METEOROLOGY. 343 



great snow of February, 1717, rose above the lower doors of dwel- 

 lini!;s, and in the winters which closed the years 1641, 1697, 1740, 

 and 1779, the rivers were frozen, and Boston and Chesapeake bays 

 were at times covered with ice as far as the eye could reach ; but the 

 like occurs at similar intervals in our day. Mild winters, too, have 

 intervened, and the other seasons are also very variable. The general 

 indications, however, give rise to the question, wliether there is a 

 cause of change of climate in the course of the sun ? 



About two thousand years ago, in the time of Hipparchus, 128 

 B. C, the obliquity of the ecliptic, or the sun's greatest declination, 

 was 23° 43'. It has now decreased to 23° 27^' ; therefore, at the 

 former epoch, the sun came firther north and rose to a higher alti- 

 tude in summer ; and went farther south and rose only to a lower 

 altitude in midwinter. There is then an astronomic cause of change, 

 of which we propose to determine more precisely the effect. 



Let the latitude be 40°, which is nearly the latitude of Philadel- 

 phia, 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 flourished, under the 

 latitude of Italy and Greece the summer was two-tenths of a degree Fah- 

 renheit hotter^ and the icinter 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 effects, 

 however, of cutting down dense forests, of the drainage and culti- 

 ration of open grounds and woodlands admit of conflicting inter- 

 pretation, 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 Geolo- 

 gical 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 astronomic arrangement which made 

 the daily intensities in tlie 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 



