46 



ASTRONOMICAL PHENOMENA AND PROGRESS. 



any determination of ingress or egress. Understand- 

 ing the uncertainty of such determinations, the Ger- 

 man astronomers have proposed to trust to measures 

 with a heliometer, made wnile the planet is crossing 

 the disk. The use of a sufficient number of heliome- 

 ters would be both difficult and expensive, and I 

 think we have an entirely satisfactory substitute in 

 photography. Indeed, Mr. De La Eue has proposed 

 to determine the moment of internal contact by pho- 

 tography. But the result would be subject to the 

 same uncertainty which affects optical observations 

 the photograph which first shows contact will not 

 be that taken when the thread of light between Ve- 

 nus and the sun's disk was first completed, but the 

 first taken after it became thick enough to affect the 

 plate, and this thickness is more variable and uncer- 

 tain than the thickness necessary to affect the eye. 

 "We know very well that a haziness of the sky which 

 very slightly diminishes the apparent brilliancy of 

 the sun will very materially cut off the actinic rays, 

 and the photographic plate has not the power of ad- 

 justment which trie eye has. 



But, although we cannot determine contacts by 

 photography, I conceive that we may thereby be able 

 to measure the distance of the centres of Venus and 

 the sun with great accuracy. Having a photograph 

 of the sun with Venus on its disk, we can, with a 

 suitable micrometer, fix the position of the centre of 

 each body with great precision. "We can then meas- 

 ure the distance of the centres in inches with corre- 

 sponding precision. All we then want is the value in 

 arc of an inch on the photograph-plate. This deter 

 mination is not without difficulty. It will not do to 

 trust the measured diameters of the images of the 

 sun, because they are affected by irradiation, just as 

 the optical image is. If the plates were nearly of the 

 same size, and the ratio of the diameters ot Venus 

 and the sun the same in both plates, it would be safe 

 to assume that they were equally affected by irra- 

 diation. But, should any difference show itself, it 

 would not be safe to assume that the light of the sun 

 encroached equally upon the dark ground of Venus 

 and upon the sky, because it is so much fainter near 

 the border. 



If the photographic telescope were furnished with 

 clock-work, it would be advisable to take several 

 photographs of the Pleiades, both before and after 

 the transit, to furnish an accurate standard of com- 

 parison free from the danger of systematic error. 

 There is little doubt that, if the telescopes and opera- 

 tors practise together, either before or afterthe transit, 

 data may be obtained for a satisfactory solution of the 

 problem in question. 



At the sitting of the Vienna Academy of 

 Sciences on March 10th, Dr. Neumayer submit- 

 ted a proposal for the preparatory arrange- 

 ments for the observation of the transit. A 

 map of the circumpolar regions shows that the 

 best points in the southern hemisphere for 

 these observations will be the region south of 

 the Indian Ocean, near the circumpolar district. 

 Dr. Oppolzer has established that the most 

 favorable localities for observing the immersion, 

 both as to parallax and altitude, can be con- 

 nected by a curve passing by the great Gulf 

 of Australia to the Macdonald Islands, and 

 from these to a point situated in 36 52' S. lat- 

 itude, 43 24' E. longitude. The points best 

 adapted for observation of the emersion will 

 also be found in a curve passing from the cen- 

 tre of the Indian Ocean to a point situated in 

 180 E. longitude and 79 S. latitude ; and 

 from there to another point, 64 55' S. latitude, 

 and 244 39' E. longitude. The point of inter- 

 section of these two curves (48 5' S. latitude, 



99 3' E. longitude) will evidently be the one 

 most favorable for the observation of the tran- 

 sit in its totality. In this case, the factor of 

 the parallax and of the altitude will be 0.6Y, 

 and 48. for the immersion ; and 0.47 and 

 62. 5 for the emersion. The nearest station 

 to this point will be the Macdonald Islands, 

 situated nearly in 53 S. latitude, and 12 E. 

 longitude (from Greenwich). M. Neumayer, 

 who visited these islands in 1857, was struck 

 with their relatively high temperature ; and 

 has ascertained, by a close examination of the 

 tables of temperature published by the author- 

 ity of the Dutch Government, that the current 

 of Agulha must terminate near them. The 

 summer and winter isotherms confirm these 

 facts, and there can be no doubt that it is un- 

 der the meridian of the islands of Macdonald 

 and Kerguelen that the most favorable region 

 must be sought for a route toward the South 

 Pole, in the same manner as Sir James Ross fol- 

 lowed, with the same object, a new current 

 which set out from the shores of New Zealand. 

 The map of the southern circumpolar regions, 

 published by Petermann, furnishes very precise 

 information for the equatorial limit of the float- 

 ing ice, the curve showing two points of 

 depression toward the pole; one under the 

 meridian of Kerguelen's land, the other under 

 that of New Zealand. It may, however, be 

 said that, because floating icebergs have once 

 or twice been found in a locality, these are not 

 sufficient definitely to fix the relations of the 

 floating ice, which depends especially on cur- 

 rents, and which secondary causes, such as 

 winds, can draw into regions ordinarily free 

 of ice. It is the frequency of the ice that must 

 settle the limits in such cases. At the points 

 which have been named, the limit of floating 

 ice bends back upon itself as high as 60 S. 

 latitude ; and this is an important fact for the 

 determination of the warm currents setting 

 from the north. The position of the limit of 

 maximum density of sea-water, and the pres- 

 ence of spermaceti whales, which, as is well 

 known, seek in preference warm waters, on 

 the coasts of Termination Land, permit the 

 supposition that the current in question con- 

 tinues toward the South Pole as far as that 

 land and Kemp Island. Admiral Sir John 

 Ross also saw spermaceti whales at the ap- 

 proach toward South Victoria; while Wilkes, 

 Dumont d'Urville, and Ross, only met with 

 few and isolated individuals in the intermedi- 

 ate seas. M. Neumayer thinks that it will be 

 advisable to dispatch a small reconnoitring 

 expedition without delay to these regions, and 

 to establish a scientific station on the Macdon- 

 ald Islands, the first object of which should be 

 to determine the absolute longitude, to serve 

 as a basis for Delisle's method. It would be 

 occupied during the months of November, De- 

 cember, January, and February, with a series 

 of meteorological observations, and with every 

 thing relating to physical geography. He pro- 

 poses that, for this purpose, the Academy 



