178 PROCEEDINGS OF THE AMERICAN ACADEMY 



Mr. Everett read a short paper on the number of primary- 

 planets belonging to our system which might be supposed 

 to remain as yet undiscovered. 



He staled that he had been " led more particularly to the inquiry 

 by the striking remark of M. Leverrier (in the Compte Rendu for 5th 

 October, 1846, p. 659) to this effect, — ' that we may hope that, after 

 thirty or forty years of observations of the new planet, we shall be 

 able to use it, in its turn, for the discovery of that which follows it, in 

 the order of distances from the sun. And so on. Unhappily we 

 shall soon fall upon stars, invisible in consequence of their immense 

 distance from the sun, but whose orbits eventually, in the lapse of ages, 

 will be traced with great exactness by means of the theory of secular 

 inequalities.' All calculations of this kind must, of course, take 

 for granted that the law of gravitation exists and operates in the remote 

 parts of the system as it does within the reach of our observations. 

 The star 61 Cygni is usually regarded as the fixed star nearest to us, 

 and this is placed by Bessel at the distance of 62,481,500,000,000 

 miles. Adopting the only supposition that we are able to make on this 

 subject, viz., that our sun and the stars are bodies of equal size and 

 density, we may conclude that the attraction of the sun extends over 

 half the interval between the sun and 61 Cygni, that is, over a space 

 of 31,240,750,000,000 miles. The law of the distances from the sun 

 at which the planets succeed each other in our system is not known ; 

 but assuming Bode's law, or, still more simply, a geometrical progres- 

 sion, as that which comes near the truth in reference to all the known 

 planets except the last discovered (regarding the asteroids as one 

 planetary system), and we should have room for ten new planets, and 

 nearly the eleventh, outside the orbit of Neptune." 



Mr. Everett also read a letter from Professor Owen, of Lon- 

 don, containing an approving notice of Dr. Meigs's paper on 

 the generation of the opossum, and expressing a strong desire 

 to receive, in behalf of the Hunterian Museum, specimens of 

 the impregnated uterus of this animal, preserved in spirits. 



" Female opossums killed between the 18th of February and the 

 6th of March would be likely to afford such specimens, — of which not 

 one exists in the museums of London, nor, I believe, in Europe. The 

 value of the specimen would be enhanced, if any of your young anat- 



