664 THE POPULAR SCIENCE MONTHLY. 



level of the water is not more than two inches. A line of clear, blue 

 ice marks the boundary of each layer, produced, in the opinion of 

 various writers, by the melting of the top portion of the snow-fall of 

 a previous winter, while the white part represents the unmelted part 

 of the snow-fall. "Wilkes estimates the snow-fall in this region at 

 thirty feet in the year, and, as only a very small part of this can be 

 melted in the course of the short summer, an immense accumulation 

 must go on. The small part of each year's fall that is melted will be 

 clear ice, and this is represented by the line of blue in the berg ; so 

 that, by counting the number of layers in a berg, some idea can be had 

 of its age. 



Sir Wyville Thomson says that the reduction in thickness of the 

 layers from top to bottom is due mostly to compression, and estimates 

 that at a depth of 1,400 feet enough heat will be generated to melt 

 the ice. On the other hand, Croll contends that the temperature of 

 the bottom of the immense icebergs, which sometimes tower 700 and 

 1,000 feet above the water, is 20 or 22 Fahr., and when we remember 

 that ice floats with from six to seven times its height below water, 

 and that the bottom of a 1,000-foot berg would be 6,000 or 7,000 feet 

 below the surface, it is easily seen that no melting would occur on 

 the bottom of an ice-sheet only 1,400 feet thick. The reason assigned 

 by Croll for the gradual thinning of the ice-layers toward the bottom 

 is, that, instead of being due solely to compression, it is due mainly 

 to what he terms dispersion. In other words, if, at 85 south, a mass 

 of ice covers one square foot of surface, it will, in its gradual passage 

 north, be made to cover two square feet at 80 latitude ; at 70 it will 

 occupy four square feet, and, at 60 south, six square feet ; that is to 

 say, a stratum which was one foot thick in 85 latitude will be only 

 two inches thick when it has reached 60 latitude. 



The discharge of icebergs from the extremity of the ice-field takes 

 place constantly, and they are sometimes huge. Croll has collected 

 notices of bergs which towered 400, 580, 720, 960, and 1,000 feet 

 above the sea-level, and, as six or seven times the bulk above water 

 floats below, some adequate idea can be formed of the contents of a 

 berg three, four, or five miles in length. As the ice-field must have 

 an onward motion to cause this discharge, it has been calculated that 

 one foot in 211 is the smallest slope which will be effectual, and that 

 the ice moves here at the rate of about one quarter of a mile per an- 

 num. As it is impossible that thirty or forty feet of snow can be 

 melted in a single short summer's season, it follows that there must be 

 a rapid accumulation all the time. This accumulation will probably 

 be greatest at the pole, and Croll estimates that here the ice has at- 

 tained a thickness of about seven miles. Allowing this estimate to 

 be correct, and it does not seem excessive, when it is remembered that 

 the thickness of the ice-sheet over Northern New England, during 

 the Glacial period, was 6,000 feet, the question arises, Upon what sort 



