On the Intensity of Light during the Recent Solar Eclipse. 305 



Comparison ivith M. Regnault's experiments on expansion at 

 constant pressure between 0° and 100 C. 



Comparison with M. Regnault's experiments on compressibility 

 at constant temperature. 

 Let Pj = initial pressure, V, = initial volume ; 

 Let P2= final pressure, ¥3= final volume. 



B' 



XXXIX. On the Intensity of Light during the recent Solar Eclipse. 

 By J. P. Joule, LL.D., F.R.S. <^r. 



To the Editors of the PhilosojMcal Magazine and Journal. 

 Gentlemen, 



I EING desirous to obtain an image of the annulus in the late 

 solar eclipse, I took a camera to the Werrington Junction 

 on the Great Northern Railway. A few minutes before the cen- 

 tral eclipse, however, it became evident that the sky would remain 

 obscured with cloud, I therefore employed the camera simply to 

 obtain, if possible, a measure of the intensity of the light. The 

 country about the Werrington Junction is an extensive plain. I 

 placed the camera on the ground, directing it to the south-east 

 horizon. The sensitive plate was exposed during the five minutes 

 which preceded the central effect, and the image was developed 

 immediately afterwards. The next day, the weather appearing 

 almost exactly similar, possibly of the two a little more cloudy, 

 I exposed near ^Manchester plates prepared with the same collo- 

 dion and nitrate of silver, and developed them with the same 

 solution of sulphate of iron ; the direction in which the camera 

 was placed, the time of the day and every other circumstance 

 being as nearly as possible the same as before. In this latter case, 

 a picture, judged by Mr. Dancer, a gentleman of great experience 



Plul. Mug. S. 4. Vol. 15. No. 100. April 1858. X 



