2 9 z THE POPULAR SCIENCE MONTHLY. 



sheet of white paper and shading its hulb with a small arch of similar 

 paper doubled, the sun-thermometer (blackened bulb in vacuo) was 

 also placed either upon white paper or upon snow, and the numbers in 

 the column headed " sun-temperature " were obtained by deducting 

 the readings of the air or shade thermometer from those of the sun- 

 thermometer. Thus, on the Diavolezza, the reading of the air-ther- 

 mometer (42'8), being subtracted from the reading of the sun-ther- 

 mometer (19'9), left the number 107'1 as the sun-warmth, or the 

 temperature, above that of the surrounding air, to which the sun's rays 

 raised the blackened bulb in vacuo. 



The very high sun-temperature observed on the Diavolezza was 

 recorded at a station surrounded by snow-fields in brilliant sunshine, 

 and the thermometer was placed upon snow. 



Not only is solar radiation much more intense at great altitudes, 

 but it is also more equable during the whole day, inasmuch as the 

 comparative absence of suspended matters in the air renders the ther- 

 mal power of the rising and setting sun more nearly equal to that of 

 the meridional sun. Thus it has been observed that, at or near the sea- 

 level, the sun-temperature increases about 15 Fahr. between 8.30 a.m. 

 and noon, and decreases to the same extent between noon and 3.30 

 p. m. ; whereas on the Riffelberg, 8,428 feet above the sea, the increase 

 and decrease between the same hours are only 9 Fahr. These obser- 

 vations were, however, made in summer ; in winter, the difference 

 between the two stations would doubtless be still greater. Moreover, 

 an elevation of 5,000 or 6,000 feet places us, especially in winter, to a 

 great extent above the region of cloud, and thus enables us to enjoy a 

 bright sun at time when clouds effectually cut off his rays from lower 

 altitudes. 



Lastly, and this is of the highest importance to invalids, the air at 

 great elevations is characterized by comparatively great freedom from 

 zymotic matters. By numerous and ingenious experiments Pasteur 

 found this to be the case at a height of 6,300 feet, on the slope of Mont 

 Blanc, and Tyndall at an elevation of 6,730 feet, on the Bel Alp, in 

 Switzerland. 



2. Reflection from Snow. Although the air-temperature in mid- 

 winter, at elevations of from 5,000 to 10,000 feet, differs but little from 

 that of much lower levels in the same localities, the low temperature 

 prevails for a longer time. Thus the valley of Davos and the sur- 

 rounding mountains are usually thickly covered with snow from No- 

 vember to the beginning of March, and the solar heat reflected from 

 this snow is an important factor in the production of the genial winter 

 climate of Davos. By laying a sun-thermometer on surfaces of differ- 

 ent materials, I have demonstrated the high reflective power of snow. 

 The following summary of the results of these experiments shows the 

 degrees to which the blackened bulb in vacuo was raised when laid in 

 the sunshine upon each of the materials experimented with : 



