TOTAL SOLAE ECLIPSE OF JULY 18, 1860. 389 
The beginning and end of the eclipse do not agree with the calculations; for example — 
, _ . , 
First contact. 
Last contact. 
Duration. 
h min. sec. 
h 
min. sec. 
h 
min. 
sec. 
De La Rue . 
1 48 3-4 
4 
10 3-6 
2 
22 
0-2 
Carrington . . . 
I 47 56 
4 
10 15-2 
2 
22 
19-2 
Farley .... 
1 47 57 
4 
10 15 
2 
22 
18 
But it is possible that the discrepancy may partly arise in consequence of the assump- 
tion of a greater angular measure for the diameters of the sun and moon than they 
in reality subtend ; and this \iew ’ is ' supported by my measures of the distances of 
the sun and moon’s centres, which", as a whole, come out greater than the computed 
distances. It will be seen that, as the reduction of my arbitrary measures to their 
equivalents in arc is dependent on the tabular value for the sun’s semidiameter, if this 
be in excess of the truth, my distances of the centres must come out greater than the 
real values ; and this is actually the case, as will be hereafter seen. If the diameter 
either of the sun or of the moon, or of both, be less than the tabular numbers, the first 
contact must happen later, and the last contact sooner than the computed times : the 
epochs of these phenomena, derived from the measure of the peripheral distances of the 
sun and moon given above, tend to show that some correction is necessary to the tabular 
diameters. 
The epoch of the middle of the eclipse, the direction of motion of the moon’s centre, 
and the nearest approach of the centres of the sun and moon may also be derived from 
the photographs, by means of the distances of the centres and the epochs of two photo- 
graphs, one before and one after totality. 
In the above diagram, let a represent the position of the sun’s centre, h the position 
of the moon’s centre previous to totality, at the epoch of No» 22 photograph, and c 
the position of the moon’s centre in photograph No. 28, after totality; he will repre- 
sent the motion of the moon’s centre across the solar disk during the interval, and the 
line f3 a, parallel to he and passing through the sun’s centre, the direction of motion 
referred to the sun’s centre ; a X shows the nearest approach of the centres of the sun 
MDCCCLXII. 3 G 
