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With anxious eye he surveys the commencement of this new stage of his journey :-But how uncertain and obscure is the "prospect" that "lies before him"-for

"Shadows, clouds and darkness rest upon it."

He in vain may say," here will I hold.”—Alas! the hand of time pushes him on, and he must rely for safety in his course on the protection of his God, and trust, for skilful guides, to his own caution and experience. If he reflect with due attention on the past; if he know how to profit by the ills and troubles which have already assailed him; if he remember the difficulties in which his feet have been entangled, and the evils that have resulted from his own folly, vice, or carelessness, he will now avail himself of the knowledge he has purchased at so dear a price, and be enabled to avoid them all in future. His steps will be governed by prudence. Every portion of ground will be examined and surveyed with solicitude before he will trust it with his weight.

He knows that the time when he shall arrive at the end of his tour cannot be hastened, and will therefore pursue his steady and undeviating way, viewing with pity the unthinking mortals who dash by him with dangerous and unavailing velocity, or the rash and giddy, who fly off from the direct, but rugged tract, in quest of a pleasanter path, and are lost for ever. Let us then, at this important and solemn period, determine to follow the wise man's counsel, though expressed in homely language, "not to make more haste than good speed." Whoever adopts this resolution, will find at the end of his pilgrimage, that he has iaid up a stock of wisdom, which may confer the most important of benefits on all who are to come after him. His grey hairs will be to him a crown of honor, and future generations shall rise up and call him blessed.

FOR THE POLTANTHOS.

A COURSE OF

LECTURES ON NATURAL PHILOSOPHY,

BY J. LATHROP, JUN. A. M.

By what we have seen of man, it is easy to observe that his frame is not adapted to drawing carriages; while, on the contrary, that of an animal upon all fours, the column of whose body, and the situation of whose muscles, act almost directly upon objects placed behind them, is perfectly fitted by nature for this kind of service. That noble animal the horse, has been subjected to the dominion of man, and his strength and speed, are made subservient to the humblest, and the most dignified transactions of life. The sturdy ox, the strong and stubborn mule, and other animals, are also employed in occupations of agriculture; but the horse is fated to suffer a variety of vicissitudes in his journey through life. He triumphantly bears a conquering hero on the field of battle, where he exults in his strength, his neck is clothed with thunder, -and the glory of his nostrils is terrible. Alas! how short lived is his pride-Another year beholds him the common hack of a livery stable, or the wind broken fay of a post boy; and the high mettled racer, from being the boast of his master, and the charm of the course, is humbled in the ignominious traces of a scavenger's cart.

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Common experience informs us, that if a horse is to convey a certain weight, he ought, that he may draw the better, to have a proportionable weight on his back and shoulders. A horse in a two-wheeled cart, in which there is a ton weight, when it is in equilibrium, will not be able to draw it; but if there be 50 or 60 pounds bearing upon his back, he will draw it with ease. A horse tackled in a waggon will draw two or three tons weight, because the line of traction is be low his breast.

Sledges were probably the first machines used in carrying loads; and in some countries they are still employed for such purposes. They certainly answer much better than

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wheels on the smooth surface of ice; but for rough or level ground, carriages that are drawn upon wheels are infinitely preferable.

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Wheel carriages were first introduced into England in the year 1564, by William Bonner, a Dutchman, who was coachman to Queen Elizabeth. In those days,' saith a quaint historian, a coach was a strange monster, and did put both man and horse into amazement. Some said it was a great crab-shell brought out of China; and some imagined it to be one of the pagan temples, in which the cannibals adored the devil: but at last these doubts were cleared, and coachmaking became a substantial trade.'

