Long-stroke air compressors are evidently objectionable on the basis of greater expense of construction. All the parts must be larger and heavier. The fly wheels are increased enormously in diameter and weight, and the strength of bearings must be enlarged in proportion. It is difficult to equalize power and resistance in air compressors with long strokes. The speed will be jerky, and when slow, the fly wheel rather retards than assists in the work of compression. This action tends to derange the parts and makes large bearings a necessity. The piston in a long-stroke compressor travels through considerable space before the pressure reaches a point where the discharge valve opens, and after reaching that point it has to go on still further against a prolonged uniform resistance. This makes rotative speed difficult. During the early part of the stroke, the energy of the steam piston must be stored up in the moving parts, to be given out when the steam pressure has been reduced through an early cut-off. With a short stroke and a large diameter of steam cylinder we are able to get steam economy or early cut-off and expansion without the complications of compounding. * * * * * [Continued from SUPPLEMENT, No. 793, page 12677.] THE POWER OF WATER, OR HYDRAULICS SIMPLIFIED. By G.D. Hiscox. CURRENT WHEELS FOR POWER AND RAISING WATER. The natural flow of water in a current is probably one of the oldest and cheapest of the methods for obtaining power, or the lifting of water within moderate elevations, for a supply for irrigation and domestic purposes; and we propose, apart from the current wheel, to treat only of self-water-raising devices in this chapter. Water wheels of various forms for this purpose have been used from time immemorial in Europe, Asia and Egypt, where the record gives examples of wheels of the noria class from 30 to 90 feet in diameter; the term _noria_ having been applied to water wheels carrying buckets for raising water; the Spanish _noria_ having buckets on an endless chain. Records of a Chinese noria, of 30 feet diameter, made of bamboo, show a lifting capacity of 300 tons of water per day to a height of ¾ of the diameter of the wheel--velocity of current not stated. For less quantity and greater elevation, these forms of wheel may have pumps attached to the shaft, by crank, that will give a fair duty for a high water supply. For power purposes, as in the plain current wheel, Fig. 23, there are two principal factors in the problem of power--the velocity of the current and the area of the buckets or blades. [Illustration: Fig. 23] Their efficiency is very low, from 25 to 36 per cent., according to their lightness of make and form of buckets. A slightly curved plate iron bucket gives the highest efficiency, thus ( to the current, and an additional value may also be given by slightly shrouding the ends of the buckets.