distance of the sun and planets, and make the diameter of the
earth's orbit the first step of a scale by which he may ascend
to the starry firmament. Such pursuits, while they ennoble the
mind, at the same time inculcate humility, by showing that
there is a barrier, which no energy, mental or physical, can
ever enable us to pass: that however profoundly we may
penetrate the depths of space, there still remain innumerable
systems compared with which those which seem so mighty to
us must dwindle into insignificance, or even become invisible;
and that not only man, but the globe he inhabits, nay the
whole system of which it forms so small a part, might be
annihilated, and its extinction be unperceived in the immensity
or creation.
A complete acquaintance with Physical Astronomy can only be attained by those who are well versed in the higher branches of mathematical and mechanical science: such alone can ap- preciate the extreme beauty of the results, and of the means by which these results are obtained. Nevertheless a sufficient skill in analysis to follow the general outline, to see the mutual de- pendence of the different parts of the system, and to compre- hend by what means some of the most extraordinary conclusions have been arrived at, is within the reach of many who shrink from the task, appalled by difficulties, which perhaps are not more formidable than those incident to the study of the elements of every branch of knowledge, and possibly overrating them by not making a sufficient distinction between the degree of mathematical acquirement necessary for making discoveries, and that which is requisite for understanding what others have done. That the study of mathematics and their application to astronomy are full of interest will be allowed by all who have devoted their time and attention to these pursuits, and they only can estimate the delight of arriving at truth, whether it be in the discovery of a world, or of a new property of numbers.
It has been proved by Newton that a particle of matter placed without the surface of a hollow sphere is attracted by it in the name manner as if its mass, or the whole matter it contains, were collected in its centre. The same is therefore true of a solid sphere which may be supposed to consist of an infinite number of concentric hollow spheres. This however is not the caseb2
