METEOROLOGY. 345 



It is evident that, in receding from the sphere of intensity of such 

 star, as a comet from the sun, the earth's annual temperature would 

 very slowly decrease in process of time, according to the temperature 

 of the space traversed. ' And, at a remote distance irora the stars, the 

 temperature of space ought to remain stationary; as the mean annual 

 temperature of the earth has remained for at least two thousand year* 

 past, and without doubt will so continue for ages to come. 



Section VIII. 



ON LOCAL AND CLIMATIC CHANGES OF THE SUN'S INTENSriT. 



As the principal topics under this head have been anticipated in the 

 former portions of the work, they need not here be repeated.^ The 

 inequality of winter, and especially of summer intensities in the 

 northern and southern hemispheres, has already been discussed in the 

 last Section, and ascribed to the changing position of the sun's perigee. 



Let us now pass to another local inequality, which consists in the 

 difference of daily intensities at two places situated on the same par- 

 allel of latitude, but separated by a considerable interval of longi- 

 tude. This difference arises solely from hourly change of the Sun's 

 Declination, while moving from the meridian of one place westward 

 to the meridian of the other ; the Sun in the interval attaining a 

 Higher or lower meridian altitude. 



For example, the latitude of Greenwich, near London, is 51° 28' 

 39". Following this parallel west to a point directly north of San 

 Francisco, in California, the difference of longitude is 122° 28' 2". At 

 the time of the autumnal equinox, the daily change of the sun^s decli- 

 nation is 23' 23". Consequently, in passing from the meridian of 

 Greenwich to that of San Francisco, the declination is diminished by 



rsr.s. 



When the Sun's Declination is 0, at apparent noon at Greenwich, 

 on Sept. 21st, it will be T 57". 3 S. at noon in the longitude of San 

 Francisco, on the same day ; the semi-diameter being ]5' 59" or 959" 

 for Greenwich, and 959". 1 for San Francisco. With these elements, 

 let the sun's daily intensity be computed for both places. The result is 

 50.13 thermal units for Greenwich, and 49.91 for the place north of 

 San Francisco, on the same latitude. The difference is .22 corre- 

 sponding to nearly + {° Farenheit ; and by so much the intensity upon 

 the zenith of Greenwich is greater, on the same day. 



At the vernal equinox, March 20, the sun's daily change of declina- 

 tion would be in the opposite direction, and the difference would be- 

 come — |-°F. The inequality of this species thus compensates itself in 

 theory, leaving the yearly intensity the same for all places having the 

 same latitude. 



For further reference on this point, the daily changes of declinationy 

 near the first of each month, are subjoined as follows: — 



January, 5' May, 18' September, 22' 



February, 18' June, 8' October, 28' 



March, 23' July, 5' November, 18' 



April, 23' August, IV December, 9' 



