332 



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



between the two temperatures being taken as a measure of the sun's 

 radiant heat operating at the time and place of the two observations. 



The chief sources of error in this method are the difficulty of 

 ascertaining the temperature of the air immediately surrounding the 

 vacuous globe containing the blackened bulb, and the placing of this 

 thermometer under exactly similar conditions at different meteoro- 

 logical stations. How considerable may be the errors arising from 

 these sources will be evident from the following observations and 

 experimental results. 



Determination of Shade Temperature. 



A thermometer merely shaded from the sun gives, in air of uniform 

 temperature, readings differing very widely from each other according 

 to its surroundings. If it be placed opposite a wall, for instance, 

 upon which the sun is shining, the temperature indicated will be 

 several degrees above what it would be if there were no such object 

 near. I have also observed a difference in its readings when, od the 

 one hand, it is exposed towards a blue sky, or, on the other, towards 

 white clouds. Again, if the thermometer be placed in a louvred box, 

 the readings will be much too high, unless the outside of the box be 

 white ; because the box becomes heated by the sun and communicates 

 its heat to the air entering the louvres. Even the colour of the ground 

 beneath the box has considerable influence upon the temperature of 

 the air inside. 



A true shade temperature means the temperature of free air in full 

 sunshine ; and, strictly, it ought to be ascertained without any shade 

 at all, for, as soon as a shade is created, conditions supervene which 

 often entirely baffle the object of the observer. The shade of a parasol 

 exhibits a different temperature from the shade of a tree, and this 

 again differs widely from that of a house. The temperature of the 

 shade of a sheet of tinfoil is quite different from that of a sheet of 

 writing-paper. Indeed, it may be truly said that every shade has its 

 own peculiar temperature. The following thermometric readings show 

 this effect of the area of shade, and of the quality of the shading 

 material : — 



Shade Temperatures. 



Beneath larch tree 19*5° C. 



,, white parasol 25*0 



„ small white paper arch 35 - 



,, small arch of bright tinfoil .... 45 '2 



Thus shade temperatures, measured during If hours of uninter- 

 rupted sunshine in the middle of the day, and within a few yards of 

 the same spot, differed by no less than 25°* 7 C. These observations 



