scrape the surface of the wall, knocked the animal down. He presented Sir Everard with a specimen weighing five ounces and three-quarters avoirdupois, which enabled him to ascertain the peculiar mechanism by which the feet of this animal can keep their hold of a smooth, hard, perpendicular wall, and carry up so large a weight as that of its body. The foot has five toes, at the end of each of which, except that of the thumb, is a very sharp and much-curved claw; on the under-surface of each toe are sixteen transverse slits, leading to so many cavities or pockets, the depth of which is nearly equal to the length of the slit that forms the orifice; they all open forward, and the external edge of each opening is serrated, like the teeth of a small-toothed comb. The cavities, or pockets, are lined with a cuticle, and the serrated edges are also covered with it. The structure just described is supplied with various muscles, whose action is to draw down the claw, open the orifices of the pockets, and turn down the serrated edges upon the surface on which the animal stands. Upon examining attentively the under-surface of the toes, when the pockets are closed, Sir Everard Home was struck with their resemblance to the surfaces of that portion of the Echineis remora, or sucking fish, by which it attaches itself to the shark, or to the bottom of ships; and it consequently suggested the probability of obtaining, from an examination of this latter apparatus, much useful information which might be applicable to the subject of the lizard, more especially as the parts of which it is composed are so much larger, and therefore more within the reach of anatomical examination. The surface on the top of the head of this fish, fitted for adhesion, is of an oval form, and bears a considerable proportion to the size of the whole animal; it is surrounded by a broad, loose, moveable edge, capable of applying itself closely to the surface on which it is placed and it is evident that, when the external edge is so applied, and the cartilaginous plates are raised up, the interstices must become so many vacua, and the serrated edge of each plate will keep a sufficient hold of the substance on which it rests to retain it in that position, assisted by the pressure of the surrounding water, without a continuance of muscular exertion. It thus ap pears that the adhesion of the sucking fish is produced by so many vacua being formed through an apparatus worked by the voluntary muscles of the animal, and the pressure of the surrounding water. From the similarity of the mechanism of the under-surface of the toes of the lacerta gecko, there can be no doubt that the purpose to which it is applied is the same: but as in the one case the adhesion is to take place under water, and is to continue for longer periods, the means are more simple; in the other, where the mechanism is to be employed in air, under greater disadvantages with respect to gravity, and is to last for very short periods, and then immediately afterwards to be renewed, a more delicate structure of parts, a greater proportional depth of cavities, and more complex muscular structure become necessary. In Having ascertained the principle on which an animal of so large a size as the lacerta gecko is enabled to support itself in its progressive motion against gravity, Sir E. Home felt himself more competent to inquire into the mechanism by which the common fly is enabled, with so much facility, to support itself in still more disadvantageous situations. the natural size the feet of the fly are so small, that nothing can be determined respecting them; Keller was the first person who made a drawing of the fly's foot in a highly-magnified state, in which the concave surfaces are visible, and which, no doubt, like those of the lizard above described, are employed to form vacua, which enable the fly to move under such disadvantageous circumstances. Mr Bauer, who has so greatly distinguished himself in microscopic researches, was judiciously enlisted into the service of Sir E. Home upon this occasion; and he has shown that this principle, on which progressive motion against gravity depends, is very extensively employed by nature in the structure of the feet of insects; and Sir Everard observes, that now this structure is known, it can be readily demonstrated by looking at the movement of the feet of any insect upon the inside of a glass tumbler, through a common magnifying glass; the different suckers are readily seen separately to be pulled off from the surface of the glass, and reapplied to another part. In consequence of the expedition to In the polar regions, Sir E. Home was ena- 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 33, p. 143. Accidental Discoveries, and of Inventions suggested by the Analogies of Nature. "Art lives on Nature's alms." 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 penduluin, 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. Again, one of the most important improvements, the condensation of steam by the injection of a shower of cold water directly into the cylinder, was discovered by a leakage, letting in the water where it was never intended to penetrate. 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. In like manner the history of pottery is mixed up with an unusual amount of tales which attribute discoveries to accident; as, for instance, the servant of a potter was boiling brine in an earthen pan, and, the fire being fierce, and she careless, the saline solution boiled over, when, by the action of the intense heat, the alkali combined with the earth; and hence the origin of salt-glaze. 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. The method of purifying sugar with clay is asserted to have been the result of accident: a hen, having her feet dirty, had gone over a pot of sugar, and the sugar under her tread was found whiter than elsewhere. The idea of mezzotinto engraving is recorded as having struck Prince Rupert while he was watching a soldier scouring a rusty breastplate. A thousand such fertile accidents might be enumerated, but sufficient has been said 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." pen, not the unfledged steel instrument of the present day, but the veritable goose-quill of our grandfathers. Are you prepared? if so, observe how the square shaft is disposed so as to impart the greatest strength to that part of the feather most exposed to the strain in flight; while, as it extends towards the body of the bird, and becomes of less importance as to its power of resistance, the rectangular is tapered down to the circular form, apparently to prevent the sharp angles from lacerating the body of its active bearer. The beam is not hollow, but, to preserve its form, it is filled with a pithy substance, clumsily imitated by art in those gusset-pieces and angle-irons introduced to add strength to the square