336 



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



Bellagio, Italy. ' 

 September 17th. Clear sky except near horizon. Air very moist 



and calm. 



Time. Position of thermometer. Temperature. 



10.30 a.m. . On black caoutchouc laid upon grass 60"0° C. 



10.45 . . On black merino 59"0 



11.15 „ . . On white paper 66'3 



11.30 „ .. On white linen 66'0 



In some observatories, the blackened bulb in vacuo is laid upon grass ; 

 but the experiments at Tosten Vierod show that its indications vary, 

 under the same insolation, as much as 4°7 C. according to the con- 

 dition of the grass ; somewhat parched, and consequently lighter 

 coloured grass, giving a higher temperature than green and mode- 

 rately long grass, whilst the latter raises the thermometer more than 

 newly-mown grass. These differences result partly from differences 

 of shade or air temperature in the immediate neighbourhood of the 

 vacuous globe of the thermometer, and partly from the different 

 reflecting power of the subjacent surface. With regard to the first of 

 these causes, it must be borne in mind that the solar intensity is 

 measured by the number of degrees through which the blackened 

 bulb in vacuo is raised above the temperature of the medium imme- 

 diately surrounding the vacuous globe. If this medium becomes 

 warmer, the temperature of the blackened bulb will rise in a corre- 

 sponding measure, and vice versa, although the solar intensity remain 

 the same. Now, the more absorbent the surface upon which the sun's 

 rays fall, the higher, cceteris paribus, will be the temperature of the 

 air resting upon that surface. Thus, with the same solar intensity, 

 the shade or air temperature on white paper was 25°*2 C, on black 

 caoutchouc 28 0, 5 C, on short grass 22 0, 7 C, and upon a rock 

 22° '6 C. ; whilst at a height of 4 feet above the grass it was only 

 17°-9 C. 



It is to the second of the causes just specified, however — the dif- 

 ferent reflecting power of the subjacent surface — that the variations 

 of the sun thermometer under the same solar radiation are mainly 

 due, as is proved by the following observations, in which the shade 

 temperature was always taken under a small paper arch close to the 

 vacuous globe : — 



Suburb of Zurich. September 19th. 



Indicated 



Position of Sun Shade solar 



Time. thermometer. temperature, temperature, intensity. 



11.0 a.m... Four feet above meadow. 47'3° C. . . 20'5° C . 26-8° C. 



11.20 „ . . On grass 53"3 . . 25 3 . . 28*0 



11.30,, .. On white paper 67"0 .. 25'3 .. 417 



12.0 noon. . On black caoutchouc . . . 59'0 . . 27'0 . . 32'0 



