40 INTENSITY OF SUN'S HEAT AND LIGHT. 



Here, for A, there is substituted its equal — ~ i— '- 960".9 ; also gene- 

 rally sin D = sin a sin T, and cosH = — tan L tan D. For secular values, if t denote 

 the number of years after, and — t before, the year 1800, 



e= 0.0167836 — .0000004163 t; P= 279°31'10" + l'.0315* ; 

 Mean obliquity o = 23° 27' 54" — 0".4645 t — 0".0000014 f. 



At the solstices of summer and winter Tis 90° or 270°, and D is a; also let the 

 latitude L be 40°, which is nearly the latitude of Philadelphia, also of southern 

 Italy and Greece. Computing now for B. C. 128, and for A. D. 1850, the daily 

 intensities at the summer solstice are 90.45 and 90.05 thermal units, and at the 

 winter solstice 28.67 and 29.04 respectively. The differences .40 and .37 must 

 correspond almost precisely to degrees of the thermometer ; and halving them for 

 the whole seasons as before described, we are conducted to the following conclusion. 

 In the time of Hipparchus, or about a century before Julius Cesar, Virgil, Horace 

 and Ovid nourished, under the latitude of Italy and Greece the summer was two-tenths 

 of a degree Fahrenheit hotter, and the winter as much colder, than at the present day. 

 The similar changes of solar intensity upon the United States in two hundred years, 

 can only be made known by theory, and are evidently very slight. There has 

 been, therefore, no sensible amelioration of climate in Europe or America from 

 astronomical causes. The effect, however, of cutting down dense forests, of the 

 drainage and cultivation of open grounds and woodlands admit of conflicting 

 interpretation, and appear but secondary to the atmospheric fluctuations which are 

 governed by the changes in the relative position of the earth and sun. 



Before leaving the subject, the inquiry may arise respecting Geological changes, 

 whether the secular inequalities have ever been of such value under the present 

 order, as to admit of tropical plants growing in the temperate or frigid zones. In 

 reply, as the annual intensity could never have varied in any considerable degree, 

 the change must consist entirely in tempering the extremes of summer and winter 

 to a perpetual spring. And this could not happen on both sides of the equator at 

 once; for the same arrangement which made the daily intensities in the northern 

 hemisphere equable, would subject those of the southern to violent alternations; and 

 the wide breadth of the torrid zone would prevent the effects being conducted from 

 one hemisphere to the other. 



Let us then look back to that primeval epoch when the earth was in aphelion at 

 midsummer, and the eccentricity at its maximum value — assigned by Leverrier 

 near to .0777. Without entering into elaborate computation, it is easy to see that 

 the extreme values of diurnal intensity, in Section IV, would be altered as by the 



multiplier (—=1), that is 1 — 0.11 in summer, and 1 + 0.11 in winter. This 

 x I ± e I 



would diminish the midsummer intensity by about 9°, and increase the midwinter 

 intensity by 3° or 4° ; the temperature of spring and autumn being nearly unchanged. 

 But this does not appear to be of itself adequate to the geological effects in question. 

 It is not our purpose, here, to enter into the inquiry, whether the atmosphere 

 was once more dense than now, whether the earth's axis had once a different incli- 

 nation to the orbit, or the sun a greater emissive power of heat and light. Neither 



