
TWO:Having obtained his categories, Aristotle proceeds to mark off the first from the other nine. The subject or substance named in answer to the question, What is it? can exist without having any quality, size, and so forth predicated of it; but they cannot exist without it. Logically, they cannot be defined without telling what they are; really they cannot be conceived without something not themselves in343 which they inhere. They are like the tail of a kite, giving greater conspicuousness and buoyancy to the body, but entirely dependent on it for support. What our philosopher fails to perceive is, that the dependence is reciprocal, that substance can no more be conceived without attributes than attributes without substance; or rather that substance, like all other categories, can be resolved into Relation.241It is to be regretted that there have not been books especially prepared to instruct mechanical students in the relations between heat, force, motion, and practical mechanism. The subject is, of course, treated at great length in modern scientific works, but is not connected with the operations of machinery in a way to be easily understood by beginners. A treatise on the subject, called "The Correlation and Conservation of Forces," published by D. Appleton & Co. of New York, is perhaps as good a book on the subject as can at this time be referred to. The work contains papers contributed by Professors Carpenter, Grove, Helmholtz, Faraday, and others, and has the advantage of arrangement in short sections, that compass the subject without making it tedious.