66 
Remarks on Mr Darnell's Hypothesis 
boldt is referred to. <( I have often,” says that illustrious traveller, 
“ endeavoured to measure the power of the sun, by two thermo- 
meters of mercury perfectly equal, one of which remained ex- 
posed to the sun, while the other was placed in the shade. The 
difference arising from the absorption of the rays never exceed- 
ed 6 # .6 Fahr ” It is unnecessary to mention, that naked ther- 
mometers are quite unfit for such experiments. For of the rays 
which impinge upon a naked ball, all except those having a per- 
pendicular incidence, will be reflected from the surface. Hence, 
the amount of heat developed must be extremely small ; and 
during a breeze *, if the instrument be not screened from its 
effects, it is nearly neutralized. 
We have made numerous observations with naked thermome- 
ters, but none of them deserve notice, except those made during 
the hot weather about the end of last July 1825. On the 27th, 
at 3 p. m., when the black thermometer was at 150°, a naked 
thermometer, exposed fully to the sun during a dead calm, rose 
to 99° ; the temperature of the slates at this time was 117°, of the 
earth 103°. On the 28th, temperature of the air 62°, wind E., 
pleasant breeze. 
At noon, black therm. 1250, naked T5°, 
— Ip. 139, 92. 
— 2 p. m. 139,-i 90, 
— 2 30' 135, — 87. 
On the 29th, temp, of the air 62°.5, wind as before. At 1 f. m., 
black thermometer 127°, naked 79°. 
We covered the latter loosely with a piece of black cloth till 
it rose to 97° ; on removing the cloth, it fell in 5' to 83°. At 2 
p. m., black thermometer 140°, naked 95°. 
Temperature of the earth 101, of the air, three inches above 
the slates of a low out-house facing the south, and sheltered from 
the wind, 90°. 
From these it appears, that, at a certain excess of temperature, 
the emission from the shaded part of the ball, owing to the high 
radiating power of the glass, more than counterbalances the 
calorific effects from the absorption. We see, therefore, that 
* These observations were not made at Cumana as Mr Daniell says, but on ship* 
board, during the voyage from Teneriffe to Cumana : consequently they cannot be 
considered satisfactory. 
