PHILOSOPHICAL SOCIETY OF WASHINGTON. 43 



Two important results were obtained from these experiments ; 

 the face of the plate was rendered very much harder than the 

 copper-plate, permitting ten times the number of impressions to 

 be taken from it; and second, the plate was rendered, by the 

 "iron-facing" process, permanently magnetic, having a polarity 

 north and south in the vertical direction of the plate when the 

 deposit was made. 



Remarks were made by Messrs. Dutton and E. B. Elliott, 

 and by Mr. Henry on the magnetic condition of these plates. 



Mr. Button made some observations on 



THE CAUSES OP GLACIAL CLIMATE, 



reviewing the various theories, geographical and astronomical, 

 on the subject; rejecting, after analysis, Mr. Croll's astronomical 

 theory as inadequate to account for the observed facts ; and pre- 

 fering Sir Charles Lyell's geographical theory based on the 

 distribution of land and water, as meeting the demands of the 

 question in a more satisfactory manner. 



Mr. Taylor, in reply to Mr. Button, thought that Mr. Croil's 

 "astronomical" theory had not been fully presented. That 

 theory did not assume any diminution of solar heat upon the 

 northern hemisphere when its winter solstice occurred at aphe- 

 lion during the period of maximum eccentricity, but merely a 

 change of distribution of the same annual amount of radiation 

 received, between a prolonged winter of increased severity, and 

 a shortened summer of proportionally increased severity. 



The great variety of physical results flowing from a slight 

 annual accumulation of snow in one hemisphere, continuing for 

 many thousands of years, producing an ice cap many thousands 

 of feet thick at the pole, and extending probably half-way to the 

 equator, displacing the centre of gravity of the earth several hun- 

 dreds of feet northward, occasioning a corresponding overflow of 

 the Northern Ocean, or an apparent submergence of considerable 

 portions of the northern continents, affecting the relative force of 

 the northern and southern trade- winds and a southward pressure 

 of the thermal equator, a similar change in the ocean currents, 

 etc., all accord well with the observed conditions of the glacial 

 epoch, while they all serve to re-enforce and intensify the aggre- 



