386 Scientific Intelligence — Meteorology, 



occurring in sudden gusts, the Northern Lights are almost always in 

 a state of flickering rapid movement ; and, while at the periods of the 

 greatest evolution of light, the storm is generally much diminished, 

 it is renewed in all its strength when the the brilliancy of the aurora 

 fades. 



2. Mr Hopkins on the Cause of the Motion of Glaciers. — Mr 

 Hopkins, at the Cork meeting of the British Association, explained 

 liis views respecting the cause of the motion of glaciers. De Saussure 

 had adopted the theory which attributes this motion to the resolved 

 part of gravity, acting along the inclined surfaces on which all gla- 

 ciers in motion are situate ; and he explained, also, how the motion 

 could be facilitated by the effects of the internal heat of the earth, 

 and of sub -glacial currents. When the attention of philosophers, 

 however, was recalled a few years ago to this subject, and more ac- 

 curate observations and admeasurements were made, the inclinations 

 of the bods of glaciers were found, in many cases, to be so small 

 (in the glacier of the Aar, for example, not exceeding three degrees) 

 that it appeared extremely difficult to conceive how the force of 

 gravity alone could be adequate to overcome the friction on the bot- 

 tom and sides of the glacier, and the numerous local obstacles to its 

 movement. Numerous experiments on the descent of bodies along 

 inclined planes had shewn, that, when the surfaces of the bodies and 

 planes were perfectly hard and polished, no motion would ensue 

 without an inclination considerably greater than that of many glaciers ; 

 and, moreover, that the inclination required to produce motion was 

 independent of the weight of the sliding body. These considerations 

 led to the very general rejection of De Saussure's theory, and to the 

 adoption, by many persons, of the dilatation theory^ of which M. 

 Agassiz had been the principal advocate. According to this theory, a 

 part of the water produced by the dissolution of the superficial portion 

 of the glacier during summer, passed, by infiltration, into the minute 

 pores and crevices of the glacier, where it was again converted into 

 ice, and, by its expansion in the process of freezing, produced a dila- 

 tation and consequent motion of the glacier. It was manifest, how- 

 ever, that the frequent alternation of freezing and thawing within the 

 glacier which this theory assumed, could not possibly take place at 

 depths beneath its Surface exceeding a very few feet, and therefore 

 could not produce any sensible effect on the motion of the whole mass. 

 This theory presented many other difficulties, of which no adequate 

 solution had been given, and the author could not but consider it as 

 contrary to the most obvious mechanical and physical principles* 



