On Solar Radiation. 117 



sun (100,000 times greater), a small addition to its reflective 

 power increases the photometric indication more than the thin 

 haze of vapour which produces that effect diminishes the 

 direct sun-light. 



Again, on the 29th June, a clear and warm day, in the same 

 position at 11 a.m. — 



The photometer standing on snow, .... 114'* 

 Shaded from the direct sun, 63"* 



Du-ect sun effect, 51 



I never observed these effects so strongly as on the 30th of 

 June, a day of the most intense solar heat, when, at a height 

 of 7600 feet, the sky exhibited a deep indigo tint, unusual for 

 that moderate elevation. I was engaged for some hours in 

 making trigonometrical observations, on an exposed promon- 

 tory of rock, with scarcely any shelter from the piercing sun- 

 beams. At length I was so exhausted as to be obliged to 

 thrust my head now and then behind a stone for protection 

 and relief. 



Now, at this time, the photometer directly exposed on the rock to the 



sun, stood only at 88° 



When shaded from the direct sun-beam, it fell to . . . 22" 

 the smallest result for diffuse atmospheric radiation and reflection 

 from the soil combined which I have witnessed. This, it will be 

 observed, leaves for direct sun-heat, 66" 



which is large, compared with the previous results. It is cer- 

 tain from these experiments, that the photometric effects 

 thus measured bear no kind of proportion to the phj-siologi- 

 cal effects of direct and reflected heat. — Prof, Forbeis Travels 

 through the Alps^ p. 416. 



