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 obstructions.

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 foam 93d.; 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 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

as a mechanical agent, has been variously estimated. Desaguliers considers that a man can raise the weight of 550lbs. ten feet high in a minute, and continue to do so for six hours. Smeaton, however, thinks that six good English labourers will be required to raise · 21,141 solid feet of sea-water to the height of four feet in four hours. In this case, they will raise very little more than six cubic feet of fresh water each, ten feet high in a minute. The labourers whom Smeaton supposes to execute this work he considers to be equal to twice the number of ordinary men. It would, therefore, perhaps, be a fair average value of a man's work to estimate it, for a continuance, at half a hogshead of water raised through ten feet in a minute.

The efforts of men differ with the manner in which these efforts are employed. It has been shown by Mr. R. Buchanan, that the same quantities of human labour employed in working a pump, turning a wheel, ringing a bell, and rowing a boat, are as the numbers 100, 167, 227, and 248. The most advantageous manner of applying human strength is in the art of rowing. The strength of an ordinary man walking in an horizontal direction, and with his body inclining forward, is, however, only equal to 271b., and it is known by experience, that a horse can draw horizontally as much as seven men.

Why is the power of a steam-engine expressed in horse power?

Because this mode was introduced when steam en gines first began to supersede horse mills, when the manufacturer naturally inquired how many horses a steam-engine would dispense with. Hence the expression is more practical than scientific.

The power of a horse is understood to be that which will elevate a weight of 33,000* pounds, the height of

* Another estimate reduces this to only 22,000 pounds, raised one foot high in a minute, equivalent to 100 pounds in two miles and a half per hour.

one foot in a minute of time, equal to about 90 pounds at the rate of four miles an hour. This is a force greater than that exerted by a common cart horse, which is not estimated at more than 70 pounds: that is to say, that a horse harnessed to a cart, weighing, with its load, forty cwt. or two tons, and drawing on a level road at the rate of four miles an hour, makes use of the same force, as if his traces, instead of being fastened to a cart, were passed over a pulley, and lifted perpendicularly a weight of 70 pounds.

A steam-engine consumes about 20 feet of steam per minute for every horse-power.-Notes in Science.

RAILWAYS.

Why are railways more economical than ordinary roads?

Because, to drag a loaded waggon up one inconsiderable hill, costs more force than to send it thirty or forty miles along a level railway; and the conclusion follows, that although the original expense of forming the level line might materially exceed that of making an ordinary road, still, in situations of great traffic, the difference would soon be paid by the savings; and when once paid, the savings would be as profit ever after.-Arnott.

By way of illustrating the great economy of machinery, we may observe, that in Sedjah, (where the Arabs obtain fine millstones) "their unskilfulness and want of proper implements adapted to their labour, with the expense of carriage from the quarry to the place of sale, each stone requiring a single camel, (wheel carriages and good roads being entirely unknown) occasion an advance above the prime cost at which they might be hewn in England, of at least 500 per cent, each pair of stones costing from ten to twenty pounds sterling."-Buckingham's Travels.

Why has a suspension railway been represented as more advantageous than a ground railroad?