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POPULAR SCIENCE REVIEW. 
determining the amount of heat radiated from the sun ; may we 
not therefore expect that the arrival in perihelion of a comet 
from most remote regions, which, if of smaller mass, often ap- 
proaches the sun much more nearly than they, may also produce 
a marked effect on the state of the solar atmosphere ? Although 
the 11-year period of solar spot frequency is too well marked to 
admit of any question, there is already much evidence to prove 
that there exists many minor disturbances, secondary maxima 
and minima of solar activity, which remain to be explained, 
and which may possibly be due to the occasional and irregular 
approaches of comets. It is certain that the solution of no 
question can be of more service to the advancement of science 
than that of the real practical utility of comets in the economy 
of the universe. Speculation on this point has already been too 
long neglected, and unless it is carried on now as far and as 
correctly as the state of modern science allows, it is certain that 
it will mask some other results and hinder progress. Surely 
none can believe that these bodies are mere ignes-fatui , coming 
and going, without being of any service to us or to other sys- 
tems ; and although much mystery has always hung about 
them and still baffles our researches, perhaps the best way of 
attaining to the solution of it is by searching for some purpose 
that they may subserve. Without attempting altogether to set 
up the nebular hypothesis in as favourable a light as before, the 
above remarks and speculations may serve to indicate the posi- 
tion in which it stands at present, and the broader basis on 
which in future the question must be argued. A certain degree 
of unity of design seems to result from these theories, which is 
consonant with the order of nature. Every known body in the 
universe appears to have an important and appropriate function 
to perform in the development or maintenance of systems like 
our own — a function that is constant in all conceivable states of 
existence of those systems ; and while my speculations do not 
contradict the various theories of others, they show a tendency 
to unite them into one consistent whole. Perhaps the most 
distinctive feature of these remarks is, that the effects I speak 
of must actually take place, whether or not they have the im- 
portance here attached to them. No one will deny that the 
nebula and the comet will constantly come into contact ; and 
were we acquainted with all the materials so meeting, it would 
be a comparatively simple problem for the chemist to deter- 
mine what compounds would result, and for the physicist to 
show in what state, gaseous or otherwise, they would afterwards 
remain. Both these questions require to be answered satisfac- 
torily before it is possible to declare whether the celebrated 
hypothesis of Laplace is or is not the true key to the solution 
of the formation and history of the Solar System; and for these 
answers, at the present, it waits. 
