Heat of the Corona.
Mr. Abbot, with aid of Mr. Mendenhall, appears to have measured the heat of the corona, and in spite of previous efforts, this is probably the first time that it has been really shown to exist. For five minutes before second contact, the bolometer was successfully exposed to the region of the sky close to the narrowing crescent of the sun where the corona was shortly to appear. A diaphragm was interposed in the beam having an aperture of only 0.4 sq. cm. Deflections, rapidly diminishing from 80 to 6 mm. were obtained, the last being about 40 seconds before totality. Then the diaphragm was opened to 290 sq. cm. and a negative deflection of 13 mm. was observed after totality, where these positive deflections had just been found, showing that the corona was actually cooler than the background which had been used at the room temperature. Next the black surface of the moon was allowed to radiate upon the bolometer, and the still larger negative deflection of 18 mm. was observed.
The important result was that the corona gave a positive indication of heat as compared with the moon.
This heat, though certain, was, however, too slight to be sub-divided by the dispersion of the prism with the means at hand.
The negatives taken to depict the outer corona show from three to four solar diameters extension for the longest streamers. The equatorial ‘wings,’ as they recede from the sun, are finally lost in an illuminated sky, without any indication of having actually come to an end.
No attempt to carefully examine the plates taken for intra-mercurial planets has yet been possible. It is, however, as has been remarked, doubtful if the very faintest objects will be found, in consideration of the considerable sky illumination during totality. However, Pleione in the Pleiades (a star of the 6.3 magnitude), is plainly seen on one of the plates, and some smaller ones are discernible.
On the whole, the expedition may be considered as promising to be very satisfactory in its results, and that it was so is largely owing not only to the efficient care of Mr. Abbot, but to the many gentlemen who have assisted me with the loan of valuable apparatus, with counsel, with voluntary service and with painstaking observation, to one and all of whom I desire to express my obligations.