1888.] Presidents Address. xxxvii 



A magnificent drawing of Saturn and his rings has been made by 

 Trouvelot by the aid of the 26-inch refractor at Washington ; and 1 

 am fortunate enough to be able to show you a copy of it. 



The rotation of the planet Uranus has recently presented a most 

 puzzling problem for the consideration of astronomers. As is well 

 known, the satellites revolve in a plane nearly at right angles to the 

 ecliptic and it might naturally be inferred that the axis about which 

 the planet rotates is situated nearly in the plane of the ecliptic. This 

 inference is supported too by observations of a slight bulging of the 

 disc in the plane of the satellites. But in 1884, from January to 

 June, two grayish parallel markmgs were distinctly seen at Paris 

 and Nice ; and these markings which must be assumed to lie in planes 

 parallel to the planet's equator were inclined at an angle of 40° to 

 the plane of the satellites' motion. This fact was also confirmed by 

 observations of a bright spot which indicated a time of revolution 

 of about ten hours. Does Uranus, then, at the present time rotate 

 in a plane differing widely from the plane of rotation at the far distant 

 time when the satellites were formed ? No solution of this difficulty 

 has yet been proposed. 



Whether a planet will ever be found beyond Neptune it is of course 

 impossible to say, but it is not at all unlikely that some day one will 

 be found in the course of the search for minor planets. Two attempts 

 have been made to fix the position of such a body from theoretical 

 consideration, one by Forbes in 1880 and the other by Todd about the 

 same time. Prof. Forbes relied on the fact that comets of short 

 |)eriod group themselves into families in such a way that when at 

 their furthest distance from the sun they are near the orbit of some 

 one of the major planets ; he found two considerable groups of comets 

 which reach their aphelia at distances of 100 and 300 times the earth's 

 distance, and he maintained that there was an unseen planet revolving 

 at each of these distances. Prof. Todd followed the method which 

 in the hands of Adams and Leverrier had led to the discovery of 

 Neptune. The perturbations of Neptune have not had time yet to 

 develope sufficiently, but there are certain anomalies in the motion of 

 Uranus or ^' residual errors," as they are called, not accounted for by 

 theory on which he based his investigation. It is remarkable that 

 both he and Prof. Forbes assigned places for the hypothetical planet 

 which agreed well with one another. 



I must not pass away from the planets without briefly referring 

 to the remarkable investigations of Prof. Gr. H. Darwin on the subject 



