142 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1919. 
We had provided ourselves with two brands of plates, but it had 
become apparent from photographs taken and developed before the 
eclipse that one of these brands was unsuitable in the hot climate, 
and it was decided to use practically only one brand of plates. 
In taking the experimental photographs it was noticed that the 
clocks and ccelostats were very sensitive to wind. We had reason 
to fear strong gusts about the time of totality, such as had occurred 
in other eclipses; and as the conditions of our locality seemed to 
render them specially probable, protective wind screens were erected 
round the hut openings at every point where it was possible without 
interfering with the field of view. Happily dead calm prevailed at 
the critical time. Screens also protected all projecting parts of the 
telescope tubes from direct sunlight. 
The performance of the 16-inch ccelostat was unsatisfactory in 
respect of driving. There was a clearly marked oscillation of the 
images on the screen in a period of about 30 seconds. For this reason 
exposure time was shortened, so as to multiply the number of ex- 
posures in the hope that some would be near the stationary points. 
13. The morning of the eclipse day was rather more cloudy than the 
average, and the proportion of cloud w^as estimated at j% at the 
time of first contact, when the sun was invisible ; it appeared a few 
seconds later, showing a very small encroachment of the moon, and 
there were various short intervals of sunshine during the partial 
phase which enabled us to place the sun's image at its assigned posi- 
tion on the ground glass, and to give a final adjustment to the rates 
of the driving clocks. As totality approached, the proportion of 
cloud diminished, and a large clear space reached the sun about one 
minute before second contact. Warnings were given 58 seconds, 22 
seconds, and 12 seconds before second contact by observing the length 
of the disappearing crescent on the ground glass. When the crescent 
disappeared the word " go " was called and a metronome was started 
by Doctor Leocadio, who called out every tenth beat during totality, 
and the exposure times were recorded in terms of these beats. It 
beat 320 times in 310 seconds ; allowance has been made for this rate 
in the recorded times. The program arranged was carried out suc- 
cessfully, 19 plates being exposed in the astrographic telescope with 
alternate exposures of 5 and 10 seconds, and eight in the 4-inch 
camera, with a uniform exposure of 28 seconds. The region round 
the sun was free from cloud, except for an interval of about a minute 
near the middle of totality, when it was veiled by thin cloud, which 
prevented the photography of stars, though the inner corona re- 
mained visible to the eye and the plates exposed at this time show it 
and the large prominence excellently defined. The plates remained 
in their holders until development, which was carried out in con- 
