Suitable Turbines. Several types of turbine are suited for these low falls, such as the Victor, the Trent, and especially the Girard, giving a good percentage of effect with water varying in height of fall, but regular in quantity.

The quantity of water required to give one effective horse-power (at 80 per cent. efficiency of wheel) is as follows:

Cubic feet per niinute.. 670 330 230 170 132 112 95 83 74 67

Beyond 14 electric horse-power the cost of accumulators so much increases, that an arrangement would be needed to drive the motor direct from the dynamo part of the time, and the balance of the time to drive it by the aid of the accumulators. By this means the accumulators for the smaller powers given above could also be decreased in cost by reducing the number of their plates.

CHAPTER XII.

FLOATING MILLS AND WATER-WHEELS.'

THESE machines cannot be said to be of high economy, but still, in cases where a rapid and ample current is always to be relied upon, they are a cheap method of obtaining a given power. An old barge may be converted into a very fair mill with overhanging wheels like a paddle steamer. A better arrangement is to employ two barges stoutly secured together, a proper distance apart, between which the wheel is arranged and revolves. The diameter of such wheels is generally 12 to 15 feet, with 9 to 11 floats 24 to 30 inches. deep, and dipping 12 inches in the water. They should be made with curved floats if possible, and the position on the boat should be such that the water has a free entry to the wheel, and no obstacle in getting away from it.

The first thing necessary is to ascertain the speed of the stream.

Stake off a length of 50 feet on both sides of the stream. Station two men, one at each end of the 50 feet chosen. Drop a light float of cork or wood in the stream a few feet above the first stake, and take the time it occupies in passing between the two men. Check this several times for the

sake of accuracy.

second.

Let equal the speed of the stream in feet per Then the velocity of the floats of the wheel in feet per second, which we will call C, will equal .4 of V = V × .4, or of V.

Let A = the dip of the floats in the water.

"B the width of the floats in the water.

=

Then the horse-power = V x C x A x B x .0028.

Only very approximate figures could be given as to the cost of a floating mill. Naturally, it would in most cases be unnecessary to construct new floats or barges, and the cost of these would therefore depend greatly on local circumstances. Even a raft might be made to do duty, if timber were plentiful and other craft unobtainable. Their position in mid-stream, in a strong current, does not make them very suitable as motors for the purposes of general industry.

TABLE OF THE PRESSURE OF FRESH WATER AGAINST A PLANE SURFACE AT RIGHT ANGLES TO THE MOTION OF THE WATER.

Pressure in lbs. per sq. ft. of surface.... 3.87 15.49 34.85 61.95 96.80 139.39 189.73

Pressure in lbs. per sq. ft. of surface. 247.81 313.63 387.20 557.57 871.20 1548.8

Undershot Water-wheels. The economy of ordinary undershot water-wheels, having straight paddles or floats, is very low, and Poncelet's improved construction should always be adopted. In this the floats are constructed on a curve bearing a relation to the angle of inflow and depth of The result is the raising of the economical use of the water from 35 per cent. to 60 per cent.

same.

The effective power of a Poncelet wheel = .00113 × the fall the quantity of water in cubic feet per minute.

The arrangement of floats should be such as that two are always covering the sluice opening.

The number of floats should equal the diameter of the wheel x 1.6 + 16.

These wheels should not be less than 7 feet or more than 16 feet diameter.

An undershot water-wheel 7 feet 6 inches diameter x 3 feet wide will, with two 3-inch pumps, lift about 3,000 gallons an hour 50 feet high, and costs complete with pumps about $400, or £80.

COST OF UNDERSHOT WATER-WHEELS.

Breast and Overshot Water-wheels.-The adoption of one or other of these types of wheels must depend largely on local necessities. There can be no question that where the fall admits of it, the higher the breast adopted, or the nearer the arrangement approaches to that of an overshot wheel, the better the results are likely to be. Well-proportioned wheels of these forms are, however, reliable, and in the case of the overshot type run closely up to the turbine for economy.

The power of either may be found as under:

Leth

Let Q

Then

the head of water in feet,

= the quantity of water in cubic feet per minute.

in low breast wheels the effective h. p. = .00104 × Q × h

Inversely, the quantity of water required to provide a given power may be found thus :

The speed of a wheel may be ascertained by the follow

These wheels are usually made from 12 to 50 feet diameter and even larger in exceptional instances, such as the great wheel at Laxey, in the Isle of Man.

Some excellent little pumping water-wheels are manufactured, which may be found very handy for small house and garden supplies, and a list of which, with prices, is therefore appended.

COST OF WATER-WHEELS WITH PUMPS.