55 



phase depend upon the relative size of the sun and moon. 

 The duration of totality is simply the time occupied by 

 the moon in passing over the excess of its own apparent 

 diameter over that of the sun. As the sun and moon 

 have nearly the same apparent diameter, the total phase 

 can never occupy but a few minutes ; in the most favor- 

 able case, when the sun is farthest away and the moon 

 nearest to the earth, the duration of totality is less than 

 eight minutes. The greatest width of the shadow is 160 

 miles. 



The establishing' of a temporary observatory for observ- 

 ing an eclipse is a work of much labor. Of course, if a 

 person wishes simply to examine the general appearance 

 of the spectacle with a portable telescope, it will not take 

 him long to get ready. But if an extensive series of 

 observations has been planned, the preparations will be 

 of corresponding extent. The instruments will include 

 telescopes of various kinds, some of which must be 

 mounted with great firmness ; others, such as meridian 

 instruments, in a particular position to be determined by 

 observations of the stars. If spectroscopes are to be 

 used, they must be properly adjusted. If photographic 

 work is attempted, the preparations become still more 

 complicated, for to secure the best results the instrument 

 must move by clock work to correspond with the motion 

 of the sun. It requires, too, a great deal of ingenuity 

 to utilize the rough means at the disposal of a scientific 

 party in the field. The man who can make boards, spikes 

 and dirt answer the purpose of stone and mortar, or who 

 can fit instruments together without either tools or mate- 

 rials for his work, is a valuable member of the party. 

 The best equipped party near Denver, that of Prof. 

 Young of Princeton, made an astronomical camp for 

 their observations, and their instruments shared the rough 



