212 
orbit. Terrestrial magnetism is, however, subject to diurnal, 
annual, and secular variations , attributable, apparently, to modi- 
fications of the ethereal streams resulting from periodic changes 
produced in the distribution of temperature in the atmospheie, and 
other matter pertaining to the earth, through solar and planetaiy 
influences. It may be noticed that the above view of the genera- 
tion of magnetic streams by impulses given to the ether by the 
earth’s revolving atoms involves the fundamental hypothesis that 
the atoms are of sensible magnitude. 
28. In deducing the foregoing inferences, (1)— (13), from the 
Newtonian physical philosophy, I have relied on mathematical 
arguments contained in three works relating (o the study of mathe- 
matics and physics in the University of Cambridge, which I pro- 
duced in the years 1869, 1878, and 1875, and on physical investi- 
gations which I have contributed from time to time to the Philoso- 
phical Magazine. Any one who wishes to be fully acquainted with 
the reasoning which, from a few intelligible hypotheses, has con- 
ducted to results so numerous and so various, would have to consult 
those publications. Perhaps, however, these results, even as stated 
above, may be considered to give, at least, primd facie evidence that 
the system of philosophy I am upholding is in character and comprc- 
hensiveness such as, to be true, it is required to be. If it does not 
embrace the whole range of physics, it tails altogether By inspec- 
tion of the words which, in the statements, (1) (18), are put m 
italics , it may be seen that the condition of comprehensiveness, as 
regards the different branches of physics, is fulfilled. . Consideung, 
therefore, that a presumption has thus been established that t ic 
principles of the Newtonian philosophy are true, I shall now pro- 
ceed to inquire what, accordingly, are the character and limits o 
physical science, and what metaphysical consequences may be 
deduced from it. , 
29. In the first place it is to be observed that this philosophy 
postulates the existence of tico kinds of realities, those of the fiist 
kind being the ether, the atoms, and tlicir intrinsic qualities, as 
already defined, and those of the other kind, consequences 
shown by mathematical reasoning to How from certain conditions 
and mutual relations which the first kind have been ascertained to 
be susceptible of. As our philosophy admits of no qualities othei 
than those the cognition of which is acquired by sensation and 
experience, the first class of realities are perfectly intelligible ; and 
such also are the other class, because they are shown by mathe- 
matics to be consequences of the first. The very term mathematics 
(from uaQnmo) implies this. There is, therefore, nothing occult in 
this philosophy. When we have arrived at the second class by 
reasoning mathematically from the first, we may be said to have 
