ICE AND GLACIERS 245 



cceds best when the pieces are already in the act of melt- 

 ing. 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. 8 



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 o that the warmth of 

 the hand is sufficient to raise it to this temperature. Very 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 huge masses resting on them, 

 often amounting to several hundred feet, and under this 

 pressure they cohere with an ever firmer and closer struc- 

 ture. 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 con- 

 tinually 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 

 effected by using a corresponding pressure. 



We have here (Fio. 113) 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 has lain for a while in ice-water, 

 so as to reduce it to the temperature of o, it is packed full 



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

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

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



* Vide the additions at the end of this Lecture. 



