G4 Royal Society :— 



atmosphere by radiation from the sun in the day-time, and cooled 

 below it by radiation into space at night. 2ndl)'. One variation of 

 temperature only has been assumed to take place every t\venty«four 

 hours, viz. that from the extreme heat of the day to the extreme 

 cold of the night; whereas such variations are notoriously of con- 

 stant occurrence during the twenty-four hours. Each cannot but 

 have caused a corresponding descent of the lead, and their aggregate 

 result cannot but have been greater than if the temperature had 

 passed uniformly (without oscillations backwards and forwards) from 

 one extreme to the other. 



Tiiese considerations show, I think, that the causes I have ' 

 assigned are sufficient to account for the fact observed. They sug- 

 gest, moreover, the possibility that results of importance in meteor- 

 ology may be obtained from observing with accuracy the descent of 

 a metallic rod thus placed upon an inclined plane. That descent 

 would be a measure of the aggregate of the changes of temperature 

 to which the metal was subjected during the time of observation. 

 As every such change of temperature is associated with a corre- 

 sponding development of mechanical action under the form of work*, 

 it would be a measure of tlie aggregate of such clianges and of the 

 work L-o developed during that period ; and relations might be found 

 between measurements so taken in different equal periods of times, 

 successive years for instance, tending to the development of new 

 meteorological laws. 



The following are the results of recent experiments f on the 

 expansion of ice : — 



Linear expansion of ice for an interval of 100° of the Centigrade 

 thermometer. 



0-005 24, Schumacher. 



00513, Johrt. 



0-00518, Moritz. 



Ice, therefore, has nearly twice the expansibility of lead, so that a 

 sheet of ice would, under similar circumstances, have descended a 

 plane similarly inclined, twice the distance that the sheet of lead 

 referred to in the preceding article descended. Glaciers are, on an 

 increased scale, sheets of ice placed upon the sloj^es of mountains, 

 and subjected to atmospheric variations of temperature throughout 

 their masses by variations in the quantity and the temperature of 

 the water, whicli flowing from the surface everywhere percolates 

 them. That they must from this cause descend into the valleys is 

 therefore certain. That portion of the Mer de Glace of Chamouni 

 which extends from Montanvert to very near the origin of the Glacier 

 de Lechaud, has been accuratel}' observed by Professor James Forbes J. 

 Its length is 22,600 feet, and its inclination varies from 4° 19' 22" 

 to 5° 5' 53". The Glacier du Geant, from the Tacul to the Col du 



* Mr. Joule has shown (Phil. Trans. 1850, Part I.) that the quantity of heat 

 capable of raising a pound of water by 1° Fahr. requires for its evolution 772 units 

 of work, 



t Vifle Archiv f. Wissenchaftl. Kunde v. Russland, Bd. vii. S. 333. 



X Travels through the Alps of Savoy. Edinburgh, 1843. 



