1882.] 



relative Thermal Intensity of the Sun. 



337 



These results show that the indications of the black bulb in vacuo 

 •are profoundly affected by the character of the surface beneath th e 

 instrument, the more perfect the reflecting power of that surface, 

 other things equal, the higher the solar intensity indicated. Of all 

 the substances tried, the highly reflecting power of white paper and 

 linen for solar heat was very remarkable, exceeding appreciably that 

 of bright metals, and even of freshly-fallen snow of dazzling white- 

 ness. Of course, lateral reflection produces the same effect, and I 

 found that the indicated solar intensity was increased by no less than 

 11° C. when the blackened bulb in vacuo was placed at a distance of 

 10 feet in front of a whitewashed wall upon which the sun was 

 shining. 



Finally the indications of the solar thermometer are also affected 

 by strong wind, the readings of solar intensity being somewhat 

 lower when the instrument is exposed to the current than when it is 

 sheltered. The cause of this is obvious ; the difference of tempera- 

 ture produced by the sun's radiant heat is really that between the 

 inner blackened bulb and the glass of the vacuous globe. Now the 

 latter is constantly receiving and absorbing obscure rays of heat from 

 the blackened bulb, and its temperature must therefore always be 

 somewhat higher than that of the surrounding air, which is measured 

 by the shade-thermometer; but the glass globe will obviously main- 

 tain a less elevated temperature when it receives the strong impact of 

 the molecules of cooler air in a breeze than when it is surrounded by 

 a still atmosphere. The error thus introduced into the observations 

 by a light breeze, however, does not seem to be serious, for I have not 

 found it to exceed o, 7 C. ; but in a high wind it would probably be 

 more considerable. 



The results of the foregoing experiments disclose the precautions 

 necessary to be observed to render such determinations of relative 

 solar intensity fairly comparable and trustworthy. They are the 

 following : — 



1. The vacuous globe should always and everywhere be placed upon 

 the same kind of horizontal reflecting surface. 



2. The temperature of the air upon this reflecting surface should be 

 taken as the shade temperature, and its observation should be syn- 

 chronous with that of the sun-thermometer. 



3. The horizon all round the instrument should be as free as possi- 

 ble, and there should be, especially, no sunlit walls in such a position 

 as to reflect heat upon the thermometer. 



4. As far as compatible with these conditions, the solar thermo- 

 meter should be sheltered from the wind. 



The white surface on which the thermometers are laid need not be 

 of large area. A square foot practically affords to the blackened 

 bulb in vacuo a reflective plane of infinite extent, for I have ascer- 



VOL. XXXIII. 2 B 



