1882.] 



relative Thermal Intensity of the Sun. 



333 



were, however, made at Pontresina, 5,915 feet above sea level, and so 

 wide a range would probably not occur at lower altitudes. 



The most effective shading material is, obviously, that which most 

 perfectly reflects solar heat ; and of all materials with which I have 

 experimented white paper is the best, white linen and zinc- white being 

 nearly equal to it. The most trustworthy shade thermometer, there- 

 fore, is one having its bulb covered with a thin layer of white paper, 

 or, in default of this, the naked bulb may be shaded by a small arch 

 of white paper. So placed, the thermometer will indicate a lower 

 temperature than any obtainable in a similar shade produced by any 

 other material. 



The foregoing temperatures were observed when the thermometer 

 was level with the ground, but the readings often rapidly become 

 lower as the instrument is raised. The ratio of the diminution of 

 temperature at increasing heights above the ground is, during sun- 

 shine, enormously great within a few feet of the earth. The ground, 

 strongly heated by the sun, powerfully warms the molecules of air in 

 immediate contact with it ; these, becoming specifically lighter, rise, 

 and at once begin to share their heat with the colder molecules above 

 them, losing temperature in proportion as they mix with larger and 

 larger volumes of supernatant cold air. The intensity of this effect 

 attains a maximum when the air is calm, and a minimum during a 

 storm. Indeed, these powerful convection currents are readily seen, 

 on a calm sunny day, rising from the ground like the heated air from 

 a stove, but they are scarcely, if at all, visible when a strong breeze is 

 blowing. 



In order to be comparable with each other, therefore, observations 

 of shade temperature, whether at the same place or at different 

 stations, should always be made under uniform conditions. That is 

 to say, the thermometers, fully exposed to the air, should be similarly 

 protected from radiant heat, and should be placed either at the level 

 of the ground or at a definite height above it, upon a surface of 

 uniform quality as regards absorbing and reflecting power. I would 

 suggest that the bulb of the thermometer and 2 inches of its stem 

 should be protected from the rain by being placed beneath a sheet- 

 zinc arch of 1-inch span and 4 inches long, painted inside and out 

 with "flatted" zinc-white. The instrument with the arch should be 

 securely fixed horizontally upon a wooden stand 1 foot square, painted 

 on both sides with " flatted " zinc- white, and, in order to avoid the 

 excessively warm air very near the ground, the stand should have a 

 height of 4 feet — an elevation convenient for observation, and one at 

 which the temperature of the air suffers comparatively small decre- 

 ments per foot of elevation. It should also be at a distance from 

 buildings or trees, and have as free a horizon as possible. By the use 

 of instruments so prepared and mounted, comparative and fairly 



