DISCOURSE ON THE STUDY OF NATURAL PHILOSOPHY.
(Cabinet Cyclopaedia. Vol. xiv.)
The arrangement of Dr. Lardner's Cyclopaedia, as it becomes more and more developed, will be proportionally appreciated. Its system is a marked contrast with the heterogeneous lists of the Family and National Libraries, which, as books of reference and authority, are little worth.
The Cyclopaedia plan is to form a series of Cabinets of the principal departments of human knowledge. Those already commenced are History, Biography, Natural Philosophy, Geography, and the Useful Arts. Each of these divisions is to be preceded by a prefatory discourse on "the objects and advantages" of the branch of knowledge which is treated of in the series or cabinet. Thus, the work before us is such a volume for the Cabinet of Natural Philosophy; that for History is promised by Sir James Mackintosh; and that for the Useful Arts, by the Baron Charles Dupin. The present Discourse is by J.F.W. Herschel, Esq., A.M. It is divided into three parts:—1. On the general nature and advantages of the study of Physics. 2. The rules and principles of Physical Science, with illustrations of their influence, in the history of its progress. 3. The subdivision of Physics. These parts are divided into chapters, and these chapters again divided into sectional illustrations, of which latter there are nearly four hundred. Such an arrangement can hardly fail to attract the listless reader. The style is lucid and popular, and the writer's reasonings and bearings are brought out with much point and vigour. Even a drawing-room reader must be caught by their attractions, and no better means was probably ever devised for bringing superficial readers into the way of knowledge, and setting forth its pleasantness. It has been said that such works as the present satisfy the reader, and disqualify him for the study of science. This opinion is hardly worth controverting: since that mind must be weak indeed which would not be stimulated as well as gratified in this case; and it is still more improbable that the great truths of science should at once take root in such a barren soil without any preparation for their reception.
We conclude with a few specimen extracts. The how, the why, the wherefore, and the because, of such wonders as they relate to, belong rather to the treatises themselves.
Mechanical Power of Coals.
It is well known to modern engineers, that there is virtue in a bushel of coals properly consumed, to raise seventy millions of pounds weight a foot high. This is actually the average effect of an engine at this moment working in Cornwall.
The ascent of Mont Blanc from the Valley of Chamouni is considered, and with justice, as the most toilsome feat that a strong man can execute in two days. The combustion of two pounds of coal would place him on the summit.
The Wonders of Physics.
What mere assertion will make any man believe that in one second of time, in one beat of the pendulum of a clock, a ray of light travels over 192,000 miles, and would therefore perform the tour of the world in about the same time that it requires to wink with our eyelids, and in much less than a swift runner occupies in taking a single stride? What mortal can be made to believe, without demonstration, that the sun is almost a million times larger than the earth? and that, although so remote from us, that a cannon ball shot directly towards it, and maintaining its full speed, would be twenty years in reaching it, it yet affects the earth by its attraction in an inappreciable instant of time?—Who would not ask for demonstration, when told that a gnat's wing, in its ordinary flight, beats many hundred times in a second? or that there exist animated and regularly organised beings, many thousands of whose bodies laid close together would not extend an inch? But what are these to the astonishing truths which modern optical inquiries have disclosed, which teach us that every point of a medium through which a ray of light passes is affected with a succession of periodical movements, regularly recurring at equal intervals, no less than 500 millions of millions of times in a single second! that it is by such movements, communicated to the nerves of our eyes, that we see—nay more, that it is the difference in the frequency of their recurrence which affects us with the sense of the diversity of colour; that, for instance, in acquiring the sensation of redness our eyes are affected 482 millions of millions of times; of yellowness, 542 millions of millions of times; and of violet, 707 millions of millions of times per second. Do not such things sound more like the ravings of madmen, than the sober conclusions of people in their waking senses? They are, nevertheless, conclusions to which any one may most certainly arrive, who will only be at the trouble of examining the chain of reasoning by which they have been obtained.
Extraordinary Property of Shadows.
An eminent living geometer had proved by calculations, founded on strict optical principles, that in the centre of the shadow of a small circular plate of metal, exposed in a dark room to a beam of light emanating from a very small brilliant point, there ought to be no darkness,—in fact, no shadow at that place; but, on the contrary, a degree of illumination precisely as bright as if the metal plate were away. Strange and even impossible as this conclusion may seem, it has been put to the trial, and found perfectly correct.