ed from ignorance of certain plain mechanical principles. For example, serious accidents have sometimes occurred from the want of acquaintance with the laws of motion. Persons have heedlessly jumped out of moving vehicles, and got their legs and arms sprained or dislocated, and from one boat to another when both were in rapid motion, and run the risk of being either bruised, drenched or drowned. But had the effects of compound motion been generally known and attended to, in all those cases where it occurs, it would have prevented many of those accidents which have happened from persons rashly jumping out of carriages when in rapid motion, or attempting to jump from the top of a moving cylinder, in which cases, they are always precipitated with violence, in a direction different from what they expected, from the obvious effects of a combination of forces. Boats and carriages have been sometimes overset by persons rising hastily when they were in danger of such accidents,-from ignorance of the principle, that the centre of gravity of the moving vehicle, by such a practice, is raised so as to endanger the line of direction being thrown beyond the base, when the vehicle must, of course, be overturned; whereas, had they clapt down to the bottom, they would have brought down the line of direction, and consequently the centre of gravity, farther within the base, so as to have prevented the accident and secured their safety. The reason of this will perhaps more plainly appear from the following explanations:-The centre of gravity is that point of a body about which all its parts are in equilibrio, or balance each other; and consequently, if this point be supported, the whole body will be at rest, and cannot fall. An imaginary line drawn from

the centre of gravity of any body towards the centre of the earth is called the line of direction. Bodies stand with firmness upon their bases, when this line falls within the base; but if the line of direction falls without the base, the body will be overturned. Thus, the inclining body ABCD, whose centre of gravity is E, stands firmly on its base CDKF, because the line of direction EM falls within the base. But if a weight, as ABGH, be laid upon the top of the body, the centre of gravity of the whole body and weight together, is raised up to I; and then as the line of direction ID falls without the base at D, the centre of gravity I is not supported, and the whole body and weight must tumble down together.

The tower of Pisa in Italy, leans sixteen feet out of the perpendicular, so that strangers are afraid to pass under it; but as the plummet or line of direction falls within its base or foundation, it is in no danger of falling, if its materials keep together; and hence it has stood in this state for three hundred years. But were an additional erection, of any considerable elevation, to be placed upon its top, it would undoubtedly soon tumble into ruins.

To a somewhat similar cause, in combination with heedlessness and ignorance, may be ascribed many of those accidents which so frequently happen at spinning mills and other pieces of machinery, by which legs and arms are torn asunder, and the human frame sometimes mangled and destroyed.

Fatal accidents have likewise happened from ignorance of the effects produced by the refraction of light. It is a well-known optical fact, that when a ray of light passes from air into water, and is again refracted, the sine of the angle of incidence is in proportion to the sine of the angle of refraction as four to three. From this circumstance it happens, that pools and rivers appear shallower than they really are their channels, when viewed from their brink, being apparently higher than their true position, in the proportion of three to four; so that a river eight feet deep will appear from its bank to be only six. This fact may be at any time perceived in a tub or pail full of water, where the bottom of the vessel will obviously appear to be raised a considerable space above its true position, and its apparent depth consequently diminished. In consequence of this optical illusion, which is not generally known, many a traveller as well as many a schoolboy has lost his life, by supposing the bottom of a clear river to be

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within his depth, as, when he stands on the bank, the bottom will appear one-fourth nearer the surface than it really is.

This will appear evident from the following illustrations:-If a ray of light AC passes obliquely from air into water, instead of continuing its course in the direct line CB, it takes the direction CH, and approaches the perpendicular PP, in such a manner, that the angle of refraction PCH is less than its angle of incidence ECA. AE is the sine of the angle of incidence, and HP the sine of the angle of refraction; and the proportion they bear to each other is as four to three. If a small body, therefore, were placed at H and viewed from the point A, it would appear as if it were raised to the point B, or one-fourth higher than it really is.

This may be farther illustrated by the following common experiment. Put a shilling into the bottom of an empty bason, at C, and walk backwards till it appear completely hid by the interception of the edge of the bason; then cause water to be poured into the bason, and the shilling will instantly appear as if placed in the point D; for, being now in a denser medium, it appears raised, or nearer to its surface. Before the water was poured in, the shilling could not be seen where it was; now it is seen where it is not. It is not the eye that has changed its place, but the ray of light has taken a new direction, in passing from the water to the eye, and strikes the eye as if it came from the piece of money. This experiment may be varied as follows:-Take an empty bason, and, along the diameter of its bottom, fix marks at a small distance from each other, then take it into a dark room, and let in a ray of light; and where this falls upon the floor, place the bason, so that its marked diameter may point towards the window, and so that the beam may fall on the mark most distant from the window. This done, fill the bason with water, and the beam which before fell upon the most distant mark, will now, by