THE FORMER EXTENSION OF GLACIERS. 1 6 I 
Paradoxical as it may appear, glaciers require 
heat as well as cold: heat to create the vapour, 
which again condenses as snow. A succession of 
damp summers would do more to enlarge the glaciers 
than a series of cold seasons. Leblanc* estimated 
that the glacial period need not have had an average 
temperature of more than 7 degrees centigrade below 
the present, and other great authorities consider that 
at anyrate a fall of even 5 0 would suffice. 
The temperature decreases 1 0 for about every 
188 metres. A fall of 5 0 would = 940 metres. 
The present snow-line being 2700 metres, would de- 
scend to 1760 metres, and the lower limit of the 
glaciers from 1 200 metres to 360 or somewhat below 
Geneva, the level of which is 375. It would indeed 
be even lower, because the greater the snow-field, the 
further the glacier descends. 
We have no evidence of the existence of Man in 
pre-glacial times, and whether he inhabited Switzer- 
land during the inter-glacial period is still uncertain. 
Rutimeyer has described certain pieces of wood be- 
longing to that period, which have been cut by some 
sharp instrument, and which are so arranged as to 
form a sort of basket-work. They certainly appear 
* Bull . Soc. Geol . France . 1843 . 
Scenery of Switzerland. I. I I 
