Purchase of this book includes free trial access to www.million-books.com where you can read more than a million books for free. This is an OCR edition with typos. Excerpt from book: - CHAPTER VII. F"ieiv of what is known respecting Gravity. 51. Gravity, considered with respect to its effects, is by far the most important of natural mechanical forces. Its cause is entirely unknown. But we know the laws by which it operates more exactly than those of any other natural force. As the illustration of its effects is one of the principal objects of mechanical philosophy, it is proper that they should be described in all their details. Most of these effects, however, can only be stated here in a historical manner; for some of the methods by which philosophers have determined the laws of gravity, must necessarily be deferred till the reader is further advanced ; and the rest do not belong to the present subject, but to astronomy. 52. The first effect of gravity which we have to consider is the pressure directed towards the earth, which each body xerts upon those which are placed beneath it. This pressure, the determinate intensity of which is called the weight of a body, may be very exactly measured by means of a balance. It is invariable, whatever changes take place in the form, position, extension, and chemical propeilhes of a body, provided that no ponderable matter is added or taken sfway. This circumstance justifies the conclusion that the weight of a body depends solely upon the quantity of ponderable matter which it contains, and consequently that the mass must be in the same proportion. 53. Experience teaches us that when several bodies are homogeneous, that is, absolutely identical as to their nature and constitution, the weights of these bodies are to each other as their bulks. But this proportionality does not hold with respect to bodies which are heterogeneous, either by nature, or in consequence of the circumstances in which they are p...