116 Oh Solar Hadiation. 



I suffered severely from a single day spent amongst the pure 

 snows of the highest summits. 



2. On the photometric effect of the diffuse light reflected 

 from the sky. Professor Kamtz first, I believe, announced the 

 startling fact, that half the photometric effect of Leslie's in- 

 strument is due to the diffuse light of the sky, the other half 

 only being the effect of the direct rays of a bright sun. This 

 singular paradox also manifests itself by the fact, that cloudy 

 weather, if the sun be not itself greatly obscured, apparently 

 increases the effect of the solar radiation. Of the truth of 

 both of these facts, I had also last year sufficient evidence, of 

 which I shall quote one or two examples. 



On the 28th June 1842, a warm and clear day, at 6000 feet 

 above the sea, at 1^ 40"^. 



The photometer in the sun placed on the snow, stood at . 121° 



An alpine pole, an inch in diameter, was then stuck into the 

 snow, so as to throw its shadow on the instrument, thus in- 

 tercepting the direct sun-beam only. It fell to ... 82* 



Leaving for the direct effect of the sun-light, .... 39** 



the remaining 82° being derived from the reflection of scattered sky-light, 



and from the snowy surface. 



At the same place and time, the photometer, in the sun, surrounded by 



grass, stood at . 78° 



Shaded by a stick as before, .... . . . . . 30° 



Direct sun effect,* 8 



Now, if we look to 78° as the total effect of the sun's light 

 and its reflection from the ground, in one of the hottest days 

 of June, in a fine climate and 6000 feet above the sea, — it ap- 

 pears to be inconceivably small, when we know that the same 

 instrument often stands at 120° in moderately fine weather in 

 Scotland. What is the reason \ The sky does not reflect so 

 much light when it is pure as when it is milky, and its sur- 

 face being immense, compared to the apparent surface of the 



* The diflference of this and the last is probably due, in a great measure, to the 

 defect in the principle of the instrument, the momentary increment of heat ne- 

 cessary to maintain the temperature 1° above 120°, being greater than what is 

 necessary to maintain it 1° above 77°, for example. There are likewise other 

 sources of error. 