beam. We might add numerous other instances of the same kind; such as the Ball and Socket-joint-the Flexible Watermain, suggested to Mr Watt by the mechanism of the lobster's tail-the plan of tunnelling through the bed of the Thames, by the operation of the teredo -the improvement in the manufacture of our paper, so as to obtain toughness by long fibres, from the process adopted by the wasp-the art of shotting silk, from the wing of the butterfly, &c.; affording a striking testimony to the celebrated aphorism of Robert Hooke, that "Nature is generally the best guide for Art;" or, in the words of the poet, that "Art lives on Nature's alms." and But let it be remarked that it is one thing to adopt the suggestions of Nature, which may be regarded as the common property of all, and another to copy appropriate the inventions of our fellows, and by ingeniously disguising them, as gipsies do their stolen children, to make them pass as their own. Such has been too frequently the case in the present day: workmen, as candidates for em Then, again, the originality of an invention has been questioned, from the fact that Nature had adopted a similar mechanism in some of her works. Let us illustrate our meaning by a reference to two of the most stupendous architectural wonders of the age-THE CRYSTAL PALACE, and THE TUBULAR BRIDGE. Sir Joseph Paxton, in describing the construction of the former, states that the large umbrella-shaped leaves of the Victoria regia suggested to him the means of giving to it stability. "If you ex-ployment, have entered the factories, amine," says he, "this leaf, and compare it with my models and drawings, you will perceive that Nature has provided it with longitudinal and transverse girders and supporters." In the construction of the TUBULAR BRIDGE it was found that the rectangular was much stronger than the circular beam. If this fact were not suggested, it was at least confirmed by Nature. But before we proceed to its explanation, be pleased, gentle reader, to take up your "Library of Useful Knowledge." and surreptitiously stolen ideas, and taken out patents for inventions to which they had no honest claim: thus realising the fable of Æatus and Polyclea, who, as our classical readers will remember, were informed by the Oracle that the one who first touched the land, should obtain the kingdom. Polyclea appeared lame and disabled, and appealed to the kindness of her brother to carry her across on his shoulders: no sooner, however, had they arrived on the opposite side, than Polyclea leaped ashore from the back of Eatus, and ex- NOTE 34, p. 144. This rarefied state is not only shown by the barometer, which is a very faithful measure of the air-pressure (p. 146), but by phenomena which may not have been duly regarded; it may therefore be advisable to explain them. That water should boil at a lower temperature in an elevated region, was a fact consistent with theory, and proved by experiment; but until the experiments of Archdeacon Wollaston, it was not known that the elevation of a few feet could be thus indicated. He constructed an apparatus by which the height of St Paul's was ascertained. Water, in vacuo, boils at 88° Fahr.; on the summit of Mont Blanc, at 1870; and it has been stated that the monks of St Bernard complain of not being able to make good bouilli-they certainly could not make good tea-in that elevated region. The town has lately been amused by Mr Albert Smith's graphic account of his ascent to the summit of Mont Blanc; and if we may be allowed to gain knowledge at the expense of our sympathies, we may tell our readers, that when this adventurous gentleman, in a state of exhaustion, anticipated the delight of a refreshing draught of champagne, the contents of the bottle were forcibly ejected and lost, the moment its cork had been withdrawn. "Tantalus a labris sitiens fugientia captat The great fatigue experienced by persons ascending the higher regions has received an additional explanation from the philosophical discovery of Dr Edward Weber, that the head of the thigh-bone cannot be separated by the mere weight of the limb from the surface of the articular cavity, to which it is accurately adapted: but that in all its motions it is retained close to the articular surface by the pressure of the atmosphere. All the muscles which surround the hip-joint may be divided, but the weight of the limb does not remove the head of the bone from its cavity; but if a hole be bored into the cavity, or it be exposed under an air-pump, the separation will take place. It is evident, therefore, Horat. Sat. i., 509. that, in all the movements of rotation, the head of the bone is retained in its place by atmospheric pressure. In ascending very high mountains, where the air is greatly rarefied, the muscular force must on that account be increased in order to maintain the head of the bone in its cavity, and hence the peculiar kind of fatigue experienced by such persons. All sounds are materially modified in such regions: the discharge of a pistol is not louder than a pop-gun; the human voice undergoes a similar change, and an opera-singer would be very much surprised on hearing her own musical notes. The rate of combustion is also modified by altitude. This was first ascertained by the difference of time in the burning of the fuses of shells at Bangalore, and other places, when it was found that combustion was retarded at considerable elevations. Quartermaster Mitchell, who communicated the fact to the Royal Society, attributes it to the deficiency of oxygen; but, according to the experiments of Sir H. Davy, we are induced to refer it to the want of heat in rarefied air. NOTE 35, p. 144. Weight of the superincumbent Ocean. If we include the pressure of the atfeet would sustain that of sixty pounds mosphere, a body at the depth of 100 on the square inch; while one at 4000 feet, a depth by no means considerable, would be exposed to a pressure of about 1830 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 further illustration of our subject:-" After the invention of the diving-bell, and its success in subaqueous 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 of 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." At the present time the subject of the ocean's pressure, at its different depths, derives a peculiar interest, from its possible bearing upon the successful submersion of the electrical submarinecable for telegraphic communication with America. NOTE 36, p. 156. 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, in consequence of the film becoming too attenuated to reflect light, 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 exem This also confirms the observation (p. 155) regarding the tendency of the lather to gravitate towards the depending part of the bubble. plified by the iridescence of mother-of- In consequence of this ner, the varying and delicate hues |