18 A JOURNEY IN BRAZIL. 



fins acting as sails rather than wings, and carrying them 

 along on the wind. They skim over the water in this way 

 to a great distance. Captain Bradbury told us he had 

 followed one with his glass and lost sight of it at a consider- 

 able distance, without seeing it dip into the water again. 

 Mr. Agassiz has great delight in watching them.* Having 

 never before sailed in tropical seas, he enjoys every day some 

 new pleasure. 



April 9th. Yesterday Mr. Agassiz lectured upon the 

 traces of glaciers as they exist in the northern hemisphere, 

 and the signs of the same kind to be sought for in Brazil. 

 After a sketch of what has been done in glacial investigation in 

 Europe and the United States, showing the great extension of 

 ice over these regions in ancient times, he continued as fol- 

 lows : " When the polar half of both hemispheres was covered 

 by such an ice shroud, the climate of the whole earth must 

 have been different from what it is now. The limits of the 

 ancient glaciers give us some estimate of this difference, 

 though of course only an approximate one. A degree of 

 temperature in the annual average of any given locality 

 corresponds to a degree of latitude ; that is, a degree of 

 temperature is lost for every degree of latitude as we travel 

 northward, or gained for every degree of latitude as we travel 

 southward. In our times, the line at which the average 

 annual temperature is 32, that is, at which glaciers may be 

 formed, is in latitude 60 or thereabouts, the latitude of 

 Greenland ; while the height at which they may originate in 

 latitude 45 is about 6,000 feet. If it appear that the ancient 

 southern limit of glaciers is in latitude 36, we must admit 

 that in those days the present climate of Greenland 

 extended to that line. Such a change of climate with 



* See Appendix No. II. 



