62 Things not generally Known. 



equal to 520 wax-candles, will render a small surface as bright 

 as when it is illuminated by mean sunshine. 



It is thence inferred, that a stratum occupying the entire 

 surface of the sphere of which the earth's distance from the 

 sun is the radius, and consisting of three layers of flame, each 

 10 ' 00 th of an inch in thickness, each possessing a brightness 

 equal to that of such an electric light, and all three embraced 

 within a thickness of -^th of an inch, would give an amount 

 of illumination equal in quantity and intensity to that of the 

 sun at the distance of 95 millions of miles from his centre. 



And were such a stratum transferred to the surface of the 

 sun, where it would occupy 46,275 times less area, its thick- 

 ness would be increased to 94 feet, and it would embrace 

 138,825 layers of flame, equal in brightness to the electric light ; 

 but the same effect might be produced by a stratum about 

 nine miles in thickness, embracing 72 millions of layers, each 

 having only a brightness equal to that of a wax-candle.* 



ACTINIC POWER OF THE SUN. 



Mr. J. J. Waterston, in 1857, made at Bombay some ex- 

 periments on the photographic power of the sun's direct light, 

 to obtain data in an inquiry as to the possibility of measuring 

 the diameter of the sun to a very minute fraction of a second, 

 by combining photography with the principle of the electric 

 telegraph ; the first to measure the element space, the latter 

 the element time. The result is that about 20 ^ 00 th of a se- 

 cond is sufficient exposure to the direct light of the sun to 

 obtain a distinct mark on a sensitive collodion plate, when 

 developed by the usual processes ; and the duration of the 

 sun's full action on any one point is about ^Wth of a second. 



M. Schatt, a young painter of Berlin, after 1500 experi- 

 ments, succeeded in establishing a scale of all the shades of 

 black which the action of the sun produces on photographic 

 paper ; so that by comparing the shade obtained at any given 

 moment on a certain paper with that indicated on the scale, 

 the exact force of the sun's light may be determined. 



HEATING POWER OF THE SUN. 



All moving power has its origin in the rays of the sun. 

 While Stephenson's iron tubular railway-bridge over the Menai 

 Straits, 400 feet long, bends but half an inch under the hea- 

 viest pressure of a train, it will bend up an inch and a half 

 from its usual horizontal line when the sun shines on it for 



* See M. Arago's researches upon this interesting subject, in Things not gener- 

 ally Known, p. 4. 



