8 



Tribune Extras lecture and Letter Scries. 



the sun looks a little larger and the planet a liitl<> 

 Ptuuller than tlieir true magnitudes. When the. light of 

 the sun is close to the plau-'t the space bet -veen Is ap- 

 parently filled by a lijrament known as the black dr >;>. 

 It is commonly supposed that tin- moment of disappear- 

 ance of the black drop is tho moment of contaci : but the 

 uncertainty thus occasioned is of M-rious moment, it is 

 not due inerelv to a had atmosphere; it is an ab.-oi nte 

 effect iiide]>LMident of this caiiM-. In oh>erving th ar i- 

 flcial Venus tin; same difficulty is more or le-s encoan- 



I red. Kven ina fairer atnnis|iliere than w,- hupe lor at 

 the time of the tran-i; we must e\[>ret this phenomenon. 



In moving toward the edge of the sun, before reaching 

 it, there is again a similar source of error. A eioinl 

 seems to pass through the thread of liirht between tho 

 plam-t and tho edge of tho sun; tlio hriirht lino crows 

 darker and darker, Rnd at last dis appears that being 

 the moment of true contact. No ordinary observer ean 



II x this time with accuracy. Another method 

 lias been suggested observation by pho- 

 tography ; hut the difficulty here is that tho ohotogr.ip h 

 is dependent upon the comparative actinic power of tho 

 thread of light, and every photographer Icnows that the 

 slightest haz ; will make a difference in the impivs - on 

 on the sen* m : plate. The light thrown on one n:a.le 

 may be live or even ten times greater than on ano'h sr 

 without auv corresponding difference in tlio fads. I 

 think we shall not attempt the photographing of the 

 interior contact. 



It was supposed that the movement of Venus into tha 

 BOD'S atmosphere oould be observed by means of tho 

 spectroscope with great accuracy; but a commit tea in 

 Germany, including Z iwlnor and Awera, came to tha 

 conclusion that this was after all one of the most un- 

 eertaln methods, and it was finally given up entirely. 

 It, has b'-en found, however, that the momeut when the 

 planet first in ikes a notch on the sun is a well defined 

 occasion, and th:- experiments of Striibner of St. 

 Petersburg agree with ours on the artificial Venus in this 

 particular. Another method of determining tbis problem 

 con-ists In observing the distances of tbe centers of the 

 two bodies. Tho Germans thought of doing this by 

 nn-ans of he I in m i- ter* ; but the use of tho necessary u um- 

 ber of these instruments is impracticable, as they are 

 cumbrous and expensive. I believe there is not a heli- 

 om (! in this country. 



Tho Ameiican method of photographing the transit 

 has been already published : this is Me method of Prof. 

 AVinloek, and experiments have shown that it is likely 

 to 1)0 eminently successful. The photograph is 

 taken bv means of a horizontal telescope into 

 which the image is deflected. It was at 

 first supp.,-d that tho measures could be taken 

 direct upon the photograph; but Ibis has elements ot 

 inaccuracy, e-.;, .-pt by tho method of phonography de- 

 \ led bv I'rof. Winlock. Tho other methods of photo- 

 graphy will not give a closer approxlmati >n, according 

 to M. l> linn-, than one minute of arc; but wo have 

 already much closer measurements of th solar parallax 

 than this would provide. Our present uncertainty does 

 not exceed thirn-i'iur hundrcdius of a second. Such 

 photogia|ihs v. .Mild In- of no value. Tho method of Prof. 



AVilllocK was (hell ilescri lied. 



M'\ MoNS AND ORGANIZATION. 



