m sea water by special apparatus provided for the
purpose. In the construction of the distilling or evaporating apparatus
advantage has been taken of two important physical facts, namely, that,
if water be heated to a temperature higher than that corresponding with
the pressure on its surface, evaporation will take place; and that the
passage of heat from steam at one side of a 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 second 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
condenser, where it is condensed, and serves to make up the loss. This
plan localizes 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they can be either withdrawn or exposed, as the apparatus is generally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. O