X PREFACE. 



ence of 24 ; and thus also our winter visitors, coming as 

 they do from the North, find our winter temperature of 

 40 to be within seven or eight degrees of the temperature 

 of the country they came from ; and we observe that the 

 more severe our winter is, the further south these our 

 winter visitors go. 



It is known that a marked difference exists in the 

 average temperature of places in similar parallels of lati- 

 tude on the western coasts of the Old World and the 

 eastern coasts of the New "World, as a glance at the 

 undulations of the isothermal lines will exhibit. Thus 

 the isothermal line at Boston gives the same temperature 

 as that of London, though Boston appears to be ten 

 degrees further south ; and Iceland appears to be as warm 

 as the south point of Greenland, though situated five 

 degrees further north. 



The undulations of the isothermal lines, and the higher 

 comparative temperature of the western shores of Europe, 

 may exercise some influence in the route of water birds 

 crossing the northern portion of the Atlantic ; while the 

 large patches of floating sea-weed, sometimes occupying 

 half an acre or more, and teeming with aquatic animal 

 life in its various stages, as observed by those who cross 

 the more southern part of the Atlantic, afford both rest 

 and food to many. Thus birds of great and enduring 

 powers of flight, able moreover to obtain both food and 

 rest on the surface of the sea, may reasonably be ex- 

 pected to have a wide geographical range ; and of these 

 powers the long-winged web-footed species are good 

 examples. 



