmed by the evidence of geological history. Hut if duty impressed with this 
limitation to the certainty of all speculations regarding the future and prc-historical 
periods of the past, we may legitimately push them into endless futurity, ana we 
ran be stopped by no barrier of past time, without ascertaining at some finite epocli 
a state of matter derivable from no antecedent by natural laws. Although we can 
conceive of 6uch a state of all matter, or of the matter within any limited space, 
and have cases of it in the arbitrary distributions of temperature, prescribed as 
'’initial” in the theory of the conduction of heat (see* Cambridge Mathematical 
Journal, vol. iv. 1843), yet we have no indications whatever or natural instances ol 
it, and in the present state of science we may look for mechanical antecedents to 
every natural state of matter which we either know or can conceive at any past 
epoch however remote. 
It is by tracing backwards the motions which are at present observed, according 
to the known laws of motion and beat, with no limit as to time, that the author 
arrives at the conclusion that the bodies now constituting our solar system have 
* " Note on some point* in ihc Theory of Heal,” a short article in which it was shown how 
to te»t the age of a distribution of heat, by applying a certain criterion of convergence to i s 
expression in the infinite series characteristic of the external circumstances of the bo y in 
which it is given. 
