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that if equal weights be put into the scales, no effect will be produced on the position of the balance, and the beam will remain horizontal. If a small addition be made to the weight in one of the scales, the horizontality of the beams will be disturbed; and, after oscillating for some time, it will, on attaining a state of rest, form an angle with the horizon, the extent of which is a measure of the delicacy or sensibility of the balance.

Why should not the weights of a balance be touched by the hand?

Because that would not only oxydate the weight, (or cause it to rust) but by raising its temperature, it would appear lighter when placed in the scale-pan, than it should do, in consequence of the ascent of the heated air. For the large weights, a wooden fork or tongs should be employed; and for the smaller, a pair of forceps made of copper; this metal possessing sufficient elasticity to open the forceps on their being released from pressure, and yet not opposing a resistance sufficient to interfere with that delicacy of touch, which is desirable in such operations.-Kater.

Why does one weight alone serve to determine a great variety of others, by the steelyard?

Because the steelyard is a lever, having unequal arms, and by sliding the weight along the longer arm of the lever, we thus vary its distance from the fulcrum, taken in a reverse order; consequently, when a constant weight is used, and an equilibrium established, by sliding this weight on the longer arm of the lever, the relative weight of the substance weighed, to the constant weight, will be in the same proportion as the distance of the constant weight from the fulcrum is to the length of the shorter arm.

Why is the spring steelyard in very general use? Because of its portability; as a spring that will ascertain weights from one pound to fifty, is contained

in a cylinder only 4 inches long and inch diameter. To use this instrument, the substance to be weighed is suspended by a hook, the instrument being held by a ring passing through the rod at the other end. The spring then suffers a compression, proportionate to the weight, and the number of pounds is indicated by the division on the rod, which is cut by the top of the cylindrical tube.-Kater.

The dial weighing machine is a modification of the same principle, connected with hands on a dial or clock-face to denote the weight.

WATER.

Why do water-wheels vary in their construction? Because of the different ways in which the mechanical force of the liquid is intended to be applied.

Why are certain of these wheels called overshot? Because the water by which they are impelled descends from its level to a lower one; its weight during the descent (falling, as it were, over the wheel) causing the wheel to turn. That this may be possible, it is only necessary that there should be a sufficient supply of water at the superior level, and that there should be a means of carrying it off after its descent, so as to prevent, by its accumulation, the equalization of the two levels. Hence the necessity of flood-gates in a mill course. On the circumference of the wheel the weight of the water is made to act in its descent, in a direction as nearly as possible at the right angles to the spokes, or radii; this pressure, however, acting only at one side of the wheel; thus making the wheel revolve, and communicate motion to its axis; and this motion being transmitted by wheel-work, and other contrivances, to the machinery which it is required to work.

Why are other wheels called undershot?

Because the flat or float boards placed at equal distances on the rim, and projecting from it, in direc

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tions diverging from its centre, are intended to receive the impulse of the water as it passes under the wheel. The wheel is thereby caused to revolve in the direction of the stream, with a force depending on the quantity and velocity of the water, and the number, form, and position of the float-boards.

The breast wheel partakes of the nature of the overshot and undershot wheels; like the latter, it has floatboards; but, like the former, it is worked more by the weight of water than by its impulse.

The power of water on wheels may be thus illustrated. If 100 gallons per minute be equal to a certain power with one foot of fall, one gallon per minute will perform the same work with 100 feet of fall.*

Why is the hydrostatic or Bramah's press, another example of the mechanical agency of water?

Because water, in common with all fluids, possesses the power of transmitting pressure equally in every direction. In this instance, too, it is materially aided by the mechanical efficacy of the lever.

Pascal demonstrated this principle and its advantages, by fixing to the upper end of a cask set upright, a very long and narrow cylinder. In filling the barrel, and afterwards the cylinder, the simple addition of a pint or two of water, which the latter was capable of containing, produced the same effect as if the cask, preserving its diameter throughout, had its height increased by the whole length of the cylinder. Thus, the increase of weight of a pint or two of water, was sufficient to burst the bottom of the hogshead, by the immense augmentation of pressure it occasioned. Now, if we suppose the water removed from the cylinder of

The mechanical force of running water is tremendous. During the great storm and flood in Scotland, in 1829, the river Don forced a mass of 400 or 500 tons of stones, many of 200 or 300 pounds weight, up an inclined plane, rising 6 feet in 8 or 10 yards. A stone of 3 or 4 tons, was likewise moved out of a deep pool of the river, 100 yards from its place.

narrow dimensions, and replaced by a solid of equivalent weight, such as a piston, it is evident that the pressure must remain everywhere the same. Again, if we suppose the weight of the piston to be multiplied by the power of a lever acting on its shaft, the pressure will be proportionally augmented, so as to produce on the bottom of the cask a pressure equivalent to an enormous weight, with the exertion of very little primitive force on the piston.-Notes in Science.

This property of liquids also enables us with great facility to transmit the motion and force of one machine to another, in cases where local circumstances preclude the possibility of instituting any ordinary mechanical connexion between the two machines. Thus, merely by means of water-pipes, the force of a machine may be transmitted to any distance, and over inequalities of ground, or through any other obstruc

tions.

Why is the hydrostatic press more advantageous than that worked by a screw?

Because between solids and fluids there is little or no friction; and, accordingly, in the hydrostatic press no force is lost by friction, except what is necessary to overcome the friction of the pistons in the cylinders. The loss of power in the screw, by means of friction, has already been explained at page 32.

ANIMAL STRENGTH.

Why does the rate of steam carriages surpass the utmost stretch of animal power?

Because the machine by which they are propelled, unlike any animal, rolls along unimpeded in any degree by the speed of its own motion.

According to some experiments, recently made by Mr. Bevan, to determine the actual force of draught of carriages upon common roads, it appears that five horses will draw with equal ease the same load upon a good hard turnpike road, as thirty-three horses can

do upon loose sand. Or, if we assume the value of draught, upon a well-formed road in good condition, at 6d. per ton per mile, the equivalent price of draught will be upon hard turf, 74d.; hard loam 94d.; ordinary bye-road, 1s. 7d.; newly gravelled road, 2s. 2d.; loose sandy road, 3s. 1d.—Philos. Mag.

The power of some dogs is very extraordinary. Nine Esquimaux dogs, belonging to Captain Lyon, dragged 1611 pounds one mile (1760 yards) in nine minutes, and worked in this manner for seven or eight hours a day.

Why is it so disadvantageous to propel boats on canals by means of horses?

Because the expenditure of animal strength takes place in a far greater proportion than the increase of speed. Thus, if a horse of a certain strength is barely able to transport a given load ten miles a day for a continuance, two horses of the same strength will be altogether insufficient to transport the same load twenty miles a day. To accomplish that a greater number of similar horses would be requisite. If a still greater speed be attempted, the number of horses necessary to accomplish it would be increased in a prodigiously rapid proportion. This will be evident, if the extreme case be considered, viz., that there is a limit of speed which the horse, under no circumstances, can exceed. In an ordinary canal one horse with a boat will be sufficient for every thirty tons.

Why is a man better enabled than a horse to carry a a weight up a steep hill ?

Because the peculiar disposition of the limbs of a man, renders him well fitted for this species of labour; whereas it is the worst method in which a horse can be employed. It has been observed that three men climbing a hill, loaded with 100lbs. each, will ascend with greater speed than one horse carrying 300lbs.

The average value of human strength, considered

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