1870.] 



Observations of the Sun. 



357 



spectrum, and at times the whole prominence spectrum was built up of 

 bright lines so arranged, indicating that the prominence itself was built up 

 of single discharges, shot out from the region near the limb with a velo- 

 city sometimes amounting to 100 miles a second. After this had gone on 

 for a time, the prominence mounted, and the cyclonic motion became 

 evident ; for away from the sun, as shown in my sketch, the separate 

 masses were travelling away from the eye ; then gradually a background 

 of less luminous hydrogen was formed, moving with various velocities, and 

 on this background the separate " bombs " appeared (I was working with 

 a vertical spectrum) like exquisitely jewelled ear-rings. 



It soon became evident that the region of the chromosphere just behind 

 that in which the prominence arose, was being driven back with a velocity 

 something like 20 miles a second, the back-rush being so local that with 

 the small image 1 am unfortunately compelled to use, both the moving and 

 rigid portions were included in the thickness of the slit. I saw the two 

 absorption-lines overlap. 



These observations were of great importance to me ; for the rapid action 

 enabled me to put together several phenomena I was perfectly familiar 

 with separately, and see their connected meaning. 



They may be summarized as follows, and it will be seen that they teach 

 us much concerning the nature of prominences. When the air is per- 

 fectly tranquil in the neighbourhood of a large spot, or, indeed, generally 

 in any part of the disk, we see absorption-lines running along the whole 

 length of the spectrum, crossing the Fraunhofer lines, and they vary in 

 depth of shade and breadth according as we have pore, corrugation, or 

 spot under the corresponding part of the slit, — a pore, in fact, is a spot. 

 Here and there, where the spectrum is brightest (where a bright point of 

 facula is under the slit), we suddenly see an interesting bright lozenge of 

 light. This I take to be due to bright hydrogen at a greater pressure 

 than ordinary, and this then is the reason of the intensely bright points 

 seen in ranges of facnlae observed near the limb. 



The appearance of this lozenge in the spectroscope, which indicates a 

 diminution of pressure round its central portion, is the signal for some, 

 and often all of the following phenomena : — 



1 . A thinning and strange variations in the visibility and thickness of 

 the hydrogen absorption-line under observation. 



2. The appearance of other lozenges in the same locality. 



3. The more or less decided formation of a bright prominence on the 

 disk. 



4. If near the limb, this prominence may extend beyond it, and its 

 motion-form will then become more easy of observation. In such cases 

 the motion is cyclonic in the majority of cases, and generally very rapid, 

 and — another feature of a solar storm — the photospheric vapours are torn 

 up with the intensely bright hydrogen, the number of bright lines visible 



2 e 2 



