104 EXPLANATION OF THE ISOTHERMAL CHARTS. 



not what they might be, respecting accuracy in detail, were we in possession of an 

 elaborate hypsometric chart. 



On each chart was plotted the mean temperature for the respective period, 

 corrected for daily variation, if necessary, for all the available stations within the 

 area of the chart. On the east of the Mississippi all series extending over five 

 years or more were given to the nearest tenth of a degree of Fahrenheit, those of 

 less than five years' duration were set down to the nearest whole degree. 



The decimal point marked the position of the place. For stations west of the 

 Mississippi the limit of 3° was adopted instead of 5°. The curves were constructed 

 with due regard to the elevations of the ground, producing a resemblance, for short 

 distances, of the thermal curves to contour lines of equal elevation. The isother- 

 mals thus constructed are not reduced to the sea level for the following reasons. In 

 the first place, we desire a knowledge of the true distribution of the temperature 

 near the surface to which we are actually exposed and which aff'ects agricultural 

 and other pursuits, and not of any artificial distribution under special, qualified 

 conditions such as the reduction to the sea level ; in fact we might as \^'ell correct 

 also for propinquity to the sea, for prevailing wind, for proximity of table-land or 

 large lakes, nature of the soil, and a variety of other disturbing causes, which 

 process would finally bring about a close conformity of the isothermals with 

 parallels of latitude, and would represent what has been called the solar climate. 

 Moreover, we do not possess the precise data for such a reduction; thus to expe- 

 rience a diminution of 1° Fah. in the atmospheric temperature, near the surface, 

 the average values vary between 250 and 500 feet of rise, and at elevations beyond a 

 mile, the change in altitude must be greater for the same diff'erence in temperature. 

 Besides, the law is different in the different seasons. It is proper to connect the 

 decrease of temperature in altitude with the decrease of pressure to which it is 

 supposed proportional (when starting from the absolute zero of temperature), a 

 fall of 1° of temperature corresponds approximately to a decrease in pressure of 

 nearly 0.25 inch, the barometric column indicating about 29 inches, and to 0.35 

 inch nearly for pressure at and below 27 inches. 



On the other hand, if the meteorological stations were sufficiently numerous and 

 equally distributed in area, the isothermal curves drawn among them would them- 

 selves furnish the best means of ascertaining the separate effects on the climate 

 (temperature) of the various modifying elements of elevation, slope, surface condi- 

 tion (wooded or barren), and many other circumstances. 



If we review the indications presented by each chart separately and notice only 

 the leading characteristic features of the distribution of temperature, we may con- 

 veniently divide the area of the United States into two parts, viz. : that east of the 

 100th meridian, of comparatively small elevation, generally below 1000 feet and 

 only exceptionally rising to 4000, and that west of this meridian, with an elevation 

 generally above 4000 feet, and not unfrequently attaining the altitude of 10,000 

 feet and above. 



When referring to the isothermal curves in the description of the charts, those 

 referring to the yearly period will simply be designated as "isothermals," those 

 referring to the winter as " isocheimals," and those referring to the summer as 

 "isotherals." 



