TRANSACTIONS OF THE SECTIONS. S79 



has proposed, it should shade away gradually into darkness on 

 this outside edge ; and he adds that the phaenomenon corresponds 

 exactly to this latter deduction. 



Explanation of the Principle and Construction of the Actino- 

 meter. By Sir John F. W. Herschel, F.R.S. 



This is an instrument, invented by Sir John F. W. Herschel, 

 for measuring at any instant the direct heating power of the 

 solar rays. 



In the ordinary methods of estimatiiig the sun's calorific 

 effect, the heating power of the sun is put in equilibrium with 

 the cooling influence of external causes, and the elevation of 

 temperature in the body (usually a thermometer,) thus heated, 

 so as to maintain the equilibrium, is noted and set down as a 

 measure of the sun's calorific effect. But the principle of such 

 a proceeding is obviously faulty : the temperature maintained 

 is just as properly a measure of the cooling as of the heating 

 influences, and, in fact, is a measure neither of one nor of the 

 other. This objection applies to all statical measurements of 

 dynamical effects, taking the word generally to mean cases 

 where power is transformed into its immediate effect, and ex- 

 hausted by such transformation. The actinometer is intended 

 to afford a dynamical measure of the solar radiation, i. e. one 

 in which the actual quantity of heat received per second or in 

 a given infinitely small time is the object of measurement, upon 

 a given surface directly exposed to the sun. The instrument 

 itself is nothing more than a very large cylindrical thermometer- 

 bulb with a scale greatly enlarged, so as to render the smallest 

 possible increase of temperature distinctly measurable. In 

 consequence, the scale is not divided into degrees, but into ar- 

 bitrary equal parts ; and as a very slight elevation of tempe- 

 rature suffices to carry the liquid up the whole column, the 

 bulb itself is furnished with a screw, by withdrawing which its 

 capacity maybe enlarged, and the power of reading off" restored. 

 The bulb is of colourless glass, and is filled with an intensely 

 blue liquid. Into the interior of this the calorific rays pene- 

 trate, and are absorbed at some sensible depth within the bulb, 

 so that the liquid is heated from within, and the whole of the 

 calorific rays (not stopped by the glass) go to dilate the con- 

 tents, and are extinguished in producing that effect. To make 

 an observation with the actinometer, the instrument must be 

 freely exposed in the shade for one minute, and the variation 

 of the reading noted ; then freely exposed to sunshine for the 

 same time, and the variation again noted ; and lastly, once more 



