408 Prof. Erman on the Structure, the Melting, 



A. A horizontal division, mentioned under (1), and due to 

 the perpendicularity of crystallographic axes, and perhaps to 

 compression in a vertical sense. 



B. Two kinds of vertical fissm-es, mentioned under (2) and (4), 

 and arising from the lateral compression mutually exerted by 

 the particles of freezing water. The rending effect of this com- 

 pression agrees with what, in the case of other substances, is 

 commonly called crushing. And — 



C. The broad fissures occasioned by contraction. 



I must own that the crushing of ice or its rending by com- 

 pression seems to me as explicable, or even more so, than the 

 rending of the same by contraction. As a phsenomenon the 

 latter is of course more famiUar to us, and therefore generally 

 acknowledged ; but on theoretical grounds we can neither expect 

 nor should be able to explain its taking place in any mass of 

 which all parts were equally cooled. 



Some instances of a dry or wet soil being rent by contraction. 



{Raise um die Erde, u. s. w., Histor. Abth. vol. ii. p. 215. 

 Travels in Siberia, vol. ii. p. 326.) 



March 25, at Kivensk in the valley of the Lena. . . . Towards 

 evening the clouds vanished, and I observed transits of the stars, 

 to determine the geographical position and the magnetic decli- 

 nation . In a fevF hours the air had again cooled down to 1 6° R. * ; 

 and in the yard where I set my instruments, the ground cracked 

 with a loud report from the rapid contraction which it underwent. 



(Raise um die Erda, u. s. w., Histor. Abth. vol. ii. p. 247- 

 Travels in Siberia, &c. vol. ii. p. 363.) 



April 8, near Yakutsk. — The Lena was now divided into a 

 number of anns ; and between the ice of these, lay islands with 

 snow heaped high above, while theii- sides, from 15 to 20 feet 

 high, were quite black. These islands consist of fine mud with 

 fragments of willow-stems and roots, as if swept together by 

 inundations of the stream ; but the unparalleled frost which 

 their naked sides have to endure every year had cracked them 

 vertically, and divided them into such rude pillars that they 

 reminded one of the limestone rocks of the upper valley. The 



• The same day I noted the temperature of the air as follows : — 

 h m o 



At 1 15 P.M. . . . — 3-5 R. 



4 45 „ ... 00 „ 



5 „ ... — 4-5 „ 

 7 30 „ ... -16-6 „ 



See Raise xim die Erde, u. s. w., Physikalischa AbtJieiluny, vol. i. ]). 372. 



