the trouble of deposit, and frees it from its dangerous character, because an evaporator cannot become overheated like a boiler, even though it be neglected until it salts up solid; and if the same precautions are taken in working the evaporator which used to be adopted with low pressure boilers when they were fed with salt water, no serious trouble should result.  When the tubes do become incrusted with deposit, they can be either withdrawn or exposed, as the apparatus is generally so arranged; and they can then be cleaned.

Screw Propeller.—­In Mr. Marshall’s paper of 1881 it was said that “the screw propeller is still to a great extent an unsolved problem.”  This was at the time a fairly true remark.  It was true the problem had been made the subject of general theoretical investigation by various eminent mathematicians, notably by Professor Rankine and Mr. William Froude, and of special experimental investigation by various engineers.  As examples of the latter may be mentioned the extended series of investigations in the French vessel Pelican, and the series made by Mr. Isherwood on a steam launch about 1874.  These experiments, however, such as they were, did little to bring out general facts and to reduce the subject to a practical analysis.  Since the date of Mr. Marshall’s paper, the literature on this subject has grown rapidly, and, has been almost entirely of a practical character.  The screw has been made the subject of most careful experiments.  One of the earliest extensive series of experiments was made under the writer’s direction in 1881, 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 ingenuity 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 experiments were then carried further with a view to determine quantitative moduli for the resistance of screws with different ratios of pitch to diameter, or “pitch ratios,” and afterward with different ratios of surface to the area of the circle described by the tips of the blades, or “surface ratios.”  As these results have to some extent been analyzed and published, no further reference need be made to them now.

In 1886, Mr. R.E.  Froude published in the Transactions of the Institution of Naval Architects the deductions drawn from an extensive series of trials made with four models of similar form and equal diameter, but having different pitch ratios.  Mr. S.W.  Barnaby has published some of the results of experiments made under the direction of Mr. J.I.  Thornycroft; and in his paper read before the Institution of Civil Engineers in 1890 he has also put Mr. R.E.  Froude’s results into a shape more suitable for comparison with practice.  Nor ought Mr. G.A.  Calvert’s carefully planned experiments to pass unnoticed, of which an account was given in the Transactions of the Institution of Naval Architects in 1887.  These experiments were made on rectangular bodies with sections of propeller blade form, moved through the water at various velocities in straight lines, in directions oblique to their plane faces; and from their results an estimate was formed of the resistance of a screw.