PAPERS ON NATURAL PHILOSOPHY. CENTRE OF GRAVITY. WE stated, in a former number, that the attraction of gravitation was to be estimated from the earth's centre, which is in fact the point about which all the other parts balance each other, or "the centre of gravity." This definition equally applies to other bodies. In the case of dense masses, we may support or suspend them by a single point, which we consider to bear all the burden, and therefore term the centre of weight or gravity; and, in the case of liquids, the like might be effected but for their less cohesive attraction. If the centre of gravity be beyond the base of the body-which is ascertained by drawing a perpendicular line from it-its upright position cannot be maintained, and its security may be estimated by the nearness of the perpendicular line to the centre of the base. It is matter of no small moment in many instances rightly to estimate the true position of the centre of gravity, particularly in the case of heavily-laden wagons, which have to traverse rough roads; we frequently observe them, and also stagecoaches, piled to such a height as to endanger the safety of all the travellers. Equally so is it with buildings, only the hazard here is somewhat abated owing to the cohesion subsisting between their several parts, and which will frequently prevent the building from falling, even though the line of direction (as this perpendicular is termed,) be beyond the base. The leaning tower of Pisa is a curious example; and, as an illustration of the nicely-balanced position of the centre of gravity, we cannot give a better than the celebrated Stonehenge. THE MECHANICAL POWERS. The powers termed mechanical, are simply contrivances for applying force in the most advantageous manner. In number they are six; and all the movements of nature, whether in reference to anatomy, astronomy, or any other branch of physics, as well as in machinery, produced by art, however complex or intricate, each movement can be reduced to the influence of the lever, the wheel and axle, the pulley, the inclined plane, the wedge, and the screw, and even some of these are but compounded of others. There is a grand axiom to be considered in the application of these powers, viz., "That what is gained in power is lost in time," which however, it must be confessed, is rather an ambiguous definition. We know that the velocity of a body is the rate of its motion over a certain space; and, therefore, we think that this axiom would be more clearly expressed by saying, "That advantage is gained in proportion as the velocity of the power exceeds the velocity of the weight," and vice verså. A few illustrations, as we proceed, will exhibit the truth of this definition. The first mechanical power then is the lever, of which there are three descriptions. 1st. There is the lever which has the fulcrum between the weight and the power. Suppose A B to represent a lever, the point c being its centre of motion; on the fulcrum of which, if the lever turn, в comes into the position D, and at the same time a to E. We perceive, in this case, no advantage is gained, because a C, and C B, being equal, they both B travel with the same velocity, or move over an equal space in an equal time. This kind of lever is, however, brought into great practical use in the generality of scales now employed. B Again, in another description of the first kind of lever, we see that the fulcrum is placed much nearer the weight to be raised, than the power (say one-third); consequently, the arm of the lever, E B, being twice as long as E a, it will describe a greater circle in the same time, as E d; or, in other words, travel with greater velocity: and, if our supposition relative to its length be correct, we shall arrive at the conclusion that one pound applied to B, will raise two pounds attached to c. W In the second kind of lever, the power is applied to one end, the other resting on the fulcrum, with the weight between. From an example, we shall see the principle acting as in the former cases. If we compare this diagram with the last, it will be obvious that in each case the weight describes a segment of an equal circumference; and the same rule in that will apply to this, viz., that a pound placed at a, will raise two at D. B D In respect to the third description of lever, it will be perceived that the weight is at one end, the fulcrum at the other, and the power in the centre. Taking it on a supposition that the length of the part of the bar, в d, is four times greater than dE, we shall find that B, with the weight attached, will travel with four times the velocity of d, the power. We must, therefore, conclude, in accordance with our golden rule, that four pounds power would, in this case, be required to move one pound weight, which is really the fact; for this kind of lever is most disadvantageous, and ap B pears seldom used, except in hoisting ladders, when we see men unable to effect their purpose by the muscles of the arm alone, and therefore call in the aid of their legs, and, standing on the bars, by this means raise the ladder. The wheel and axle, the next in order to the lever, is manifestly based on the same principle; but whereas a straight bar can only be moved a few