1889.] XEW YORK ACADEMY OF SCIENCES. 53 



tie over 100 miles. Under the most favorable conditions possible, 

 the cross-section of the central shadow in any total eclipse can 

 have a maximum diameter of 167 miles, and totality may last 

 7m. 58s. In latitude 40°, under the same circumstances, total- 

 ity can barely equal 6^m.' 



The central shadow, however, does not move uniformly over 

 the face of the waters, and it may be interesting to consider how 

 variable its motion is. The variability is shown, sufficiently for 

 my purpose, by measurements from a map, as follows: — The 

 central shadow on December 22d moves from where it first strikes 

 the earth to the island of Trinidad, a distance of about 820 

 miles, in 2m. From that island to the equator it moves in 20m,, 

 a distance of about 1,495 miles; thence to the lowest point on 

 the earth, a distance of 2,760 miles, in Ih. and 40m. Then 

 it sweeps to the African coast in about 50m., a distance of 1,380 

 miles, and leaves the earth 31m. later, traversing in this last half- 

 hour a distance of 2,800 miles. 



"Summing up what we have learned about the corona dur- 

 ing some 45m. of scrutiny in as many years, we may state, to 

 begin with, that it is not a solar atmosphere. It does not gravi- 

 tate upon the sun^s surface and share his rotation, as our air 

 gravitates upon and shares the rotation of the earth; and this 

 for the simple reason that there is no visible growth or pressure 

 downward (such as the spectroscope would infallibly give notice 

 of) in its gaseous constituents; whereas under the sole influence 

 of the sun's attractive power their density should be multiplied 

 many million times in the descent through a mere fraction of 

 their actual depth. 



"The corona is properly described as a solar appendage, and 

 may be conjecturally defined as matter in a perpetual state of 

 efflux from, and influx to, our great luminary, under the stress of 

 electrical repulsion in one direction and of gravity in the other. 

 Its constitution is of a composite character. It is partly made 

 up of self-luminous gases, chiefly hydrogen, and the unknown 

 substance giving the green ray 1474; partly of white-hot solid 

 or liquid particles, shining with continuous light, both reflected 

 and original. There is a strong probability that it is affected 

 by the periodic ebb and flow of solar activity, the rays emitted 

 by the gases contained in it fading, and the continuous spec- 

 trum brightening, at times of minimum sun-spots, as if by a 

 fall of temperature producing, on the one hand, a decline in 

 luminosity of the incandescent materials existing near the sun, 

 and, on the other, a condensation of vapors previously in- 

 visible into compact particles of sun-reflective capacity. The 



' "X Dung's " General Astronomy," p. 238. 



