Prentk National Inshtiiie^ 3^1 



Tills new planet, therefore, so difficult to be seen, and 

 the theory of wlsich still announces greater difficulties to 

 those who may wish to determine it^ by calculating all the 

 perturbations it may experience, seems sufficiently known 

 to be found without much trouble, when disengaged so far 

 from the rolar rays as to be again visible. 



Solsiiiial Equhw£C3. 



We gave an account the preceding year of the observa- 

 tions made with Borda's circle at the observatory Rue dc 

 Paradis, to determine the equinoxes and the solstices. Since 

 our last public sitting the same astronomer has observed two 

 new equinoxes and two solstices ; for th-e sky was too often 

 obscured during the latter part of Frairial : yet, as he let slip 

 no opportunity, we have already collected a great number of 

 observations of the solstice which took place in the night 

 between the second and third of" Messidor, to be assured 

 (hat the observations which we hope still to make may pro- 

 duce great changes in ihe definitive result. 



The observation of the solstices was employed by the an- 

 tient astronomers to determine the length of the longest 

 day in every climate, and the height of the pole for the 

 place of observation. At jiresent, we have means much 

 more precise for ascertairiing the height of the pole; and 

 in regard to the length of the longest day, we already know 

 it with more than sufficient exactness. But the solstices 

 are still no less interesting to astronouiy, which has no other 

 method so natural of determining the obliquity of the eclip- 

 tic ; that is to say, the angle formed by the planes in u-hicli 

 the annual and diurnal revolutions of the earth are effected ; 

 the fundamental element v.'hich enters into all our calcula' 

 tions, and the fixing of which is a matter of so much deli- 

 cacy, that observations cannot be multiplied too much to 

 determine properly either the preciseextent it hadat a given 

 period, or the variation which it annually evj)cricnces. 



It results from the observations of which we here give an 

 account, that by a mean of twelve solstices, both of winter 

 and bunimcr, the mean obliquitv must have been 23"^ '2/' 

 57" at the conunenctment of the IQth century, and that 

 it would l>e less by l" or 2" if we referred merelv to the 

 last summer solstice. The annual diminution is still much 

 more dilhcult to be known, since it supposes excellent ob- 

 servations made at two periods sufficiently distant from each 

 other. Theory would give it with more precision, were \\c 

 not obliged to suppose a mass respecting which there still 



remains 



