want stations where the acceleration is greatest and where the retardation is greatest. The first lie at the point where 'accelerated ingress' is written. We must not choose a station at this point, because there the Sun would be on the horizon, and therefore distorted; but taking the line representing the places where ingress is beginning a minute or so later we see that it passes near Woahoo and Hawaii. These, then, are good stations for observing this phase. Three or four minutes later the line passes Jeddo, Bonin, Marquesas, Otaheite, and so on; and these, therefore, though not such good stations as Hawaii or Woahoo, are still excellent. We may note, too, that at these stations the Sun will have a greater elevation, the actual elevation at different stations being indicated by the concentric circles marked with degrees. As respects retarded ingress, we see that the best station is Crozet Island, close by the point marked retarded ingress. But Kerguelen's Island, Macdonald Island, Amsterdam Island, as also Rodriguez, Mauritius, and Bourbon, are all good stations for observing this phase. Now let us turn to Plate X. to determine what stations are best for observing accelerated and retarded egress. It will be seen that the place marked 'accelerated egress' falls inconveniently near to the south pole. Only when we reach the cross-lines marked 11m. and 10m. do we come on places where stations could be conveniently taken. The lines marked 9m. and 8m. bring us past several excellent stations in New Zealand; and then we come to stations in South Australia; and on the other side of the arrow-line we find Kemp Island, and (inferior, but still serviceable) the Macdonald Islands, Kerguelen, and Crozet. Lastly, as respects retarded egress, we find an abundance of excellent stations, the best being in Siberia and Eastern European Russia; but there are several excellent stations in India; while Alexandria will supply a very suitable place of observation. *The very best station in English territory, namely Peshawur-far superior to Alexandria both as respects the amount of retardation and Tables at the close of this Appendix exhibit the actual amount of acceleration and retardation at the several stations indicated in these maps, and at some others not named in either plate. Now, as respects Halley's method, it will be remembered (see page 33) that we have to consider that the whole transit (or at least the beginning and end)* should be visible. We want stations (these will obviously be northern ones) where the transit will last as long a time as possible, and other stations where the transit will last as short a time as possible. We should therefore naturally look for northern stations where the transit begins as early as possible and ends as late as possible, and vice versâ as respects southern stations. But when we consider Hawaii and Woahoo, where the transit begins at the earliest, we find, on turning to Plate X., that these stations will not suit our purpose; for in Plate X. they are not visible; in other words, before the end of transit Hawaii and Woahoo pass to the un-illuminated side of the Earth, the Sun sets, in fact, and the end of the phenomenon cannot be seen. In like manner, if we take those stations shown in Plate X. which are most suitable for observing the retarded egress, we find that at the epoch represented in Plate X. they are not visible; in other words, they are on the un-illuminated side of the Earth, or the Sun has not yet risen at these stations when transit begins. It needs but a brief study of the two Plates to see that the stations which give the longest duration are those in Man solar elevation—had wholly escaped notice until my construction of Plate X. (reduced from the sixth plate of the original series drawn by me for the Royal Astronomical Society, and published in vol. xxix. of their Proceedings) exhibited the advantages of this station. * This parenthetical remark may seem strange at first sight; but it must be remembered that there are southern stations (though I do not say any in this case are available) where the beginning of the transit can be seen before sunset and the end after sunrise. It is only necessary to study Plates IX. and X. thoughtfully in order to see that this is the case. |