26 



The Temperature of the Solar Radiation. 



sphere. But here are two first class instruments, of the 

 same maker, made on same principle — reading in shade, 

 &c, alike ; placed side by side, under precisely similar circum- 

 stances, differing J0° or 12° in their indications of solar 

 radiation. I have seven or eight of these instruments in 

 store, and I intend to try them all, — I will let you know 

 the result, and in the meantime should be glad to have your 

 opinion as to cause. Is it that the one reading the higher 

 is in a more perfect vacuum, or the black glass different, 

 one less absorbent than the other, or may the smaller outer 

 bulb have anything to do with it ?" 



From inquiry, I find Mr. Todd ' formerly used a 

 thermometer with a blackened bulb, hence the higher solar 

 radiation obtained in Adelaide, and the apparently low 

 temperature in Melbourne and Sydney. 



The differences between his new black glass bulb thermo- 

 meters must, I think, depend on the different diathermancy of 

 the glass of the bulbs, for if black glass is not opaque to heat 

 rays, the thinner the glass the more diathermanous it must 

 be, and it will be interesting to learn if coating these bulbs 

 will render their indications similar or more comparable. 



The highest temperatures in the sun published in the 

 meteorological reports of the Melbourne Observatory up till 

 the 1st of January 1868, have all been obtained from the 

 ordinary black glass bulb thermometer, and do not therefore 

 represent the true temperatures of the greatest solar radiation. 

 The table first given furnishes an approximate correction, 

 which, when applied, brings up the temperatures to what 

 they would have been if measured by a blackened bulb 

 thermometer — and I here append the maximum solar radia- 

 tion for each year since 1860, as observed with the black 

 glass bulb, as well as the true temperatures obtained by 

 applying the correction : — 





Black Glass Bulb. 



Correction. 



Coated Bulb. 



I860 



136-8 



15-4 



152-2 



1861 



129 



15-1 



144-1 



1862 



144-3 



15-7 



160 



1863 



133 



15-3 



148-3 



1864 



124 



14-8 



138-8 



1865 



126-1 



15-1 



141-2 



1866 



130-8 



15-3 



146-1 



1867 



131 



15-3 



146-3 



(?) 1868 



127-4 



15-2 



142-6 



