In the construction of the distilling or evaporating apparatus advantage has been taken of two important physical facts, namely, that if water he heated to a temperature higher than that corresmionding with the pressure on its surface, evaporation will take place; and that the passage of heat steam at one side of as plate to water at the other is very rapid. In practice the distillation is effected by passing steam, say from the first receiver, through a nest of tubes inside a still or evaporator, of which the steam space is connected either with the Seer rnd receiver or with the condenser. The temperature of the steam inside the tubes being Higher than that of the steam either in the second receiver or in the condenser, the result is that the water inside the still is evaporated, and passes with the rest of the steam into the C1.11?1011S01*, Wilcro it is condensed and serves to make np the loss. This plan localizes the trouble if deposit and frees it from its dangerous character because an evaporator can not become ovorheutcd like at holler, even though it be neglected until it salts lip solid. When the tubes do become incrusted with deposit, they can he either withdrawn or exposed, as the apparatus is generally so arranged, and they pan then he cleaned.
Serele-Provellers.—An extensive series of experiments on screw-propellers was made, Meier the direction of Mr. IIIcchynden in ItittI, with a large number of models, the primary object being to determine what value there was in a few of the various twists which inventive inge nuity can give to a screw-blade. The results led the experimenters to the conclusion that in free water such twists and curves are valueless as serving to augment efficiency. The experi ments were then carried further, with a view to determine quantitative modiiii for the resist ance of screws with different ratios of pitch to diameter, or "pitch ratios," and afterward with different ratios of surface to the area of the circles described by the tips of the blades, or "surface ratios" One of the most important results deduced from experiments on model screws is that they appeal' to have practically equal efficiencies throughout a wide range both in pitch ratios and in surface ratio, so that great latitude is left to the designer in regard to the form of the propeller.
Another important feature is that, although these experiments are not a direct guide to the selection of the most efficient propeller for a particular ship, they supply the means of analyzing the performances of screws fitted to vessels, and of thus indirectly determining what are likely to be the best dimensions of screw for a vessel of a class whose results are known.
Thus a great advance has been made on the old method of trial upon the ship itself, which was the origin of almost every conceivable erroneous view respecting the screw-propeller. The fact was lost sight of that any modifications in form, dimensions, or proportions referred only to that particular combination of ship and propeller, or to one similar thereto. and so something like chaos was the result. This, however, need not be the case much longer.
In regard to the material used for propellers, steel has been largely adopted for both solid and loose bladed screws, but unless protected in some way the tips of the blades are apt to corrode rapidly and become unserviceable. One of the stronger kinds of bronze is often judiciously employed for the blades in conjunction with a steel boss. Where the first extra expense can be afforded bronze seems the preferable material; the castings are of a reliable character, and the metal does not rapidly corrode; the bronze blades can therefore with safety be made lighter than steel blades, which favors their springing and accommodating them selves more readily to the various speeds of the different parts of the wake. (References: Trans. Inst. _Ararat Architects, 1886-'87; Proc. Inst. Engrs., 1890; Northeast Coast lost. of Engm. and Ship-builders, vol. vii, 1890-'91.) Twin Screws.—The great question of twin-screw propulsion has been put to the test upon a large scale in the mercantile marine. While engineers, however, are prepared to admit its advantages so far as greater security from total breakdown is concerned, there is by no means thorough agreement as to whether single or twin screws have the greater propulsive efficiency. What is required to form a sound judgment upon the whole question is a series of examples of twin and single screw vessels, each of which is known to be fitted with the most suitable propeller for the type of vessel and speed; and until this information is available little can be said upon the subject with any certainty.
The following table shows sonic recent examples of twin-screw steamers: Twin screws offer an opportunity for reducing the weight of all that part of the machinery of which the weight relatively to power is inversely proportional to the revolutions for a given power. This can be reduced in the proportion of 1 to 52—that is, to 71 per cent of its weight in the single-screw engine; for, since approximately the same total disk is required in both cases with similarly proportioned propellers, the twins will work at a greater speed of revolution than the single screw.