Solar Radiation 387 



and provision must be made for the exact measurement of 

 the thermal effect produced by them in a given time. The 

 thermal effect produced is measured by a mass of some 

 substance and either by the change of temperature produced 

 in it or by the change of its state of aggregation. Actino- 

 meters, such as those of Herschel, Pouillet, Violle, Crova, are 

 instruments of the first kind. The ice calorimeter used by 

 Exner and Rontgen and the steam calorimeter of the writer 

 are instruments of the second kind. The thermal mass of 

 the substance affected is conveniently expressed in terms 

 of the thermally equivalent weight of water, which is called 

 its water value. In the actinometer the change of tem- 

 perature is either measured by a separate thermometer or 

 the actinometer is itself a thermometer the calorimetric 

 constants of which have been ascertained. In instruments 

 of the second class no thermometer is required ; the thermal 

 effect is measured by the mass of water-substance which 

 changes its state in a given time either from ice to water 

 or from water to steam, both being at the same tempera- 

 ture. In the ice calorimeter the quantity of liquefaction 'is 

 measured by the change of volume, as in Bunsen's calori- 

 meter ; in the steam calorimeter the generation of steam is 

 measured by the weight or volume of the distilled water 

 produced. The steam calorimeter was described recently in 

 Nature (see above, p. 337), and it is unnecessary to repeat it 

 here. It acted quite satisfactorily in the writer's hands in 

 Egypt in May 1882, and it has since been giving good 

 results in the hands of Mr Michie Smith at the observatory 

 of Kodaikanal in South India, at an elevation of about 

 7000 feet above the sea. Theoretically, the ice calorimeter 

 is as good as the steam calorimeter, but in applying it to 

 the measurement of the sun's radiant heat it has a practical 

 defect. At the moment before exposure, the ice in the calori- 

 meter is frozen to the inner surface of the metal plate, the 

 outer surface of which receives the sun's rays. The first 

 effect of exposure to the sun is that the ice is detached from 

 the plate. The intervening water introduces perturbations 

 which are not easily allowed for. 



252 



