exposed and shaded Thermometers in upper Air. 215 



nothing to protect the thermometer from the influence of cold 

 stellar space. It is true that the air above is at a temperature 

 little below that of the thermometer itself; but then the air is 

 dry, and, owing to its diathermancy, it does not absorb the heat 

 radiated from the thermometer, and consequently the instru- 

 ment radiates its heat directly into the cold stellar space above, 

 some hundreds of degrees below zero, almost the same as it would 

 do were the air entirely removed. The enormous loss of heat 

 which the thermometer now sustains causes it to fall in tempe- 

 rature to a great extent. The molecules of the comparatively 

 dry air at this elevation, being very bad radiators, do not throw 

 off their heat into space so rapidly as the bulb of the exposed 

 thermometer ; consequently their temperature does not (for this 

 reason) tend to sink so rapidly as that of the bulb. Hence the 

 shaded thermometer, which indicates the temperature of those 

 molecules, is not affected to such an extent as the exposed one. 

 Hence also the difference of reading between the two instruments 

 must diminish as we rise in the atmosphere. 



This difference between the temperature of the two thermo- 

 meters evidently does not go on diminishing to an indefinite 

 extent. Were we able to continue our ascent in the atmosphere, 

 we should certainly find that a point would be reached beyond 

 which the difference of reading would begin to increase, and 

 would continue to do so till the outer limits of the atmosphere 

 was reached. The difference between the temperatures of the 

 two thermometers beyond the limits of the atmosphere would 

 certainly be enormous. The thermometer exposed to the direct 

 rays of the sun would no doubt be much colder than it had been 

 when at the earth's surface ; but the shaded thermometer would 

 now indicate the temperature of space, which, according to Sir 

 John Herschel and M. Pouillet, is more than 200° Fahrenheit 

 below zero. 



It follows also, from what has been stated, that even under 

 direct sunshine the removal of the earth's atmosphere would 

 tend to lower the temperature of the earth's surface to a great 

 extent. This conclusion also follows as an immediate inference 

 from the fact that the earth's atmosphere, as it exists at present 

 charged with aqueous vapour, affects terrestrial radiation more 

 than it does radiation from the sun ; for the removal of the 

 atmosphere would increase the rate at which the earth throws 

 off its heat into space more than it would increase the rate 

 at which it receives heat from the sun ; therefore its tempera- 

 ture would necessarily fall until the rate of radiation from 

 the earth's surface exactly equalled the rate of radiation to the 

 surface. Let the atmosphere again envelope the earth, and ter- 

 restrial radiation would instantly be diminished ; the tempera- 



