of the Total Eclipse, 159 



If the plate or paper be so sensitive to photogenic action 

 that an image can be formed in a fraction of a second of 

 time, no equatorial movement will be required. If an 

 image can be formed in one or two seconds, a rude equato- 

 rial motion, such as may be given to a temporary stand, will 

 probably suffice. If this motion is given 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 

 accurate 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 computation 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 compu- 

 tations 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 points 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 pro- 

 ceeding, 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 phenomena about the 

 time of total obscuration are so striking that the most per- 

 fect 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. 



