344 PBOFESSOR TYNDALL OK THE ACTION OF FEEE MOLECULES ON 
explain. This, for example, I hold to be the case as regards the celebrated observa¬ 
tions of Patrick Wilson of Glasgow, made a century ago. Wilson brought strongly 
into light the great differences which sometimes exist between the temperature of the 
earth’s surface, and of the air at a small elevation above the surface. His letter to 
Dr. Maskelyne on this subject is published in the Philosophical Transactions for 1780 
under the title :—“ An account of a most extraordinary degree of cold at Glasgow , 
together with some new experiments and observations on the comparative temperature of 
hoar frost and the air near it A On the afternoon of the 13th of January, 1780, the 
cold was intense, a thermometer at the high window of the observatory pointing, at 
7 p.m., to 0° Fahr. At 8 p.m. Wilson and Dr. Irvine laid two thermometers upon the 
snow, and hung up two others in the air 2 feet above the snow. Here follow the 
temperatures observed on the evening of the 13th and on the morning of the 14tli of 
January:— 
'ime of observation. 
Snow temperature. 
Air temperature. 
8^ P.M. 
-12° 
-0° 
9 „ 
o 
>—1 
I 
— 2° 
10 „ 
-14° 
— 4° 
11 „ 
-17° 
-6° 
ni „ 
-18° 
— 6° 
4 A.M. 
-20° 
— 8 C 
1 „ 
-23° 
-7° 
The sign — signifies that the temperatures were all below zero Fahrenheit. These 
temperatures amply justify Wilson’s statement that the cold was “ extraordinary.” 
Coexistent moreover with, the general cold we have a difference of 16° between the 
temperature of the surface and that of the air 2 feet above it. Had the air thermo¬ 
meter been hung 10 feet, instead of 2 feet, above the surface the difference would 
have been still greater. The thermometer, moreover, must have been chilled, not only 
by its immersion in cold air, but also by its own radiation against the intensely cold 
snow. The chilling of the superficial snow was purely an effect of radiation. Beneath 
the surface its temperature reached +14°. Wilson filled a bread-basket with this 
warm snow at 2b a.m. on the 14th. Within half an hour it had fallen 24°, and in two 
hours 32°. 
I venture to predict that if Wilson’s experiment be repeated during the cold of a 
Canadian winter the same result will be obtained; and it seems to me that until the 
action of water vapour upon radiant heat had been discovered no explanation of the 
phenomenon could have been given. It was accepted but not accounted for. On the 
night of Wilson’s observations “alight air was felt coming from the east.” With 
such an “ air ” and such a temperature the quantity of water vapour in the atmosphere 
must have been infinitesimal. Dry air being a practical vacuum to the rays of heat, 
were the vapour-screen entirely removed, the earth would find itself exchanging tern- 