inches at a time, a wheel, through the means of a rope or handle, can continually be turned round, and thus act the part of a perpetual lever, although, of course, for certain offhand purposes, it is not so convenient as the simple bar. It will be necessary, in order to see its analogy to the lever, to exhibit it endways, thus: It will now be immediately perceived that the radius, a b, is simply a lever, having the power at one end, the weight at the other, and the fulcrum c, (the common centre to both wheel and axle,) between. power; and The pulley is the next simple mechanical one of its greatest advantages is, to enable power to be applied through the agency of man, when he himself a could not conveniently exert that power. Thus we find a weight is hoisted to the topmast of a vessel by an individual on deck,which could not be done but by the aid of the pulley. In reference to the single fixed pulley, as in the lever, no advantage is gained, because the power and the weight travel with equal velocity, or pass over the same space in the same time-just in proportion as the weight ascends the power descends; and this is so, because the wheel acts as a lever with equal arms, so that the weight and power must necessarily balance. In the compound moveable pulley we shall find, as previously, that the power required will be less in proportion as the velocity of the power, is greater than the velocity of the weight. By referring to the diagram, we see that while the power a requires that all the rope to c should be hoisted, B is only elevated once. So many moveable pulleys are requisite only on account of distributing the weight over a large surface, or the length of the rope from the fulcrum. If we examine the figure of the lever of the first kind, we find the weight distributed over the whole bar, because it is inflexible; it is thus that so many moveable pulleys are required, and, of course, by adding fresh blocks, the weight is still more distributed, and less power becomes necessary to move it, although the space to be travelled by the power is thus extended. B ANASTATIC PRINTING.-We have been favoured with an opportunity of inspecting, at the offices of Mr. Joseph Woods, No. 3, Bargeyard Chambers, Bucklersbury, a process of reprinting to which this name has been given. We are aware that many attempts have, at different times, been made to arrive by similar means at an available result. These have been attended by various success, but in no case amounting hitherto to anything profitable. To describe the present result, in as few words as possible,—it is the reproduction of any form of letter-press, or any quality of print, drawing, engraving, or lithograph, in unlimited quantity, in an inconceivably brief space of time. Any journal for instance, say the Times, might in twenty minutes be prepared for reprinting merely from a single number, and worked off with the ordinary rapidity of the steam-press. It is our purpose fully to describe, in the next number of the ART-UNION, the process whereby this is effected, and to show the admirable applicability of the invention to all those kinds of croquis drawings, sketches, &c. &c., which have hitherto been presented to the public eye as wood-engravings, by giving, as a specimen, a page of drawings, by distinguished artists, printed in this manner. The proprietors are scarcely yet prepared to work their patent on the extensive scale which they contemplate. We have, however, seen a set of drawings, fresh from the hands of the artists, prepared for printing, and printed off in little more than a quarter of an hour! In less than a quarter of an hour from the time of receiving the sketch, the printer will present to the artist proofs of his work, which shall resemble the original as perfectly as if it had been reflected on the paper touch for touch. In presenting these specimens, we shall describe the process at length in the next number of the ART-UNION. In the meantime it must be observed, that it is impossible to define the development of this, to say the least, truly wonderful invention, whereby the work of the artist is reproduced in fac-simile without the slightest point of difference, the finest and rarest engravings may be reprinted ad infinitum,—and, last, though not least, books may be reprinted, as from stereotypes, in unlimited quantity.Art-Union. BIBLICAL INTERPRETATION. To the Editor of the Student. In the October number of your magazine, a writer on Biblical Interpretation closes a paper written with some ingenuity, and much candour and modesty, with the following words :-"We will readily submit to an unfavourable judgment on the merits of the rather novel explanation which we offer, should the acuteness of some critical student, one who may be 'nullius addictus jurare in verba magistri,' pronounce it inadmissible." Now, though I have not the vanity to consider myself a critical student, I must yet declare that I think the interpretation put upon the verse in Eccles., rendered in the common version, "and the grasshopper shall be a burden," totally inadmissible. The writer referred to, states the well known fact, that both the Athenians and the Egyptians (from whom the former borrowed the custom) were in the habit of wearing