172 



SCIENCE. 



[Vol. I., No. 6. 



if the formation of glaciers depended exclu- 

 sivel3' on precipitation, it would not be greatly 

 influenced by a general change of temperature. 

 The actual influence is exerted chiefly through 

 the agencies of dissipation ; to the considera- 

 tion of which we now iiass. 



The dissipation of the snow is accomplished 

 partly by evaporation and partlj' bj' melting. 

 Whether one process or the other preponder- 

 ates, depends upon circumstances ; and in the 

 case under consideration we do not know their 

 relative importance. We have therefore made 

 separate computation of the ratios of melting 

 and evaporation. Melting takes place onlj' 

 during the period we have designated ' sum- 

 mer ' ; and its rate during that period is meas- 

 ured by the mean temperature, expressed in 

 centigrade degrees. If, therefore, we multiply 

 the length of the ' summer ' in each case by 

 its mean temperature, we obtain a number in- 

 dicative of its relative, power to melt snow and 

 ice. These numbers are given in line XIV., 

 and exhibit a wide range ; the rate of melt- 

 ing with a general temperature 6° higher than 

 the present being nearly three times as great 

 as the present, and the rate with a general 

 temperature 6° lower than the present being 

 less than the sixtieth part of the present 

 rate. 



Evaporation is not restricted, like melting, 

 to the ' summer ' period, but goes on during 

 the entire j'ear whenever the atmosphere is not 

 saturated with vapor. Strictly speaking, its 

 rate is measured by the difference between the 

 amount of moisture actually in the air and the 

 amount necessary to produce saturation. We 

 have no direct means of ascertaining this rate 

 for our assumed cases ; but it seems reasona- 

 ble to suppose that the relative humiditj', or 

 the ratio of mean actual vapor-teiJsion to the 

 tension due to saturation, would be the same 

 in all the cases ; and upon this postulate the 

 rate of evaporation for each case is measured 

 by the tension of saturation dne to the mean 

 annual temperature. These tensions are given 

 in line XVI., and the deduced rates of evapo- 

 ration in line XVII. These numbers do not 

 increase so rapidlj- as those expressing the 

 melting-power ; but they indicate that the rate 

 of dissipation bj^ e^■aporation is doubled b}^ a 

 general rise in temperature of 9°. 



Since, then, a rise of general temperature 

 diminishes slightlj' the solid precipitation, and 

 at the same time increases greatly both the 

 rate of melting and the rate of evaporation, it 

 is evident that It is not favorable to the forma- 

 tion of glaciers ; and we shall obtain the same 

 qualitative result, whatever we assume to be 



the relative importance of melting and evapo- 

 ration. For the sake of reaching a definite 

 quantitative result, we will make the arbitrary 

 assumption that the snow now precipitated at 

 St. Bernard loses two-thirds of its volume by 

 evaporation and onl}- one-third by melting. 

 This gives for the ratios of dissipation the 

 numbers contained in line XVIII. Dividing 

 the relative snowfall (XI.) by the relative 

 dissipation (XVIII.), we obtain the ratio of 

 snowfall to snow-dissipation (XIX.) which 

 may be taken to express the tendency to the 

 formation of glaciers. This tendeucj'^ appears 

 to be increased two and one-half times bj' 6° 

 lowering of general temperature, and dimin- 

 ished nearlj^ two-thirds bj- a corresponding 

 advance of temperature. Considering the en- 

 tire range of temperature indicated b}' the 

 hj'potheses, each increment of 4^° doubles the 

 conjoint power of evaporation and melting to 

 remove the precipitated snow. 



It is, of course, not imagined that this anal- 

 ysis takes account of all the climatic factors 

 afiecting the problem ; but it is believed that 

 no omitted factor can modify the qualitative 

 I'esult. One of the most important of the 

 ignored considerations is that of the influence 

 of rain upon the rate of melting. There is no 

 waj' in which the heat of a warm current of 

 air is communicated so rapidly to a bed of 

 snow or ice as by means of the precipitation 

 of rain ; and, since rainfall is necessarilj' in- 

 creased bj- rise of temperature,' our results 

 would be somewhat strengthened if this factor 

 were taken into account. 



Another factor of possible importance is 

 connected with the velocit}' of air-currents. 

 The circulation of the atmosphere is caused 

 by diff'erences of temperature, and these difler- 

 euces arise from solar heating ; so that an aug- 

 mentation of solar heat tends to accelerate the 

 aerial currents. This acceleration would prob- 

 ably not be great for the range of temperatures 

 here considered ; nevertheless, it would be 

 worthy of consideration if we were able to give 

 a quantitative expression to its effects. One 

 of these effects would be an increase of pre- 

 cipitation, including an increase of snowfall ; 

 another would be an increase of the rate of 

 melting ; and a third would be an increase in 

 the rate of evaporation. In their relation to 

 our results, these effects might perhaps neu- 

 tralize one another. 



The problem we have thus examined is b^- 

 no means simple, and it is not impossible that 

 some meteorologie fallacy lurks behind our 

 figures ; but, until it shall be pointed out, we 

 are constrained to believe that one of Pro- 



