SUGGESTIONS TO ASTRONOMERS. 365 
by hand, it must be done by turning a winch in accordance with 
the beats of a chronometer or the vibrations of a pendulum. 
11. The observers at each station should be prepared with accu- 
rate computations of the local times of beginning and ending of 
the general eclipse, and of beginning and ending of total darkness, 
with particular attention to the accuracy of computation of the 
duration of total darkness. It will be remarked that the compu- 
tation of duration admits of great exactness for places near the 
central line of shadow, but that it is liable to considerable errors 
for places near the north or south boundary. They should also 
have accurate computations of the position, with respect to the 
vertical, of the points of the sun’s limb at which the general eclipse 
begins and ends, and of the points of the moon’s limb at which the 
sun disappears and reappears: the latter will be liable to sensible 
error. 
Every observer should be furnished with one or more cards, upon 
each of which a circle is described: upon one of these the poimts 
of beginning and ending of the general eclipse and of the totality 
are to be marked; the others are to be used for hasty records of 
the places of any remarkable phenomena during the eclipse. 
12. The observations to be made, and the mode of proceeding, 
should be arranged some days before the eclipse, and should be 
fully described in written instructions, with which each observer 
should be so perfectly acquainted as to have little need to refer to 
them at the time. Two cautions, however, must be borne in mind. 
The phznomena about the time of total obscuration are so striking 
that the most perfect discipline will then probably fail, and it will 
be almost useless to prescribe any observations which will prevent 
the observers from looking about them for a few moments to see 
the wonderful spectacle. And the whole time is so short, that. it 
will be very desirable that each observer’s attention be confined to 
very few phenomena. No party, probably, will be able. to make 
all the observations mentioned below; it will be desirable, there- 
fore, carefully to select those which can be made with the greatest - 
probability of success, and to give the utmost attention to those 
only. 
13. A quarter of an hour before the first contact of the sun and 
moon, the observations of radiation with the actinometer, &c. should 
be begun. [These should be continued through all stages of the 
eclipse.] The commencement of an eclipse is a very indistinct phe:- 
- nomenon, and perhaps for the principal part of the time before the 
total obscuration little can be done except to make, from time to 
time, observations of radiation and meteorological observations. 
“But when the limb of the moon crosses that of the sun at right 
angles, and afterwards, the observers will be well able to estimate 
(as far as can be done by the eye) the difference in the intensity of 
light on different parts of the sun’s disc. From this time also it 
will be proper to examine whether the entire circumference of the 
