338 



Dr. E. Frankland. On Measuring the [Feb. 2,. 



tamed that the indicated solar intensity is not augmented when the 

 area of white surface is increased fourfold. 



There is not much use in having self-registering shade and sun 

 thermometers, because the highest temperature of the blackened bulb 

 does not necessarily occur at the time of maximum shade tempera- 

 ture ; and, co nsequently, the maximum solar intensity during any 

 period cannot be found by merely deducting the maximum shade 

 from the maximum sun temperature. Correct observations of maxi- 

 mum solar intensity are, therefore, very laborious with these instru- 

 ments ; and are, I believe, never made in the routine work of a 

 meteorological station. As they afford, however, very interesting 

 data, I have endeavoured to simplify them by contriving an instru- 

 ment which allows them to be recorded for each day with one 

 reading only. 



A Differential Self -Registering Temperature, for measuring relative solar 



intensity. 



This instrument, as seen from the accompanying figure, has consider- 

 able similarity to a Leslie's differential air thermometer ; but in the 

 new instrument the differential changes in the elasticity of the air of 

 the two bulbs are measured by their action in elevating a column of 

 mercury. 



C 



