Regarding, then, the meridian of 180° as the neutral point, the most rational system, so far as British settlements are concerned, is to reckon longitude both ways, from 0° to 180°, east and west from Greenwich; and to regard all west longitude as in arrear of British time, and all east longitude as in advance of it. And this is the method practised by modern navigators. It is not, however, in obedience to any preconceived system, but by pure accident, that our settlements in Australia and New Zealand happen to be in accordance with this rule. The last-named country is very close upon the verge of eastern longitude, but still it is within it, and its day is rightly in advance of our day. But the first settlers to Botany Bay, in 1788, were actually under orders to go out by Cape Horn, and were only forced by stress of weather to adopt the opposite course by the Cape of Good Hope. Had they kept to their prescribed route, there cannot be a doubt that the day of the week and month in Australia would now be a day later than it is. The best proof of the truth of this assertion is, that a few years afterwards a missionary expedition was sent out to Otaheite, with respect to which a precisely similar accident occurred; they could not weather Cape Horn, and were forced to go round, some twice the distance out of their way, by the Cape of Good Hope; consequently they carried with them what may be called the eastern day, and since then that is the day observed at Otaheite, although fully two hours within the western limit of longitude. From this cause an actual practical anomaly has recently arisen. The French authorities in Tahiti, in accordance with the before-mentioned rule, have arranged their day by _western longitude_; consequently, in addition to other points of dissent, they observe the Sabbath and other festivals one day later than the resident English missionaries. I have extended this explanation to a greater length than I intended, but the subject is interesting, and not generally well understood; to do it justice, therefore, is not compatible with brevity. Much of what I have said is doubtless already known to your readers; nevertheless I hope it may be useful in affording to H. the information he required, and to ESTE more fixed notions on the subject than he seems to have entertained when he wrote the answer referred to. A. E. B. Leeds. * * * * * PHOTOGRAPHIC CORRESPONDENCE. _Aceto-Nitrate of Silver._--I have collected together several ounces of aceto-nitrate of silver that has been used to excite waxed paper (iodized by MR. CROOKES' method), and should be glad to know whether it can be used again for the same purpose. JOHN LEACHMAN.