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_. * * * * * 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.