Now, as the metallic weights are of greater specific gravity than the diamonds, the interests of the dealers—so far as they are affected by change of atmosphere—must be to buy when the air is heavy and sell when it is light. An increase of density in the air would, of course, reduce the gravity of both diamonds and weights, but not equally: the diamonds, being the more bulky, would lose gravity more than the weights, and consequently would weigh less. If it were possible that the air should increase in density till it became as heavy, bulk for bulk, as the diamonds, they would float therein, or, in other words, weigh nothing at all.
I well remember when, as a boy, I first read this admirable story, how much I was puzzled by the mistake in question.
An error, occasionally met with in novels, is as follows. A wonderful marksman has to exhibit his powers, which he does thus:—He throws into the air two birds—or perhaps inanimate objects—as two apples; then, waiting till both are in a line with himself, sends his arrow or bullet through both. A slight consideration will show that, in a vast majority of cases, no amount of waiting would suffice.
Another prevailing error is, that a person simply standing by the side of a pool can see his own reflection from the surface—Narcissus must have found some support which enabled him to lean over the fountain.
But it is in books especially intended to teach elementary science that such errors are most to be regretted.
A few years since I purchased for some of my grandchildren the eighth edition of “The Seasons,” by Mrs. Marcet. It is an admirable work, highly interesting and useful; but before placing it in the hands of my grandchildren, I thought it necessary to read it myself—a very pleasing task, by-the-by—and to correct any errors I might find. As examples, I may mention that in Volume I. snow is described as frozen rain; that in Volume IV. both stones in a flour-mill are said to revolve; and that the description in the same volume of a marine steam engine is very incorrect.
Again, few books are better calculated to interest boys than Dr. Parris’s “Philosophy in Sport,” but when, in the year 1829, I bought a copy for the School-Library at Bruce Castle, I found it necessary, before placing it there, to make numerous corrections to which I drew the attention of the author, who, in a letter dated March 18th, 1829, still in my possession, thanks me for my communication, and admits some of the errors, though not all.
As a specimen of the admitted errors, I give the following:—“Mr. Seymour now informed his young pupils that he had an experiment to exhibit, which would further illustrate, in a very pleasing manner, the truth of the doctrine of vis inertiæ. He accordingly inverted a wine-glass, and placed a shilling on its foot; and having pushed it suddenly along the table, the coin flew off towards the operator, or in a direction opposite to that in which the glass was moving.”[369]
My correction is as follows: “The coin would fall nearly in a perpendicular direction, but inclined a little towards the direction in which the glass was moving, owing to the friction between the glass and coin.”
As a specimen of the non-admitted errors, I give the following: “He had ignorantly fired a quantity of oxygen and hydrogen gases in a tin vessel; the consequence of the combustion was the immediate formation of a vacuum; and what happened? Why, the pressure of the external air, not being any longer balanced by elastic matter in the interior of the apparatus, crushed it with violence, as any other enormous weight might have done; and so ended the accident, which report magnified into a most awful catastrophe.”[370]