514 Prof. Tyndall on the Action of Free Molecules on 



permitting the beam from the lime-light (produced by coal-gas 

 and oxygen) to pass through the mixture, a prompt rise of 

 65 millim. was the consequence. Cutting the beam off, the 

 column rapidly returned to zero. The double of 65, or 130 

 millim., gives the difference of level in the two legs of the 

 U tube. 



I have done my best to render these determinations correct. 

 They have been repeated both by myself and my assistant* a 

 great number of times. The first measurements were made 

 in the early part of last year, and were made known in the 

 Royal Institution on the 8th of April, 1881. Difficulties were 

 encountered in obtaining a powerful, and at the same time 

 constant, source of heat. The mixture of coal-gas and oxygen 

 issuing from independent holders was finally resorted to. 

 The sounds are classified into "very strong/' strong, " &c.j 

 but it is, of course, impossible to say where one class ends 

 and another begins. They shade gradually into each other. 

 But if the middle members of any class be compared with the 

 corresponding members of another class, the difference of 

 sounding-power will appear. 



§ 10. Application of Results to Meteorology. 

 If it be at length conceded that aqueous vapour exerts upon 

 radiant heat the action which I so long ago ascribed to it, 

 1 think the knowledge of this action will prove of importance 

 to the scientific meteorologist. Meteorology, as connected 

 with heat, seems to me to abound in facts which it has hitherto 

 been incompetent to explain. This, for example, I hold to 

 be the case as regards the celebrated observations of Patrick 

 "Wilson of Glasgow, made a century ago. Wilson brought 

 strongly into light the great differences which sometimes 

 between the temperature of the earth's surface and of the air 

 at a small elevation above the surface. His letter to Dr. Mas- 

 kelyne on this subject is published in the Philosophical Trans- 

 actions 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 Tem- 

 perature of Hoar Frost and the Air near it." On the after- 

 noon 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 14th of January : — 



* Who has aided me in this investigation with his usual zeal and 

 intelligence. 



