EXPLANATION 



OF 



THE ISOTHERMAL CHARTS ACCOMPANYING THIS PAPER. 



The three accompanying charts have been constructed to show the distribution 

 of the atmospheric temperature within the limits of the United States, on the 

 averao-e during the year, and for the winter and summer seasons. 



The o-reat vakie of the graphical method consists in its capacity of bringing into 

 a connected view the result of a large mass of apparently disconnected figures, and 

 thus presenting their relations to the eye. In the present case, these relations 

 depend on the geograpliical and hypsometrical features of the country. 



The results brought out in these tables form the basis of the charts. They are 

 laid down by means of curves connecting places of equal temperature. These 

 curves may be conceived as forming the intersections of the earth's surface by a 

 series of thermal surfaces of equal temperature one above the other and for equal 

 differences of temperature. The difference, here adopted, is 4° Fah., and is the 

 same for all the charts. During the winter season the decrease of temperature 

 between the southern and northern limits of the United States is greater than 

 during the summer season, hence a greater number of curves appear on the chart 

 showing the distribution of temperature in the winter than on that for the year, 

 and the chart for the distribution in summer has the least number of curves. The 

 limiting curves are as follows : For the cold season 4° to 72° Fah., for the yearly 

 average 36° to 76° Fah., and for the warm season 56° to 88° Fah. 



From the above designation of the isotherraals it follows that each curve must 

 be continuous no matter how tortuous its course may be, that is, it cannot abruptly 

 come to an end ; of this instructive examples are presented on the chart for the 

 year by the curve of 48° Fah., and on the chart for the summer by the curve of 

 68° Fah. The construction of the curves for the yearly distribution was found 

 slightly more troublesome than those for either of the other charts, owing to the 

 way in which the mean temperature results, from the monthly means, are influenced 

 by the annual variation. Som.e difficulty was experienced in tracing out the 

 summer curves for the western part of California, owing to the well-known excep- 

 tional and remarkable distribution of its temperature, of which more will be said 

 further on. 



The want of a reliable hypsometric chart of the United States was seriously felt, 

 not one only on which the existence of hills and mountains should be correctly 

 indicated as regards position, but one, on which the actual elevations are indicated 

 by contour lines. A rough hypsometric chart of the latter description was con- 

 structed bv me to aid in the tracing out of the thermal curves, but the latter are 



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