806 
MR J. M. WORDIE ON THE NATURAL HISTORY OF PACK-ICE 
sudden changes of temperature. Though some cracks, therefore, were possibly 
temperature cracks, they were never proved as such ; and in the Weddell Sea, 
at any rate, they are not considered important. 
Elsewhere in the Antarctic they apparently do occur, and in the Arctic, accord- 
ing to Weyprecht, their occurrence is extremely common. Since their forma- 
tion, however, except at very low temperatures, has been called in question by 
Pettersson, a detailed case must be mentioned. Priestley and David describe 
how in the Ross Sea a crack formed with a fall of temperature from — 16° C. to 
— 29° C., and then opened and closed according as the temperature fell or rose. 
With the final rise of temperature to - 18° C. the young ice which had formed 
in the open water of the crack was overthrust a distance of 5 feet 6 inches. The 
importance of knowing the actual temperature comes in, in view of the remark- 
able conclusions arrived at experimentally by Pettersson ; for he illustrates by 
means of curves how. instead of being contracted by a fall of temperature, the 
volume of sea-ice increases down to —20° C. Between — 4’4° C. and — 6'4° C., 
for instance, the ice he experimented with (comparable to frozen water of the 
open ocean) expanded its volume 0'002957. Figures such as these, he says, are 
only comparable to the changes of volume effected in a gaseous body. According 
to Pettersson, therefore, sudden falls of temperature down to — 20° C. will not 
contract but will bend the ice upward and rupture it in that way. Many con- 
traction cracks, however, such as that of Priestley and David, are claimed at 
higher temperatures than -20° C., and field experience, therefore, is so much 
at variance with laboratory experiment that the question must be left an open 
one.* In any case, detailed descriptions of other temperature cracks are desir- 
able, and Pettersson’s experimental methods certainly want confirming. 
(ii) For the second and by far the most important and commonest type of 
crack it has been hard to find an entirely suitable name, and, though the term 
“ strain crack ” has finally been adopted, it is realised that it is not an exact term, 
as all cracks are a relief from strain. 
That the ice and its overlying load of snow were seldom in a state of equilibrium 
was a commonplace during all observations. The effect, for instance, of the irregular 
distribution of the snow-covering was always very noticeable, for there were even 
some places where the ice-surface was entirely below water. Consequently it is 
next to impossible to determine the specific gravity of sea-ice from the proportion 
above and below water-level ; even in places bare of snow there must still be 
stresses of some sort. This seems obvious, and yet the specific gravity of sea-ice, 
as given in Krummel’s Handbuch der Ozeanographie, is based on just such unsuit- 
able measurements as these made by Makaroff. 
* This discrepancy was pointed out to Mr Priestley in May 1919, who till then did not know of Pettersson’s 
work. Although without access to his notes, he was most emphatic in saying that contraction cracks were possible 
with a fall of temperature not reaching so low as — 20° G. ( — 4° F.). 
