380 THIRD REPORT — 1833. 



in the shade. The mean of the two variations in the shade 

 being subducted from the variation in the sun, the excess gives 

 the dilatation per minute due to the sun's rays, the quantity sub- 

 ducted being the effect of the other causes in action at the time. 

 Many years' experience and repeated trial of the instrument 

 under a great variety of circumstances have demonstrated the 

 complete dependence which may be placed on its indications, as 

 well as its extreme sensibility. It has been used by its inventor, 

 Sir John Herschel, on the Puy de Dome ; in an imperfect state 

 also on Etna by him, and since by Mr. Lunn, in its improved 

 form, on the same volcano ; and by M. Nicollet and Professor 

 Forbes on the Alps in Switzerland. In a series of experiments 

 on the absorptive powers of glasses on heat, and on the reflec- 

 tive powers of metallic and other polished surfaces, as well as 

 for the purpose of ascertaining the rate of increase and dimi- 

 nution of solar radiation throughout the day, from sunrise to 

 sunset, it has been found of great utility. In the latter inquiry, 

 the increase or diminution of altitude of the sun near the ho- 

 rizon is marked by the actinometer nearly as well as by actual 

 measurement with a sextant*. 



Sir John F. W. Herschel also communicated some results 

 obtained by Professor Forbes on the diminution of the intensity 

 of the solar rays in traversing the atmosphere by means of 

 the actinometer. He pointed out by direct comparison with 

 two instruments the degree of dependence to be placed on these 

 results, which, from experiments made by Professor Forbes at 

 the observatory of Paris, seem to indicate not more than —^ of 

 the whole effect of radiation as the probable error under fa- 

 vourable circumstances. But the point particularly deserving 

 of attention, and upon which observations strictly comparative 

 have now been made for the first time, is the numerical estimate 

 of the loss of intensity sufl^ered by the radiant heat of the sun 

 in passing through a definite thickness of the atmosphere. Out 

 of twenty series, each consisting of several (often many) indi- 

 vidual comparisons made by Professor Forbes, with the assist- 

 ance of M. Kamtz, of Halle, at the top and bottom of the 

 Faulhorn, an elevated mountain of the Bernese Oberland, and 

 under favourable circumstances, not one failed to indicate the 

 diminution of heat alluded to. The difference of height at the 

 different stations at which simultaneous observations were made, 

 varied from 5000 to near 7000 English feet, and these obser- 



* The actinometer, as used by Sir John Herschel, is made and sold by 

 Mr. Robinson, optician, Devonshire-street, Portland-place, London. 



