216 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 qucesitce, the latitude and the local time are most easily 

 determined. Portable transit-instruments will suffice for the first de- 

 termination, and for the second there are various adequate means. 



The American parties are each to be provided with a small port- 

 able transit-instrument and zenith-telescope combined, which instru- 

 ments 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 ad- 

 vantages. In accordance with a suggestion first proposed by Stein- 

 heil, 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 con- 

 duce to a constant personal equation, as it has been shown by the 

 director of the Albany Observatory, and others, that personal equa- 

 tions 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 qucesitce. 



The parties of other nations will use similar methods for this pur- 

 pose. 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- 