Let us now coiisiilcr the maUer of stations. Suppose 

 for insiaiie-- that \ve li.nl tunr ;-,t Ltlons, two northi'rn and 



two si. nth T;I. li't!:e-e were divided lu . t \vo classes of 



n'.'-i r\ .ii i A and 1: at the north and A and B at t he 



south, a.- A i.s not comiiara'ile, with I:, Ih.- lailurool' 

 eilbcrAat the uorth and li at tho south, or li at the 



no-th and A at the south, would ren-lcr all tho four ob 

 s'-ivations valueless. Therefore, all tho observations 

 will b > of the same character. Tho chief clement in BO- 

 lectiiiir stations has been their meteorolnirv, yhe 

 i| ae>tion at issue being their liability to 

 bad weal her at the time of the transit. About 

 two years ago circulars were sent to Am -riraii Consuls 

 in almost every part of the world where tin- transit is 

 visible, to ascertain the condition of tho weather at 

 thos^ points in November and D -comber, and every 

 otliT M'liive of .-imdar information was utilized. We 

 Had taoiig'it of selecting from a numb'-r of others Kurd's 

 I>lands in the Southern Indian Ocean as being one 

 among tli-- iicst stations. Northern stations with proba- 

 bilities i.f e;ood weal her are easily to be had. E-peciallv 

 favorabl' 1 in t.'iis view is Peking in China and Vladivo- 

 stok in Siberia. In Japan tho wcathir i- ^earccly as 

 favorable. Ilakodadi was very objectionable in respect 

 to weather; Yokohama was just as bad; Nagasaki was 

 rather bolter. 



Tin- only satisfactory station in the southern bemis- 

 p!ii re in i e- pee t to weather was found to be Hobart Town, 

 in Tasmania. New-Z -aland is nearly as iavorable. But 

 from all the other proposed Southern stations tho 

 accounts were very l:a I ; notably at the proposed Station 

 at Kurd's Islands the almost uniform report was " clouds, 

 r.iin, tempests, and snow :" the chances of observation 

 there di'i not exceed two-tenths; this station was then- 

 lore given up. The most favorable station left at tho 

 South was Kersueleu Island, though somewhat n-'iirhbor- 

 ing to Hum's Islands, and that, was selected. A party 

 will also be landed, it practicable, at Cmzei's Island. In 

 st-adof sending lour parties to each hemisphere, we 

 shall eend three to the north and live to the south, to 

 equalize the chances as to weather. We hope to get 

 complete results from two parties in each hemisphere. 



Tue following is a list of the chiefs of parties and their 

 stations: Northern stations, Wladivostock, Siberia, 

 Prof. A. Hall, U. S. N.; Nagasaki. Japan. Geo. Davidson, 

 U. S. Coast t>ui\vy ; Peking, China, Prof. James C. Wat- 

 son of Ann Arbor, M'.cb. Southern staiions, Crozet's 

 Island, South Indian Ocean, Capt. Kiymond, U.S.A.; 

 Kerguelcn Island, South Indian Ocean, L our.-Commau- 

 dcr George 1'. llyan, U. S. N.; lloliart Town, Tasmania, 

 Prof. Win. IlarUnos, U. S. N.; New Z -aland. Prof. C. H. 

 Peters of Clinton, N. Y.; Chatham Island. Somh Paci.ie, 

 Ivlwin Miiitli, U. S. Coast Survey. The coustitution of 

 each pirty Is such that in case of disability on the part 

 of its chief, the second officer can tak 1 his place. Kadi 

 parly will have three photographers a chu-f photo- 

 grapher, who must have been of long experience in tho 

 business; an assistant that has had practice, 

 and a second as.Mstaiit trained only for the o.-c.ision. 

 Nearly all the second as-istants' positions have been 

 l! ed by >tmlcnts or graduates of vaii us .schools and 

 technological colic ires throughout the country. Tne par- 

 lies lor t ho southern station \\ill sail, \ve expect, about 

 June 1. Tlies are all ready ; the photographers are to 

 ne in mil practice, hen- next week. Tne northern parties 

 \\ ill go later and not all together. The Navy l).-p.irl mem 

 h i- fmm.Vii .1 a ship, the Swatara, to go :o the southern 

 .stations. The longitudes of the stations will bo deter- 

 inii.ed by occuliat ions wherever t.de;;i aph communica- 

 tion is impra.-tieablo; bat already tin-re is such com- 

 munication between Vladivostok and Ilobart Town. 

 Arrange me 1 1 Is are made with tin- ( !' vei iiiin-nis for cx- 

 ohanglug longitude signals, and tne pro-peci of the ex- 

 ten.-.i .ii of cables to New Z -aland and other points gives 

 fair hope tha^ tin-re will bo only a few points where the 

 method of (.'ccultutioua will be tho solo resort. 



