384 PLEASANT IVA YS IN SCIENCE. 



in the eighteenth century. " In the evening of December 

 9," he says, "the air began to be so very sharp that we 

 thought it would be curious to attend to the motions of a 

 thermometer; we therefore hung out two, one made by 

 Martin and one by Dolland" (sic, presumably Dollond), 

 "which soon began to show us what we were to expect; 

 for by ten o'clock they fell to twenty-one, and at eleven to 

 four, when we went to bed. On the loth, in the morning 

 the quicksilver in Dolland's glass was down to half a degree 

 below zero, and that of Martin's, which was absurdly 

 graduated only to four degrees above zero, sank quite into 

 the brass guard of the ball, so that when the weather be- 

 came most interesting this was useless. On the roth, at 

 eleven at night, though the air was perfectly still, Dolland's 

 glass went down to one degree below zero !" The note of 

 exclamation is White's. He goes on to speak of "this 

 strange severity of the weather," which was not exceeded 

 that winter, or at any time during the twenty-four years of 

 White's observations. Within the last quarter of a century, 

 the thermometer, on more than one occasion, has shown 

 two or three degrees below zero. Certainly the winters 

 cannot be supposed to have been ordinarily severer than 

 ours in the latter half of the last century, when we find that 

 thermometers, by well-known instrument makers, were so 

 constructed as to indicate no lower temperature than four 

 degrees above zero. 



Let us return, after this somewhat long digression, to the 

 levelling action of rain and rivers. 



If we consider this action alone, we cannot but recognize 

 in it a cause sufficient to effect the removal of all the higher 

 parts of the land to low levels, and eventually of all the 

 low-lying land to the sea, in the course of such periods as 

 geology makes us acquainted with. The mud-banks at the 

 mouths of rivers show only a part of what rain and river 

 action is doing, yet consider how enormous is the mass 

 which is thus carried into the sea. It has been calculated 

 that in a single week the Ganges alone carries away from the 



