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.

MR. W.K. BURTON'S TABLE OF COMPARATIVE EXPOSURES
------------+--------------------+-------------------+-------------------------
| | Badly lighted| Portraits in bright
| | interiors,| diffused light
Aperture | +------------+ up | out of doors.
calculated | Landscape with | Fairly | to | /
on the | heavy foliage in | lighted | | / Portraits in
standard | foreground. | interiors | | / studio light
system +------+-------+ +------+ | | | /
of the | Sea |Open | | Under| | | | / Portraits
Photographic| and |land- | |trees,| | | | | in ordinary
Society. | sky. | scape.| |up to | | | | | room.
------------+-