464 THE POPULAR SCIENCE MONTHLY. 



ing a single room, was erected for my benefit npon the wild moorland 

 of Hind Head. From the plateau on which the hut stands there is a 

 free outlook in all directions. Here, amid the heather, I had two 

 stout poles fixed firmly in the ground eight feet asunder, and a stout 

 cord stretched from one to the other. From the center of this cord 

 a thermometer is suspended with its bulb four feet above the ground. 

 On the ground is placed a pad of cotton-wool, and on this cotton-wool 

 a second thermometer, the object of the arrangement being to deter- 

 mine the difference of temperature between the two thermometers, 

 which are only four feet vertically apart. 



Permit me at the outset to deal with the subject in a perfectly 

 elementary way. In comparison with the cold of space, the earth 

 must be regarded as a hot body, sending its rays, should nothing in- 

 tercept them, across the atmosphere into space. The cotton-wool is 

 chosen because it is a powerful, though not the most powerful, radiator. 

 It pours its heat freely into the atmosphere, and by reason of its floc- 

 culence, which renders it a non-conductor, it is unable to derive from 

 the earth heat which might atone for its loss. Imagine the cotton- 

 wool thus self-chilled. The air in immediate contact with it shares 

 its chill, and the thermometer lying upon it partakes of the refrigera- 

 tion. In calm weather the chilled air, because of its greater density, 

 remains close to the earth's surface, and in this way we sometimes 

 obtain upon that surface a temperature considerably lower than that 

 of the air a few feet above it. The experiments of Wilson, Six, and 

 Wells have made us familiar with this result. On the other hand, the 

 earth's surface during the day receives from the sun more heat than it 

 loses by its own radiation, so that, when the sun is active, the tem- 

 perature of the surface exceeds that of the air. 



These points will be best illustrated by describing the course of 

 temperature for a day, beginning at sunrise and ending at 10.20 p. m. 

 on the 4th of last March. The observations are recorded in the an- 

 nexed table, at the head of which are named the place of observation, its 

 elevation above the sea, and the state of the weather. The first col- 

 umn in the table contains the times at which the two thermometers 

 were read. The column under " Air " gives the temperatures of the 

 air, the column under " Wool " gives the temperatures of the wool, 

 while the fourth column gives the differences between the two tem- 

 peratures. It is seen at a glance that, from sunrise to 9.20 a. m., the 

 cotton-wool is colder than the air ; at 9.30 the temperatures are alike. 

 This is the hour of " intersection," which is immediately followed by 

 " inversion." Throughout the day, and up to 4 p. M., the wool is 

 warmer than the air. At 4.5 p. m. the temperatures are again alike ; 

 while from that point downward the loss by terrestrial radiation is in 

 excess of the gain derived from all other sources, the refrigeration 

 reaching a maximum at 7.30 p. m., when the difference between the 

 two thermometers amounted to 10 Fahr. When the observations are 



