Nor is progressive motion, the only function in which Nature avails herself of the pressure of the atmosphere for the accomplishment of her purposes. The act of feeding is continually effected in this manner. The operation of sucking is too familiar to require comment. It may not, perhaps, be so generally known, that it is by the very same process that bees reach the fine dust and juices of hollow flowers, like the honeysuckle, and some species of foxglove, which are too narrow to admit them. They fill up the mouth of the flower with their bodies, and suck out the air, or at least a large portion of it, by which the soft sides of the flower are made to collapse, and the juice and dust are squeezed towards the insect, as completely as if the hand had pressed it externally. It is by a similar process that the oyster is enabled to close its shell so firmly; for, if a hole be bored in it, it may be opened without the least difficulty.
Note 29, p. [191].--Accidental discoveries.
Those who are not acquainted with the operations by which the mind is enabled to arrive at truth, are too apt to attribute to accident that which is the result of great intellectual labour and acuteness. Observation, analogy, and experiment are the three great stepping-stones by which the philosopher is enabled to ascend from darkness to light: it is true that his foot may accidentally be placed upon the first, but his own efforts are required to complete the ascent. To the mass of mankind the preliminary step is obvious, and they at once conclude that the succeeding ones are equally easy and simple. In this view of the subject, it was by accident that Sir Isaac Newton discovered the laws of gravitation, for his mind was directed to the investigation by the accidental fall of an apple from its tree; it was by accident that Galileo discovered the isochronous movement of the pendulum, for it was suggested by the vibration of a chandelier: but how many persons might have witnessed the fall of an apple, or the vibration of a chandelier, without arriving at similar truths? It has been said that we are indebted for the important invention in the steam-engine, termed hand gear, by which its valves or cocks are worked by the machine itself, to an idle boy of the name of Humphrey Potter, who, being employed to stop and open a valve, saw that he could save himself the trouble of attending and watching it, by fixing a plug upon a part of the machine which came to the place at the proper times, in consequence of the general movement. If this anecdote be true, what does it prove? That Humphrey Potter might be very idle, but that he was, at the same time, very ingenious. It was a contrivance, not the result of accident, but of acute observation and successful experiment. Glass is said to have been discovered by persons having accidentally kindled a fire on the sandy shore with sea-weed, when the alkali from the ashes united with the silex of the sand; and Pliny tells us that minium, or red lead, was first recognised, in consequence of a fire that took place at the Piræus at Athens, where some ceruse, which had been exposed to the fire, had been found converted into a red substance. A thousand such accidents might be related, were we not affording a sample rather than a catalogue. We are endeavouring to combat a popular but mischievous error; and we are happy at finding the same feeling expressed in a work which, from its extensive circulation, must prove highly useful in correcting it. “Very few discoveries,” says the author, “have been made by chance and by ignorant persons; much fewer than is generally supposed. They are generally made by persons of competent knowledge, and who are in search of them. The improvement of the steam-engine by Watt resulted from the most learned investigation of mathematical, mechanical, and chemical truths. Arkwright devoted many years, five at least, to his invention of spinning-jennies. The new process of refining sugar, by which more money has been made in a shorter time, and with less risk and trouble, than was perhaps ever gained by an invention, was discovered by Mr. Howard, a most accomplished chemist, and it was the fruit of a long course of experiments, in the progress of which, known philosophical principles were constantly applied, and one or two new principles ascertained.”--Library of Useful Knowledge.
Note 30, p. [193].--Weight of the superincumbent ocean.
If we include the pressure of the atmosphere, a body at the depth of 100 feet would sustain that of 60 pounds on the square inch; while one at 4,000 feet, a depth by no means considerable, it would be exposed to a pressure of about 1,830 pounds. We need not, therefore, feel surprised, that on the foundering of a ship at sea, though its timbers part, not a spar floats to the surface; for if the hull has sunk to a great depth, all that is porous is penetrated with water or greatly compressed. Captain Scoresby states that when, by the entangling of the line of the harpoon, a boat was carried down with the whale, it required after it was recovered two boats to keep it at the surface. Sir J. Herschel has recorded a melancholy anecdote, which may well be adduced in farther illustration of our subject:--“After the invention of the diving-bell, and its success in sub-aqueous processes, it was considered highly desirable to devise some means of remaining for any length of time under water, and rising at pleasure without any assistance. Some years ago an ingenious individual proposed a project by which this end was to be accomplished. It consisted in sinking the hull of a ship made quite water-tight, with the decks and sides strongly supported by shores, and the only entry secured by a stout trap-door, in such a manner, that by disengaging from within the weights employed to sink it, it might rise of itself to the surface. To render the trial more satisfactory, the projector himself made the first essay. It was agreed that he should sink in twenty fathoms water, and rise again without assistance at the expiration of twenty-four hours. Accordingly, making all secure, and provided with the means of making signals to indicate his situation, this unhappy victim of his own ingenuity entered and was sunk. No signal was made, and the time appointed elapsed. The pressure of the water at so great a depth had, no doubt, been completely under-estimated, and the sides of the vessel being at once crushed in, the unfortunate projector perished, before he could even make the signal concerted to indicate his distress.”
Note 31, p. [199].
Hence pecunia from pecus. Opes quasi Oves. See [Note 6].
Note 32, p. [207].--The cause of iridescence.
If a soap-bubble be blown up, and set under a glass, so that the motion of the air may not affect it, as the water glides down the sides and the top grows thinner, several colours will successively appear at the top, and spread themselves from thence in rings down the sides of the bubble, till they vanish in the same order in which they appeared; at last a black spot appears at the top, and spreads till the bubble bursts. Hence it follows that the colours of a body depend in some degree upon the thickness and density of the particles that compose it; and that, if the density be changed, the colour will likewise be changed. That the production of colours depends upon the nature of the surfaces upon which light falls, is beautifully exemplified by the iridescence of mother of pearl; and which has been satisfactorily shown to depend upon a singular peculiarity in the structure of that substance. On its surface, which to the unassisted eye, and even to the touch, appears to be finely polished, there are innumerable little lines, or grooves, in some places as many as two or three thousand in the space of an inch, which, lying parallel, regularly follow each other in all their windings; by the edges of which the rays of light are reflected, and the continual change of colour arises from their continual bendings. Whatever doubts might have existed upon the subject, some late experiments of Dr. Brewster have dissipated them, by showing that the colours which play so beautifully on the surface of mother of pearl, may be communicated by pressure to sealing-wax and several other substances. The discovery of this fact was in some measure accidental; he had stuck a piece of mother of pearl on a cement made of rosin and bees-wax, and on separating this cement he found that it had acquired the property of exhibiting colours. Several persons who witnessed the effect, concluded that it arose from the presence of a thin film of the mother of pearl, which might have scaled off and adhered to the wax: but such an explanation was at once refuted, by plunging the wax in acid, which must have dissolved the mother of pearl, had any been present; but the acid had no effect, and the colours of the impression remained unimpaired. It is clear, then, that it is the grooves, as Dr. Brewster conjectured, which occasion the iridescence in the mother of pearl, as well as in the waxen impression. In consequence of this curious discovery, Mr. Barton succeeded in producing the same appearance on glass, and on different metals, by simply cutting grooved lines on their surface. These lines are so fine that, without a microscope, they are scarcely visible, and the glass and the metal appear to retain their polish: yet they and the colours also may be communicated by an impression, like those from the mother of pearl, to the wax. In like manner the varying and delicate hues exhibited by the wings of certain butterflies, arise from the action of light upon the parallel and equidistant striæ upon their surfaces.
Note 33, p. [224].--Vegetable barometers.