31 



great care must be exercised with photographic plates, for which reason 

 they are kept in tin boxes, water and air-tight. 



During the past season about seven hundred photo-topographic 

 plates were obtained, covering an area of nearly five thousand square 

 miles; besides nearly a hundred 8xio plates, making a collection of 

 photographs showing glaciers, glaciation, and glacial action of the greatest 

 interest and value. 



I will dwell for a moment upon climate, with special reference to 

 an ice age or glacial period. The absolute amount of heat received 

 annually from the sun is not known, nor the fluctuations in the emana- 

 tions from the sun, nor his rate of cooling ; all of which affect both the 

 meteorological and climatic conditions upon the earth. However, 

 certain it is, that astronomical conditions, periodic in their function, 

 must produce some effect on climate. Climate, and its offspring, metero- 

 logy, are complex subjects. They are the effects or phenomena of various 

 causes interlinked and interwoven to such a degree that up to the 

 present time their true history has not been written. We know that 

 summer and winter are due to the obliquity of the axis of the earth to 

 the plane of its orbit. By summer we understand the time from the 

 vernal to the autumnal equinox, and winter from the autumnal 

 to the vernal. The proportion of heat received in summer is to 

 that received in winter as 63 is to 37 ; and this is practically constant 

 for all time ; for the obliquity changes but very little. 



If there were no other changes in the relative position of the earth 

 towards the sun, there would be no change of climate further than that 

 indicated above ; but, as a matter of fact, the earth, revolving in an ellip- 

 tic orbit around the sun, does not preserve the same orbit through all 

 times, that is, the eccentricity changes. Furthermore the line of equi- 

 noxes passes around the ecliptic, and this, combined with the change of 

 eccentricity of the earth's orbit, produces a change in the climate by 

 changing the lengths of summer and winter. 



For instance, at present our summers are seven days longer than 

 our winters, there being 186 days between the vernal and autumnal 

 equinoxes, and 179 between the autumnal and the vernal. The time 

 required for the line of equinoxes to make a complete revolution is, in 



