1883.] on Thoughts on Badiation, Theoretical and Practical. 261 



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

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

 cord a thermometer is susj)ended 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 determine the difierence 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 

 intercept 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 flocculence, 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 

 refrigeration. 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 temperature 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 March 4. The observations are recorded in the annexed table, at 

 the head of which is named the place of observation, its elevation 

 above the sea, and the state of the weather. The first column 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 temperatures. 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 downwards 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 thermo- 

 meters amounted to 10° Fahr. When the observations are continued 

 throughout the night, the greater cold of the surface is found to be 

 maintained until sunrise, and for some hours beyond it. Had the air 

 been perfectly still during the observations, the nocturnal chilling of 



