30 PROCEEDINGS OF SECTION A. 



The more fruitful investigations concerning our satellite relate 

 chiefly to questions of temperatures, existence of atmosphere, etc. 

 Although it seems impossible at present to arrive at any certainty 

 about lunar temperatures it appears probable from the most 

 recent investigations that the lunar surface under a vertical sun 

 is hotter than boiling M'ater, and that under lunar night the tem- 

 perature gets much below any ever known on the earth, and 

 perhaps 200 or 300 below zero. All investigations with respect 

 to the existence of a lunar atmosphere have tended to a negative 

 result. 



Concerning our own planet, astronomers and physicists have 

 given us the results of some important investigations as regards 

 its density, its true figure, and the eftects of secular change in 

 the ellipticity of its orbit. The earth is generally regarded 

 by geologists as a sphere of molten matter, surrounded by a 

 comparatively thin solid crust ; mathematicians and physicists, 

 however, demand a much greater rigidity of the general mass 

 than this would admit. To resist the tide-producing action of the 

 sun and moon as the mass of the earth apparently does, would 

 require a rigidity equal to that of steel. The theories of the 

 geologist and mathematicians are therefore widely at variance 

 with respect to the condition of the earth's mass. 



While the figure of our planet is regarded as an ellipsoid of 

 revolution, tliere has always been some doubt as to the exact 

 regularity of its figure, as well as of the precise ratio of the polar 

 and equatorial diameters. Accurate goedetic measures of its 

 surface have been made only in comparatively a very few places, 

 and although the results have revealed no great departure from 

 the figure of the ellipsoid, they are insufiicient to give us certainty 

 on the point. The most recent investigations by Colonel Clarke, 

 however, point to it being an " ellipsoid of three unequal axes," 

 for he states he finds from an examination of all geodetic 

 measui'es available that different meridians have different radii of 

 curvature. But, having personal experience of the great diffi- 

 culties and uncertainties of measures of this kind with our most 

 modern instruments and methods I should regard this part of the 

 question as still unsettled. The ellipticity of the earth's figure 

 given by Colonel Clarke as between 1-293 and 1-294 is now I 

 believe generally accepted as correct. 



A very interesting point has been discussed by Dr. Croll with 

 a view of accounting for the secular climatic change indicated by 

 geological facts, tropical deposits in polar regions, and vice versa. 

 The earth revolves round the sun in an elliptical orbit, whose 

 eccentricity is now about at a minimum, yet we are nearer that 

 body in January than in July by about 3,000,000 miles — the 

 amount of eccentricity changes, and Dr. Croll savs that 850,000 

 years ago this difference was 12,000,000 instead of 3,000,000 

 miles. Under such circumstances the hemisphere, whose winter 



