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Page:Popular Science Monthly Volume 4.djvu/228

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THE POPULAR SCIENCE MONTHLY.

At all the American stations the photo-heliograph, the contact method, and the method of cusps, will be used. The American stations will be eight in number. These will be principally in the southern latitudes, in the Indian and Pacific Oceans, except one in Siberia, and, perhaps, a photographic station in the Sandwich Islands.

Stations in Japan and China will be established also by the Americans.

Most of the English parties are to be in northern stations, though the Challenger exploring expedition is instructed to examine eligible stations in the South Pacific. Of the stations of French observers little is definitely known, although they will occupy a few posts.

Each party must be provided with instruments to observe the actual transit, and it must further have the means of determining accurately time, longitude, and latitude.

Of these quœsitœ, the latitude and the local time are most easily determined. Portable transit-instruments will suffice for the first determination, and for the second there are various adequate means.

The American parties are each to be provided with a small portable transit-instrument and zenith-telescope combined, which instruments are now making by Stackpole, of New York.

These are intended to be of the simplest possible construction and of the greatest attainable stability, and they combine several advantages. In accordance with a suggestion first proposed by Steinheil, of Munich, the tube proper of the telescope will be reduced to one-half of the usual length. A prism will be placed at the end of the tube opposite the object-glass, by which the rays which enter the telescope will be turned at right angles through the perforated axis of the pivots of the instrument, thus utilizing the necessary length of this axis by making it an integral part of the telescope.

The observer will thus occupy one position, no matter to what part of the meridian his telescope is pointed, which is, in itself, a great advantage, on the score of convenience. This also will doubtless conduce to a constant personal equation, as it has been shown by the director of the Albany Observatory, and others, that personal equations vary with the altitude of the observed star.

These instruments are provided with fine spirit-levels and with micrometers, which fit them to be used as zenith-telescopes, and thus to determine two of the three important quœsitœ.

The parties of other nations will use similar methods for this purpose. The coordinate which is most difficult of exact determination is the longitude, and the problem of its determination will be attacked in various ways.

The English parties, true to the traditions of Greenwich, are to be provided with portable altitude and azimuth instruments with which to observe moon transits, both in the meridian and out of it. A long series of such moon-culminations was observed between Harvard Col-