The utility of wheels arises from their turning on their axes, by which means the resistance arising from friction is much diminished, and the draught rendered more easy. It is easily shown that it requires considerably less force to draw a carriage, when its wheels turn freely, than when they are chained, so that they cannot revolve. According to Helsham, a carriage with four wheels will be drawn with five times as small an effect, as one that slides over the same surface on a sledge. A sledge passing over a plane, undergoes a friction or rubbing against it, equal to the distance through which it moves but if an axis be applied, whose circumference is six inches, and on that a wheel be placed, whose circumference is eighteen feet, it is evident that in moving the carriage 18 feet, the wheels will make but one revolution; and as there is no sliding of the parts between the plane and the wheels, but only a mere change of surface, by one part of the wheel rising and the other descending nearly perpendicularly to the plane, no friction will take place there, the whole being transferred to the nave acting on the axis; which nave, having made but one revolution in the same time, there has been only a sliding of the parts equal to the circumference of the hole in the nave, here supposed to be about 6 inches; so that the friction is lessened about as 1 to 36; besides the advantage gained by confining it to so small a surface, whereby the parts are more easily kept smooth and fited to each other,

and substances applied and retained, to diminish the remaining friction.

By the application of wheels to a carriage, the friction is lessened in proportion of the diameters of the axes, and concave parts of the naves to those of the wheels.

Large wheels have the advantage of small ones in overcoming obstacles, because they act as levers, in proportion to their sizes. In general, the centre of gravity should be as near as may be to the axis of the wheel, for if the body be sus. pended below it, and turned forwards, as is the case with two wheeled carriages descending hills, then will the greater part of the weight be thrown before the axis, and must be partly borne by the horse that draws. In ascending, the same proportion will be thrown backwards and tend to lift the animal.

I shall conclude the subject of wheel carriages, with the result of Mr. Vince's experiments on the subject, as particularly made on them, when moving on plane, hard ground.

If the wheels be all equal and narrow, it requires the same weight to draw it whether it be loaded before, or behind.

If two wheels be low, and two be high, it requires a greater force to draw the carriage, than when all are high. In this case, it makes no sensible difference, which go before. The common opinion, that great wheels drive on the small ones, when they go forward, is not true. The only advantage of placing the small wheels before, arises from the convenience of turning, and the adjustment of a low line of traction for the horses. If the wheels be all equal, the smaller the wheels are, the greater power is necessary to draw the carriage. The disadvantage of small wheels, arises from hence, that the resistance of the ground which turns the wheels about, more easily overcomes the friction at the axle, in a large, than a small wheel, because it acts at a greater distance. Hence, also, the impropriety of laying the load upon the low wheels, as it increases the friction where there is the least power to overcome it. When the load is light, and the friction of consequence small, there is but a trifling difference, between the great and the little wheels; but when the load

is heavy, the difference becomes considerable. The use of high wheels, when going over obstacles, is very manifest; and in sinking into holes, they have a double advantage, as they do not go so low as small ones; and after sinking, they ascend again with more ease, and require less power to effect their extrication.

In treating of simple and compound machines, it has been shewn, that as much as was gained in force, so much was lost in time. In order, therefore, to obtain a just idea of the advantages gained by machines, suppose that a man, by a fixed pulley raises a beam to the top of a house in two minutes, it is clear that he will be able to raise six beams in 12 minutes; but by means of a tackle, with 3 lever pullies, he will raise the 6 beams at once, with the same ease that he before raised one, but then he will be 6 times as long about it; that is, 12 minutes. But the convenience gained by the power, is very great; for if the 6 beams be joined in one, they may be raised by the tackle, though it would be impossible to move them by the unassisted strength of one man.

Mechanics do not teach us to make, but to apply powers, such as we find them in nature; for we deceive ourselves if we think, that by means of any engine, one man shall do the work of two, in the same time, supposing him to employ the same strength. But, by mechanics, we can modify the energy of the moving power, so as to obtain effects, which it would not produce without modification. In the performance of several works, where we have sufficient strength, we often want time; and where we have time to spare, we want strength. In these cases, the mechanic has an opportunity of directing the application of the powers according to time. Thus, in raising great blocks of marble, or other heavy goods to lay them on a wharf, many hands cannot be employed; here then an engine may be used where one man may do the work of ten, but he shall be ten times as long in performing it. Or in a manufactory, suppose that in winding silk, 50 men being employed, should only move a weight equal to half a pound, while each carries his hand round; whereas one