Prof. Draper's Description of the Tithonometer. 221 



stand. It will be easily understood that the great increase of 

 light arises from the circumstance that the flame is drawn vio- 

 lently through the aperture in the plate by the current established 

 in the cylinder. 



As much radiant heat is emitted by this flame, in order to 

 diminish its action, and also to increase the tithonic effect, I adopt 

 the following arrangement. Let A B, fig. 4, be the lamp ; the 

 rays emitted by it are received on a convex lens D, four inches 

 and three quarters in diameter, that which I use being the large 

 lens of a lucernal microscope. This, placed at a distance of twen- 

 ty one inches from the lamp, gives an image of the flame at a dis- 

 tance of thirteen inches, which is received on the sentient tube 

 of the tithonometer F ■ between the tithonometer and the lens 

 there is a screen E. 



Things being thus arranged, and the lamp lighted so as to give 

 a flame about three inches and a half long, we may proceed 

 with the experiments. It is convenient always to work with the 

 flame at a constant height, which may be determined by a mark 

 on the glass cylinder. At a given instant, by a seconds watch, 

 the screen E is removed, and immediately the tithonometer be- 

 gins to descend. When the first minute is elapsed the position 

 on the scale is read off and registered ; at the close of the second 

 minute the same is done, and so on with the third, &c. And 

 now, if those numbers be compared, casting aside the first, they 

 will be found equal to one another, as the following table of ex- 

 periments, made at different times and with different instruments, 

 shows : — 



Table I. — Showing that when the radiant source is constant, the amount 

 of movement in the tithonometer is directly proportional to the times 

 of exposure. 



