TOTAL ECLIPSES OF THE SUN. 241 



day before the event Avhether the sky will be clear or clouded. Under 

 these circumstances it is not surprising that many people do not travel 

 to the scenes of total eclipses. Expeditions to eclipses were practically 

 unknown until half a century ago. Before that time man received with 

 varied emotions those which I'rovidence sent him, but did not travel 

 far to seek them. 'Now, expeditions half way round the earth are com- 

 mon. This is not due entirely to the greater scientific zeal of thei 

 present day; probably few living astronomers would care to journey 

 to the antipodes for an eclipse, under the conditions of travel which 

 prevailed one or two centuries ago. 



About seventy total eclipses of the sun occur each century. The 

 average duration is, perhaps, three miniites, which amounts to about 

 three and a half hours per century. If some Wandering Jew, at the 

 beginning of the Christian era, had started to observe total eclipses of 

 the sun, and had visited every one possible since that time, he would 

 have had less than three whole days for observation. The time, indeed, 

 would have been much less, since many of these eclipses occurred on 

 the ocean, or at inaccessible regions of the earth, and clouds un- 

 doubtedly obscured the sky during half the time of totality. During 

 the last half century, since spectroscopic observations have been carried 

 on, the time during which an individual could have obtained favorable 

 observations has been little, if any, more than a single hour. Under 

 these circumstances the wonder is that it has been possible to accom- 

 plish so much. Many men, however, have worked at different stations 

 along the narrow but extended path of totality, and every device which 

 ingenuity could suggest has been utilized in order to obtain as much as 

 possible in the brief seconds of totality. ISTothing has contributed so 

 much to increase the amount and accuracy of the results as photog- 

 raphy. There is hardly a line of investigation which cannot "be done 

 more quickly and better by photographic than by visual methods. 

 Nevertheless it would be a mistake to abandon visual observations 

 altogether. 



It may hardly need to be stated that for the most part scientific 

 observations of total eclipses have for their object the promotion of 

 our knowledge about the sun. No one, who understands at all how 

 intimate is our dependence upon that great body, will question the 

 wisdom of such efforts. In order to understand why certain problems 

 can be better studied when the sun's face is covered by the moon, it 

 may be well to outline our knowledge on the subject. 



The sun, the center of our system, is an exceedingly hot, intensely 

 bright, highly condensed, gaseous body. Its distance is a little less 

 than 9.'\,000,000 miles. Its volume is more than a million times that 

 of the earth. Its specific gravity is somewhat greater than that of 

 water. A gaseous body, denser than water, is something very different 



VOL. LX. — 16. 