grasshoppers (TέTTIYES) in their hair; but does this prove that the Jews ever made use of a similar adornment? And unless this can be proved, the rather fanciful, and in my eyes, rather ludicrous notion, that the burden caused by the grasshopper mentioned in Scripture, is the inconvenience resulting to irritable old age, from an article of adornment and decoration, is most certainly built on a foundation of sand. It may be said, however, that the Jews derived the practice of wearing these golden ornaments from the Egyptians, when captive in their country; but it is not likely, in the first place, that the Israelites would adopt a badge, which distinguished those whom they regarded with the greatest enmity, as being their cruel task-masters and tyrannical oppressors; nor is it probable, on the other hand, that these lords and masters would suffer their bondsmen, mere captive sojourners in their land, to assume an emblem which they regarded with peculiar pride, as it metaphorically represented them as being earth-born, or as their descendants the Athenians expressed it, Auróxoves, and Tŋyɛvɛis: and, therefore, of great antiquity and nobility. I had never seen Dr. Boothroyd's translation of the passage in question, till I met with it in the pages of The Student; and, as I am not a Hebrew scholar, I am not competent to judge of its accuracy or incorrectness. But, I must confess, that it appears to me very plausible :-" And the locust shall be a burden to itself." The note also, which is subjoined, seems to me to have taken the right view of the metaphor:-" An old man is compared to a locust, on account of his emaciated frame and its bending posture; and how true is it, that in this state a man is a burden to himself." I entirely differ from the writer of Fragments of Biblical Interpretation, when he says, "Such a metaphor, however, will not be found to be natural or appropriate." There is hardly a metaphor, allegory, or simile, in the whole round of poetical literature, which would bear to be carried out and dwelt upon in all its minute features. A few prominent points alone are seized upon by the poet; and in these alone will the metaphor, allegory, or simile, be found correct or just. Thus, in the case now before us, the "emaciated frame and bending posture" of the locust, or grasshopper, together with its feeble chirp, are taken to express (more powerfully than could be done by simple description) the wasted body of an old seller near Temple-bar," from the Dutch of Foersch, a surgeon stationed at Batavia. During his residence there, his curiosity being greatly excited by the various statements he received respecting the Bohun-Upas, as it is termed in the Malay language, he resolved to investigate the subject for himself. Having accordingly procured a pass to travel through the country from the Governor-general, and an introduction to an old priest who resided at the nearest habitable spot to the tree, and prepared for eternity the souls of those malefactors that preferred the attempt of procuring the poison, to the certainty of a public execution, he made the tour all around the dangerous spot, at about eighteen miles distant from the centre, and found the land entirely barren on all sides,-not even the least plant or grass to be seen. He states that each criminal was sent to the house of the old ecclesiastic, and furnished with a box for the poison, a long leather-cap, and a pair of leather-gloves, and instructions were given him to travel with the utmost dispatch, and always before the wind. During thirty years, the priest assured Forsch that he had dismissed upwards of seven hundred criminals, but that scarcely two out of twenty returned. When questioned about the origin of the tree, he replied:-"We are told in our new Alcoran that above a hundred years ago, the country around the tree was inhabited by a people strongly addicted to the sins of Sodom and Gomorrah; when the great prophet Mahomet determined not to suffer them to lead such detestable lives any longer, he applied to God to punish them; upon which God caused this tree to grow out of the earth, which destroyed them all, and rendered the country for ever uninhabitable." Foersch also states, that in consequence of a rebellion in 1755, four hundred families were compelled to settle in the uncultivated vicinity of the tree; but their number, in less than two months, was reduced to about three hundred, all of whom had the appearance of being tainted with an infectious poison. A description is likewise given of the execution of thirteen fair delinquents, in all of whom life was extinct sixteen minutes after they had been lanced in their breasts by an instrument poisoned with the gum of the Upas. Some hours after death, he observed their bodies full of livid spots, their faces swelled, their colour changed to a kind of blue, their eyes yellow, &c. This account afforded to Darwin too excellent a subject for poetic embellishment, to render him anxious to investigate its authenticity with any very great degree of severity; his personification of the tree, indeed, in the Loves of the Plants, may be well ranked amongst the most beautiful and striking passages in the whole poem :— "Fierce in dread silence on the blasted heath |