sheet of white paper and shading its bulb 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-thermometer (42·8°), being subtracted from the reading of the sun-thermometer (149·9°), 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 thermal 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 observations 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 midwinter, 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 surrounding mountains are usually thickly covered with snow from November 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 different 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:
