might proceed direct to the region indicated, each conveying two or three well-provided observing-parties, and combining reconnaissance with the occupation of stations as they were successively selected. That it perfectly in the power of this country and America to ensure the requisite number of Southern observations of the coming transit, I am satisfied. There is, it is true, no time for delay. Energy and skill will be wanted; but they have never been looked for in vain in such circumstances. The expeditions which would have to be made would be no pleasure-parties, nor would they be free from difficulties and dangers sufficient to tax the courage even of British and American seamen. But these very considerations encourage the students of science in both countries to believe that the required effort will be made. That it should be made, if failure is to be averted, does not seem to me to be open to the slightest question. From the Monthly Notices of the Astronomical Society for May 1873. ON A STEREOGRAPHIC CHART OF THE TRANSIT OF 1874. ACTION OF THE GREENWICH BOARD OF VISITORS. At the Visitation of the Greenwich Observatory on June 7, 1873, it was proposed by Professor Adams, and carried unanimously, that Government should be applied to for the means of organising parties of observers in the Southern Ocean, with the view of finding additional localities in the sub-Antarctic regions for applying Halley's method to the observation of the transit of 1874. PLATE XX. is intended to illustrate a relation to which Professor Adams called attention at the last meeting of this Society. If we disregard the rotation of the Earth during ingress or egress, and also neglect the curvature of Venus's shadow-cone where it crosses the Earth, it is manifest that the points where the shadow touches the Earth first and last, at ingress or at egress, are respectively antipodal, and may be regarded as the poles of a series of circles of equal acceleration and retardation, of equal value therefore for applying Delisle's method. Moreover, these circles manifestly indicate a value proportional directly to their distance from the plane of the great circle having the before-mentioned points as poles. The intersections of these circles indicate points of a particular value for Halley's method, the excess or defect of duration being (i) the sum of the corresponding accelerations or retardations where each of two intersecting circles indicates a time-difference of the same kind, or (ii) the excess of acceleration over retardation, or of retardation over acceleration where the time-differences are of different kinds. It is readily seen that if points of equal value for Halley's method are connected, the connecting curves are a series of circles, having as poles the points midway between the poles of maximum retardation and those of maximum acceleration. Moreover, these circles, like those of equal value for Delisle's method, indicate values directly proportional to their distance from the plane of the great circle having these mid-points as poles. In my chart the several curves corresponding to these circles have been drawn; but all the corrections depending on the Earth's rotation during ingress and egress, and on the curvature of the shadowcone, have been carefully taken into account. The dotted red curves are those indicating the loci of points of equal value for Delisle's method, and the red curves indicate the loci of points of equal value for Halley's method. The actual accelerations or retardations in minutes, and the differences of duration, have been indicated in red letters. The interpretation of the chart, and the regions indicated as suitable for the various methods proposed to be employed, will be manifest even on a very slight inspection of the chart. As a rule, I prefer to have only one printing in a chart of this sort, where every line has been laid down with scrupulous accuracy, because the coloured lines may not be printed quite correctly. But in the present case, the map would have been overcrowded by black lines, unless red had been used. To learn the amount of error in registering,' it is only necessary to compare the red and black impressions of the small cross lines indicating the points of maximum acceleration and retardation. From the Monthly Notices of the Astronomical Society for May 1873. |