The First Principles of Natural Philosophy

Abstract

IT is a little difficult to see what useful purpose is served by this work, or why a second edition should be called for, seeing that it is neither of the popular nor yet of the properly scientific order of text-book. Its modes of regarding and describing the facts of dynamics are antiquated and incorrect, and it is extremely barren in numerical illustrations of the kind most helpful to elementary students. The author begins by telling the reader that “a pulling force takes the name of a tension,” whilst “a pushing force” takes “that of a thrust.” He then gives in abbreviated form Duchayla's proof of the parallelogram of forces, “because it is the foundation of the whole theory of statics,” in spite of its essential faultiness in requiring more assumptions in the course of the argument than if the simple rule of composition of vectors were assumed outright. The author is probably now the only surviving writer on dynamics who persists in muddling up force and acceleration by calling acceleration (a purely kinematical quantity in itself) an “accelerating force,” and he adds to the muddle by writing v=ft where all modern writers would put v=at. What the student is to understand by “a force capable of generating in one second a velocity represented by D E” (p. 27) is difficult to see, when the mass on which the force is to act is nowhere stated, and when it is not even stated or hinted that there is any mass at all to be acted on. On p. 41 the author states that “in this country the ounce avoirdupois is so taken that one thousand of them will just balance a cubic foot of distilled water.” This is not so, at least in this country, for the mass of the ounce depends on the standard pound, and this was established without any reference to a standard volume of water. The definition is wrong; the fact it states is a mere coincidence; and the coincidence itself is not exact: a cubic foot of water does not weigh 1000 ounces. On the same page the author tells the reader to ascertain with respect to a certain mass the velocity which “a given pressure or impulse” will impress upon it; “the mass being inversely proportional to this velocity”. The confusion between pressure, which cannot be expressed except in terms of force divided by area, and impulse, which is expressed as force multiplied by time, is truly amazing. Is time the reciprocal of an area? Again, on page 42 the author is speaking of a certain force capable of sustaining a certain weight for one second of time, and he says “it would require twice as powerful a force to enable it to resist the action of gravity for two seconds, three times for three, and so on.” This is news indeed. In the section on hydrostatics, no sooner has the student learned that a pressure of one pound per square inch is equal to 100 Ibs. per 100 square inches, than he comes to such a statement as the following (p. 52); “The pressure therefore exerted by a mass of fluid upon the bottom of a vessel containing it is proportional to the area of the base,” &c. Here the author jumps, without one word of warning to the student as to his change in the use of words, from using the word pressure in its proper sense of so many pounds-per-square-inch, to using the word in the sense of so many pounds, in which case it is no longer a ' pressure” but a “force.” It may be said perhaps that these things are but slips of the pen. Perhaps they are; but in a teacher who undertakes to write a text-book of “first principles” slips of such a kind are unpardonable. No such confusion of thought would be tolerated in the pupil who had read WormelFs “Dynamics,” or Lodge's “Mechanics,” or Maxwell's “Matter and Motion,” or Thomson and Tait's lesser volume. If Mr, Lynn does not intend his text-book to be cast aside as worse than useless, he must at once correct blundering modes of thought that can only mislead the student.

The First Principles of Natural Philosophy.

By W. T. Lynn Second Edition. (London: Van Voorst, 1884.)