298 SCIENCE IN SHORT CHAPTERS. 



pressed together as similar to the sticking together of two pieces of 

 cobblers' wax, or softened sealing-wax, or beeswax, or the welding of 

 iron or glass when heated to their welding temperatures i.e. to a 

 certain degree of incipient fluidity or viscosity. 



If a leaden bullet be cut in half, and the two fresh-cut faces 

 pressed forcibly together, they cohere at ordinary atmospheric tem- 

 peratures, but we have no occasion for a regelation theory here. The 

 viscosity of the lead accounts for all. At Woolwich Arsenal there is a 

 monster squirt, similar to my little one. This is charged with lead, 

 and, by means of hydraulic pressure, the lead is squirted out of the 

 nozzle as a cylindrical jet of any required diameter. This jet or 

 stick of lead is the material of which the elongated cylindrical rifle 

 bullets are now made. 



But returning to the point at which we started, on the subject of 

 ice viz. its Alpine accumulation above the snow-line. If the snow- 

 fall there exceeds the amount that is thawed and evaporated, it must 

 cither go on growing upward until it reaches the highest atmospheric 

 region from which it falls, or is formed, or it must descend some- 

 how. 



If ice can be squirted through a syringe by mere hand-pressure, we 

 are justified in expecting that it would be forced down a hill slope, or 

 through a gully, or across a plain, by the pressure of its own weight 

 when the accumulation is great. Such is the case, and thus are 

 glaciers formed. 



They are, strictly speaking, rivers or torrents of ice ; they flow as 

 liquid water does, and down the same channels as would carry the 

 liquid surface drainage of the hills, were rain to take the place of 

 snow. Like rivers, they flow with varying speed, according to the 

 slope ; like rivers, their current is more rapid in the middle than the 

 sides ; like rivers, they exert their greatest tearing force when 

 squeezed through narrow gullies ; and, like rivers, they spread out 

 into lakes when they come upon an open basin-like valley, with nar- 

 row outlet. 



The Justedalsbrae of Norway is a great ice-lake of this character, 

 covering a surface of about 500 square miles, and pouring down its 

 ice-torrents on every side, wherever there is a notch or valley de- 

 scending from the table-land it covers. The rate of flow of such 

 downpouring glaciers varies from two or three inches to as many feet 

 per day, and they present magnificent examples of the actual fluidity 

 or viscosity of an apparently solid mass. This viscosity has been 

 disputed, and attempts have been made to otherwise explain the 

 motion of glaciers ; but while it is possible that it may be assisted by 

 varying expansion and contraction, the downflow due to viscosity is 

 now recognized as unquestionably the main factor of glacier motion. 



Cascades of ice may be sometimes seen. In the course of my first 

 visit to Norway, I wandered alone over a very desolate mountain 

 region toward the head of the Justedal, and unexpectedly came upon 

 a gloomy lake, the Styggevand, which lies at the foot of a precipice- 

 boundary of the great ice-field above named. Here, the ice having 

 no sloping valley-trough by which to descend, poured over the edge 

 of the precipice as~a great overhanging sheet or cornice, which bent 

 down as it was pushed forward, and presented on the convex side of 

 the sheet some fine blue cracks, or " crevasses" as they are called. 

 These gradually widened and deepened, until the overhanging mass 



