ICE AND GLACIERS. 121 



-when the pieces are already in the act of melting. 1 They need 

 only be strongly pressed together for a few minutes to make them 

 -adhere. The more plane are the surfaces in contact, the more 

 complete is their union. But a very slight pressure is sufficient 

 if the two pieces are left in contact for some time. 2 



This property of melting ice is also utilised by boys in 

 making snow-balls and snow-men. It is well known that this 

 only succeeds either when the snow is already melting, or at 

 any rate is only so much lower than that the warmth of the 

 hand is sufficient to raise it to this temperature. Yery cold 

 snow is a dry loose powder which does not stick together. 



The process which children carry out on a small scale in 

 making snow-balls takes place in glaciers on the very largest 

 .scale. The deeper layers of what was originally fine loose neve 

 are compressed by the hugh masses resting on them, often 

 amounting to several hundred feet, and under this pressure they 

 cohere with an ever firmer and closer structure. The freshly 

 fallen snow originally consisted of delicate microscopically fine 

 ice-spicules, united and forming delicate six-rayed, feathery stars 

 of extreme beauty. As often as the upper layers of the snow- 

 fields are exposed to the sun's rays, some of the snow melts; 

 water permeates the mass, and on reaching the lower layers of 

 still colder snow, it again freezes ; thus it is that the firn first 

 becomes granular and acquires the temperature of the freezing- 

 point. But as the weight of the superincumbent masses of snow 

 continually increases by the firmer adherence of its individual 

 granules, it ultimately changes into a dense and perfectly hard 

 mass. 



This transformation of snow into ice may be artificially 

 fleeted by using a corresponding pressure. 



We have here (Fig. 22) a cylindrical cast-iron vessel, A A ; 

 the base, B B, is held by three screws, and can be detached, so as 

 to remove the cylinder of ice which is formed. After the vessel 



1 In the Lecture a series of small cylinders of ice, which had been prepared 

 <by a method to be afterwards described, were pressed with their plane ends 

 -against each other, and thus a cylindrical bar of ice produced. 



2 Vide the additions at the end of this Lecture. 



