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.[[1]]
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.
------------+------+-------+-----+------+-----+------+------+-----+------
| sec | sec | sec | m s | m s| h m | sec | m s| m s
No. 1, | 1/160| 1/50 | 1/8 | 0 10 | 0 10| 0 2 | 1/6 | 0 1| 0 4
or f/4 | | | | | | | | |
------------+------+-------+-----+------+-----+------+------+-----+------
No. 2, | 1/80 | 1/25 | 1/4 | 0 20 | 0 20| 0 4 | 1/3 | 0 2| 0 8
or f/5.657 | | | | | | | | |
------------+------+-------+-----+------+-----+------+------+-----+------
No. 4, | 1/40 | 1/12 | 1/2 | 0 40 | 0 40| 0 8 | 2/3 | 0 4| 0 16
or f/8 | | | | | | | | |
------------+------+-------+-----+------+-----+------+------+-----+------
No. 8, | 1/20 | 1/6 | 1 | 1 20 | 1 20| 0 16 | 1-1/3| 0 8| 0 32
or f/11.314 | | | | | | | | |
------------+------+-------+-----+------+-----+------+------+-----+------
No. 16, | 1/10 | 1/3 | 2 | 2 40 | 2 40| 0 32 | 2-2/3| 0 16| 1 4
or f/16 | | | | | | | | |
------------+------+-------+-----+------+-----+------+------+-----+------
No. 32, | 1/5 | 2/3 | 4 | 5 20 | 5 20| 1 4 | 5-1/3| 0 32| 2 8
or f/22.627 | | | | | | | | |
------------+------+-------+-----+------+-----+------+------+-----+------
No. 64, | 2/5 | 1-1/3 | 8 |10 40 |10 40| 2 8 |10-1/2| 1 4| 4 15
or f/32 | | | | | | | | |
------------+------+-------+-----+------+-----+------+------+-----+------
No. 128, | 4/5 | 2-2/3 | 16 |21 0 |21 0| 4 15 | 21 | 2 8| 8 30
or f/45.255 | | | | | | | | |
------------+------+-------+-----+------+-----+------+------+-----+------
No. 256, |1-1/2 | 5-1/2 | 32 |42 0 |42 0| 8 30 | 42 | 4 15|17 0
or f/64 | | | | | | | | |
------------+------+-------+-----+------+-----+------+------+-----+------
The estimation of this correct exposure is probably the greatest difficulty in photography, and it is particularly discouraging to find plate after plate useless because the guess has been wide of the mark. There are some here to-night who have spoiled so many plates that at last they are prepared by experience for almost any contingency, and to those I nave very little to say; but there are also many who are still in their troubles, and I propose to tell them how the amount of guesswork required may be reduced to a minimum.
The factors which govern exposure are: the subject of the picture, the lens and its aperture, the rapidity of the plate, and last, but not by any means least, the quality of the light by which the work is to be done.
Let us consider each of these separately, and see if we cannot reduce any of them to rule. In this respect the subject will be found somewhat intractable. Scarcely two subjects will be found to send exactly the same amount of light through the lens. However, a broad classification may be made, and this has been done by Mr. Burton in his Table of Comparative Exposures. A glance at this table will show how greatly the character of the view may influence the time of exposure. Thus, with full aperture of a rapid symmetrical, the exposure for open landscape is given as one-twelfth of a second; when heavy foliage appears in the foreground, half a second will be required; while, under trees, as much as forty seconds may be needed.
The first aid I have to suggest is the use of such a table as Mr. Burton's. Before we do anything more in this direction, we must consider the influence of the lens and its diaphragms. In theory the single landscape lens is more rapid than the doublet of equal aperture, but the difference is so little that it may be disregarded in practice, and my remarks will apply to both.