CHAPTER IX.
THE BOUNDARIES OF ASTRONOMY.
T is proposed in the following chapter to trace some parts of the boundary line which divides the truths that have been established in astronomy from those parts of the science which must be regarded as more or less hypothetical. We intend therefore to select certain prominent questions, and to discuss those questions with such fulness as the circumstances will admit. It is desirable to commence with that great doctrine in astronomy which is often regarded as almost universally established. The doctrine to which we refer is known as the law of universal gravitation. It is customary to enunciate this law in the proposition that every particle of matter attracts every other particle with a force which varies directly as the product of the masses and inversely as the square of their distance. It is no doubt convenient to enunciate the great law in this very simple manner. It might seem awkward to have to specify all the qualifications which would be necessary if that enunciation is to assert no more than what we
