18 Br. E. Franhland [Feb. 10, 



zinc-white being nearly equal to it. The most trustworthy shade 

 thermometer, therefore, is one having its bulb covered with a thin 

 layer of one of these materials ; or the naked bulb may be shaded by 

 a small arch of white paper. 



The term " sun temperature," as commonly emj)loyed, has a very 

 vague meaning. If a body could be j)laced in sunlight under such 

 circumstances as to absorb heat rays and emit none, its temperature 

 would soon rise to that of the sun itself. But, as all good absorbers of 

 heat are also good radiators, the elevation of temperature caused by the 

 exposure of even good absorbers to sunlight is comparatively small. 

 Thus an isolated thermometer, with blackened glass bulb, placed in 

 sunshine, will rarely rise more than 10° C. above the temperature 

 which it marks when screened from direct sunlight. Under these 

 circumstances, however, the thermometer loses heat not merely by 

 radiation, but also by actual contact with the surrounding cold air. If 

 the latter source of loss be obviated, a much higher sun temperature is 

 obtained. Thus, the blackened bulb enclosed in a vacuous clear glass 

 globe will sometimes, when placed in sunlight, rise as much as 60° C. 

 above the shade temperature, and a still higher degree of heat may be 

 obtained by exposing to the sun's rays the naked blackened bulb of 

 a thermometer enclosed in a wooden box padded with black cloth, 

 and closed by a lid of clear plate glass. Thus he obtained with such 

 a box, on the 22ud of December, in Switzerland, when the air was 

 considerably below the freezing point, a temperature of 105° C, and 

 a still higher temperature could doubtless be obtained by surrounding 

 the thermometer with a vacuous globe before enclosing it in the 

 padded box. These widely different temperatures, produced under 

 different conditions by the solar rays, show that such observations 

 can be comparative only when the thermometer employed to measure 

 them is always surrounded by the same conditions. All the sun 

 temperatures here mentioned were measured when the "blackened 

 bulb in vacuo " was laid horizontally upon a sheet of white paper 

 with its stem at right angles to the direction of the sun's rays. 



" Solar intensity " is relative only, and means the number of 

 degrees through which the sun raises the temperature of a blackened 

 bulb in vacuo over the shade temperature. Hence the two tempera- 

 tures must be observed simultaneously, which is a laborious operation 

 when continued half-hourly throughout the day. By the use of a 

 peculiar self-registering differential thermometer, however, which he 

 had recently described to the Royal Society,* the maximum solar 

 intensity during the day is recorded by one reading only. The solar 

 intensities commented upon in this discourse were ascertained by 

 subtracting, in each case, the shade temperature from the sun tem- 

 perature taken synchronously. The precautions necessary are 

 described in the paper to the Eoyal Society just quoted. 



The chief things affecting climate are the following : — (1) The 



'Proceedings of the Royal Society,' 1882, p. 331. 



