-1859] WRITINGS OF JOSEPH HENRY. 71 



from Lake Winnipeg to the Gulf of Mexico, will divide the 

 whole surface of the United States into two nearly equal 

 parts. This statement, when fully appreciated, will serve to 

 dissipate some of the dreams which have been considered as 

 realities as to the destiny of the western part of the North 

 American continent. Truth however transcends even the 

 laudable feelings of pride of country; and in order properly 

 to direct the policy of this great confederacy, it is necessary 

 to be well acquainted with the theatre on which its future 

 history is to be enacted and by whose character it will mainly 

 be shaped. 



Temperature. — Let us now consider the distribution of tem- 

 perature of the wide belt across the continent of North 

 America which forms the territory of the United States. 

 To illustrate this, attention is requested to the lines drawn 

 from east to west across the small map so frequently refer- 

 red to. These it will be seen, are of three kinds: first, 

 the full line, indicating the mean or average temperature 

 of the year; second, the broken line, denoting the mean 

 temperature of summer; and third, the dotted line, that of 

 winter. These lines are drawn through portions of the 

 earth's surface having equal temperatures for the periods 

 mentioned, and are plotted from the result of numerous 

 observations. They do not however in all cases exhibit the 

 actual temperature of the surface ; for in order to show their 

 relations and render them comparable with each other and 

 with similar lines in other parts of the world, it is necessary 

 that the observed temperatures in elevated positions should 

 be reduced to that of the level of the sea; and in the con- 

 struction of this map allowance has consequently been made 

 for decreasing temperature of one degree for every 333 feet of 

 altitude. The map therefore will present to the eye the lines 

 along which the temperature of the air would be equal for 

 the periods mentioned, were we to suppose the mountain 

 ranges entirely removed and the air brought down to the 

 level of the ocean. 



These lines, at a glance, exhibit remarkable curvatures, 

 particularly in the western portion of the United States, indi- 



