INTENSITY OP SUN'S HEAT AND LIGHT. 31 



if computed intensities and observed temperatures both follow the same law of 

 change, their delineated curves will be symmetrical. 



Therefore, taking the latitudes for ordinates, and the Annual Intensities in the 

 table for abscissas, we obtain the curve of Annual Intensity (Plate III.) ; and, in 

 the same manner, the curve of Annual Temperature. It will be seen, no doubt with 

 interest, that the curve of annual intensity is almost symmetrical with that of 

 European temperatures, observed mostly on the western side of that continent. 

 But the curve of American temperature based on the U. S. Army Observations for 

 places on the eastern portion of the continent, diverges from the curve of intensity, 

 and indicates a special cause depressing these temperatures below the normal 

 standard due to their latitudes. 



At Key West, on the southern border of Florida, the divergence commences, and 

 on proceeding northwardly, continually increases in magnitude ; that is, so far as 

 reliable observations have been made along the expanding breadth of the North 

 American continent. 



It were natural to suppose that the annual temperature would be defined by the 

 annual number of heating rays from the sun. Indeed on and near the tropical 

 regions, the curves of annual temperature and solar intensity are symmetrical. But 

 in the polar regions, the irregularity of the intervals of day and night, and of the 

 seasons, and various proximate causes, introduce a discrepancy, which the principle 

 of annual average does not obviate. The laws of solar intensity, however, have 

 been determined; the laws of terrestrial temperature will require a special and 

 apparently more difficult analysis. 



It has been inferred that there are two poles of maximum cold about the latitude 

 of 80° north, and in longitudes 95° E. and 100° W. The fewness of the observa- 

 tions, however, in that remote Hyperborean region, leave this question still open 

 to investigation. The more recent " isothermal lines of mean annual temperature" 

 published by Prof. Dove of Berlin, in 1852, indicate but one pole of cold, and that 

 is very near the geographical Pole. 



SECTION VI. 



AVERAGE ANNUAL INTENSITY OF THE SUN UPON A PART OR THE WHOLE OF THE EARTH S 



SURFACE. 



Having determined the value of 2 u representing the Sun's vertical intensity 

 upon a single unit or point of the Earth's surface, let us next ascertain the average 

 annual intensity upon a larger area, a zone, or the entire surface of the globe. 

 After which, we shall glance at some of the climatic alternations which are most 

 clearly made known and interpreted by the mechanism of the heavens. 

 • Begarding the earth as a sphere whose radius is unity, cos L will be the radius, 

 and 2 7i cos L the circumference of the parallel of latitude L. It is evident that 



