The ignition for other than Diesel or Semi-Diesel engines is now generally by magneto as being the most generally satisfactory, and in general use.

Assuming that the conditions favour the installing of an internal combustion engine, the choice for the small power required lies between either a Town Gas, Suction Gas, Paraffin or Petrol engine. The first cost of a Town Gas, Paraffin or Petrol engine is about the same for these sizes, while that of a Suction Gas engine is about 50 per cent, greater. The average fuel running cost per hour for a 6 h.p. of each type will be as follows : Suction Gas, say threepence, Paraffin, say sixpence, Town Gas or Petrol, say ninepence. The running cost of the Suction Gas engine is very low in comparison, but against that must be set the cost of attending frequently to the producer, and that if the power is not constantly required then one or other of the engines is preferable, on the score of easy starting as and when required, with no firing up beforehand. If, however, the engine power is required all the time, and the required attention to the gas producer can be given without special cost, then the suction gas plant should be chosen. Otherwise the Paraffin engine will probably be decided upon.

Water

It is very frequently assumed that power can be got from a waterfall for practically nothing per horse power. This is very far from correct. In the first place the machinery is costly, and its installation may be still more costly, owing to natural difficulties of the situation. When the plant is installed, interest must be paid on the money spent, in addition to which it will require attendance, and also the power available will have to be transmitted to wherever it may be required. If a large amount of power can be obtained from the fall, then factories will more or less soon be started up in the neighbourhood. Now in this country there are many small water falls, and also fast-running streams that may give some the idea that power can be obtained from them, and that only want of enterprise on the part of the owner prevents this. There are several points to be considered before this can be agreed upon, such as : What amount of power is available, is it constant all through the year, what can be taken as a fair average through out the year, is the district accessible for factory purposes, what would be the cost of the necessary plant, what would be the cost (if necessary) of transmitting the power to a place where it could be profitably used, etc. To determine the horse power available will take some time and therefore cost some money, and therefore having such a fall or stream in mind, the first thing to determine is : Is the site at all possible for the profitable use of " X " horse power? or could it be made so at a reasonable cost ? The question must be settled definitely before going into other points. Assuming that the answer is in the affirmative, then the next point is to settle the horse power available. Firstly, determine the speed of the water passing per minute. Select a part of the stream above the fall, where the banks are parallel if possible, and where the depth of water is fairly uniform, take a large glass bottle, partly fill it with water so that it will float with its neck above water and let it float down the stream, time it over a distance of at least 100 feet, more if possible. Repeat this several times, both in mid-stream and near the banks, take the average time, and use that only. Make due allowance as to whether the stream varies much in different times of the year. Next measure the average width between banks at different places on the water line, take the average, and make due allowance for any shelving banks, if necessary. Measure the stream depth in as many places as ever possible, take the average, and use that only. Now the average speed of the water in feet per minute, multiplied by the average width and depth also in feet, will give the number of cubic feet passing per minute. Next the available fall or head of the water must be determined, this may require surveyor's instruments, but can possibly be done by carefully levelling a long straight-edge firmly secured near one end of the fall, and sighting along it to a rod held vertically at the other end of the fall; care must be used if this is done, or 10 per cent, errors can be introduced easily; measure the head in feet. The theoretical or water horse power available will be

Cubic ft. x Head x 621/2. 33,000

If the efficiency of the water wheel and transmitting plant combined be 50 per cent., then the brake horse power available will be half that found by the above expression, and, of course, even this depends for accuracy upon the accuracy of the figures obtained for the speed, and volume of water in the stream, and also the measurement of the head of water. It would be better therefore on these figures to allow for some errors, and instead of assuming that 50 per cent, of the water horse power was available, work on 40 per cent. only. Due allowance must be made as to what occurs in the winter; does the stream freeze up sometimes, and at times of thaws and storms increase to say ten times its average volume. All these things must be allowed for. Assuming that the figure of 40 per cent, of the water horse power is attractive for the project in view, and that all else is favourable, then it might be desirable to more accuratelv measure the stream volume, and also the head. The first could be done by causing all the water to be discharged over a weir or notched plank. The notch should be about two-thirds of the width of the stream, it should have a thin metal edge all round it, with edges not more than 1/2in. wide, and the bottom edge of the notch must be quite level, with the sides at right angles, and truly vertical; the sides must of course be high enough to take all the water within them that passes. The stream must then be dammed up so that a sjaff can be driven into the bed and marked exactly with the top of the bottom edge of the notch, the water flow over the notch is measured from this mark on the staff, not on the notch itself. A right-angled V notch is sometimes used for smaller flows, and the quantities that apply for any height of flow over these notches can be found in engineering reference books.