THE VARIATIONS OF GLACIERS. 281 
to state that since then, and presumably before, the climate has 
oscillated about its mean ina period of thirty-five years. Professor 
Richter* by a careful study of the historical records of Alpine 
glaciers, has confirmed this, and has carried it back another 
hundred years. He has shown that the average period of the 
variations of glaciers for the last three hundred years has been 
the same as Briickner’s period. He has also shown that during 
the present century the larger number of glaciers have begun to 
advance very soon after the beginning of a cold period of increased 
precipitation, though some of the larger ones have started only 
twenty or even thirty years later. We wish now to know the 
relation connecting the time of increased precipitation with 
HhemtimeTvOnyadvancesot  olaciers as. a Tunetion ol, theim size, 
Sloper eee? 
Two theories have been advanced to explain the modus 
operandi of glacial variations. Professor Richter? thinks that 
there must first be an accumulation of snow in the xévé regions, 
which at last produces a great enough pressure to overcome the 
resistance, due to the friction against its bed, of the glacier’s 
tongue; which is then pushed forward with a greater velocity, 
resulting in an advance of the glacier; this continues until the 
drain on the wévé regions, on account of more rapid flow, exhausts 
the accumulation of snow; after this the motion almost entirely 
ceases, and the glacier melts back until another advance begins. 
Professor Forel3 calls this the ‘‘theory of intermittent flow,” 
and points out that according to it variations in the size of 
glaciers would occur entirely independently of meteorological 
changes. He had, in 1881, offered a “theory of continuous 
flow.” 4 According to this theory the glacier responds immedi- 
ately to increased snow-fall, but only in its upper part is there an 
immediate increase in velocity; this pressing forward of the ice 
™ Geschichte der Schwankungen der Alpengletscher. Zeits. des deut. u. Oest. A-V., 
1891. This is a most important paper. 
? Der Obersulzbach Gletscher, 1880-82. Zeits. des deut. u. Oest. A-V., 1883. 
3 Jahrbuch des Schw. Alp. Club, 1887-88. 
4 Bibliot. Univ. de Geneve, 1881. © 
