WEATnER AND GLACIATION — REEDS 297 



The temperature at which water reaches its maximum density is 

 39.2° F. (3.945° C), accordinpj to the researches of Joule and Play- 

 fair. When cmiled below this temperature water expands instead 

 of contractingr, and the expansion goes on to the total extent of 

 ^j^^ of its bulk, until freezinjr occurs, at 32° F. (0° C), when 

 there is a sudden expansive leap of neai-ly a tenth of the whole 

 volume of the water as it freezes to form ice. 



The X-ray study of ice crystals by Sir William Bragg (1925) 

 shows that ice contains an open hexagonal lattice structure consisting 

 of four molecules of water. This lattice structure reveals why ice 

 can be melted under pressure and how it is possible for water (with 

 the molecules closely compressed) to occup}' less space than ice. It 

 also accounts for the sudden and relatively enormous expansion 

 which occurs when water freezes. On becoming ice, the w^ater has 

 increased in bulk by 9 per cent, and this increase occurs instanta- 

 neously and with enormous, well-nigh irresistible, force. It also 

 explains why ice is lighter than water and why ice floats on lakes, 

 rivers, and the open sea with about one-tenth of its volume above 

 water level. 



The fresh water of lakes continues to contract with increasing cold. 

 The surface waters, being colder, and thus heavier, sink, and the 

 warmer waters from the bottom rise to the surface. The convection 

 currents thus set up continue until all of the water in the lakes has 

 been reduced to the maximum density of water, after which the 

 circulation stops, for the surface waters, when cooled to lower 

 temperatures, remain on top and grow steadily colder until they 

 suddenly freeze at 32° F. (0° C). 



SOLAR RADIATION VARIATIONS 



The sun's rays supply nearly all of the heat of the earth's surface. 

 They affect not only the weather, but also insolation and tempera- 

 lure, two of the agencies conditioning ice masses; hence a brief 

 consideration of solar radiation is important in discussing weather 

 and glaciation. 



In 1837 Pouillet invented the pyrheliometer, with which he en- 

 deavored to measure the heat of the sun. Violle, Crova, Chowlson, 

 and the Angstroms followed him and increased the knowledge of 

 the subject. Langley invented the bolometer for studying the selec- 

 tive absorption and the scattering of light in the atmosphere. With 

 this instrument he measured, during 1002. 1903, and 1904, the solar 

 radiation, corrected for atmospheric influence, and announced that 

 the intensity of solar radiation is subject to irregular variations due 

 to conditions in the sun itself. 



More recently Dr. C. G. Abbot, Secretary of the Smithsonian 

 Institution, has improved the instruments and methods of Langley 



