S. Newcomb on observing the coming Transits of Venus. 81 
TaBLE—continued, 
Character of 
Observer. Place. a iime. of —— POWar, 1 fore 
hm 8. é 
Ventosa,* Madrid, 21-0 10 150 |B. D. (Inst.) 
Kaiser, Leiden, 23°8 7 ie A.C. b. 
Lindeman, Ee 24°7 4-2 85 
i Uckfield, 28°5 3g A. C. (2) 
ohl, Kahlenberg, 29°6 4 135 .|V¥. 6 
Lynn,t Greenwich, 311 170 j|A.C 
Penrose | imbledon, 52°2 2°2 70 |R R 
In the preparation of this table, the original intention was to 
exclude all ney ations unaccompanied with any statement of 
the sieeve: hee exception was afterward made in favor of 
the observations of Bonn and Pulkowa, so that in this respect 
there is a lack of homogeneousness in the table. The few obser- 
vations of “ apparent contact,” have also been added, in the be- 
i that they would not be devoid of interest. 
Dew cipal conclusion, to be drawn from the comparison 
here exhibited, is that there is no discoverable relation between 
the time of observation on the one hand, and the size of the tel- 
escope, the magnifying power, (so it exceed 50 or 60) or the 
character of the phenomenon on the other. We find the phenom- 
enon of the apparently instantaneous formation of the “ black 
drop” to range from 205 59m 58s (Le Verrier r) to 21 0m 210 
(Ventosa). The times of “ Vanishing Contact ” on which Wolf 
and André lay so much stress, range from 205 59™ 57s to 21h Qm 
29s. If wereject Pohl’s observation, the range will still be 178. 
I think if the observations of external contact were collected and 
compared in the same way, it would be found that their agree- 
ment was as good as in the case of internal contact. So far as 
we have data for judging, these differences would seem to be 
due to the accidental errors of observation. Their amount in 
are may be inferred from the fact that 15s of time co d 
ag a change of 1” in the relative position of Mercury and the 
se Saran ore that we shall fail if we rely mainly on 
observations ternal contact. Still, there are two measures 
by which de. STEEN of the determination of ingress and 
egress may be greatly increased. The first consists in having 
the observer occupy the entire time of partial ingress and egress 
in making 5 came eeiawe. of cusps with 
such cpa. as may be best adapted for the purpose. The 
second consists in bringing the observers at opposite stations to- 
gether, both before and after the transit, and causing them to 
make observations on artificial transits with the same instrn- 
* Contact seemed to be established simultaneously at several points. 
+ Doubtful. Limbs “certainly in contact by at least that time.” 
Am. oe Sc1.—Suconp Series, Vou. L, No, 148,—Juny, 1870. 
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