If the water in the harbor, as is not improbable, is muddy, some method of filtering it before pumping it into the boilers ought, if at all practicable, to be resorted to, for the twofold reason of preserving the boiler plates from muddy deposit, and also to prevent priming, which would certainly ensue from the use of muddy water.  No doubt the medical staff take care that the distilled water is alike thoroughly aerated and efficiently filtered.  The most successful method of aerating is, we believe, to cause the current of steam as it enters the condenser to suck in air by induced current along with it.  The filtering ought not to present any difficulty, as at all events sand enough can be had.  Charcoal, however, is another affair, and all distilled water ought to be brought into contact with this substance.

Simple, however, as such an arrangement as this appears to be, practical difficulties, which it is said are insurmountable, stand in the way of its adoption, and the distilled water produced for Egypt is made in special apparatus, and various forms of condenser are employed, made under various patents.  The principle involved is, however, in all cases the same.  Steam is generated in one of the ships’ boilers, and condensed, filtered, and aerated in a special apparatus.  The great objection to the use of the ordinary surface condenser is that the main engines would, in the majority of cases, have to be kept going, in order to pump the distilled water out of the condenser, and to supply circulating water.  But it is easy to see that if engineers thought proper, this difficulty could be readily got over.  Separate circulating pumps, usually centrifugal, are now freely used, and the addition of a special pump for lifting the condensed water presents no difficulty whatever.  While the main engines are running, the withdrawal of much condensed water would no doubt risk the safety of the boiler; but in the case of so-called “distilling” ships, there need be no trouble incurred on this score.—­The Engineer.

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AIDS TO CORRECT EXPOSURE ON PHOTOGRAPHIC PLATES.

[Footnote:  We take from the Br.  Jour. of Photo. the following interesting paper read by W. Goodwin before the Glasgow and West of Scotland Amateur Association.]

With good plates, and intelligent development, a practiced photographer may within certain limits correct the effects of an over or under exposure; but you have all, doubtless, found out that there is a correct exposure, and that you cannot trespass very far on either side of it without sacrificing something in the resulting negative.