24 president's address. 



In this marvellous manner, Neptune, the fourth of the major 

 planets, was added to the known members of the solar system^ 

 and the perturbations observed in the motion of Uranus were 

 ever after duly accounted for. Thus it was Sir Isaac Newton 

 who explained the laws of universal gravitation, by which the 

 heavenly bodies move in space ; while it was reserved for Adams 

 and Le Verrier to interpret these laws, and to indicate where a 

 hitherto unknown planet could be found. Newton recognised 

 laws not previously explained, and Adams and Le Verrier, by 

 the highest mathematical analysis, inferred from them the 

 existence of a world that had never before been seen as a planet 

 by the human eye. 



Prom the preceding remarks we may easily conclude that 

 the science of astronomy must be considered as pre-eminently 

 one of calculation and prediction — calculation of the past and 

 prediction of the future. The first object that enters the 

 astronomer's mind is therefore to extract laws and numerical 

 elements from the phenomena that have occurred; while his 

 second object is to apply these laws on the assumption of their 

 invariability to the phenomena that will occur. By this means, 

 any error that may have been committed in these fundamental 

 assumptions can, by a comparison of the predicted with the 

 corresponding observed results, be accurately ascertained. If 

 we examine successive stages in the history of physical astronomy, 

 we shall find that in all the various forms which the science has 

 taken at different periods, we have certainly presented to us, 

 either the struggle of reducing laws and elements to agreement 

 with new phenomena, or the anxious search for some hitherto 

 neglected causes of discordance, such as the effect of the 

 perturbations on the motion of Uranus, produced by the powerful 

 attraction of Neptune ; or finally, the triumph of finding that 

 assumptions were well founded, and that the agreement between 

 observation and theory is sufiiciently exact. The last of these 

 conditions has been amply verified by the most recent invest- 

 igations of the lunar and planetary theories, which now represent 

 the motions of the sun, moon, and planets, sufficiently near for 

 all practical purposes. 



This intellectual advance in theoretical astronomy is owing, 

 in a great measure, to the noble work of M. Le Yerrier, who 



