150 CARNEv-IE INSTITUTION 



III. Tpie Reskaschics Now Most Needed. 



The heavenly bodies, to the investigation of which astronomical 

 research is directed, are 01 two classes, the one comprising the bodies 

 of the solar sA'stem, the other the fixed stars. 



With respect to the first, it may be said that such subjects as the 

 phenomena presented by the sun and the question of the physical 

 constitution of this body, the a.spects and rotation of the planets, the 

 nature of the comets, and celesti&.l physics geuerallj' either require 

 onlj' individual effort or are, in most cases, adequately provided for. 

 Cases in which an investigator is in urgent need of more help than 

 be command? can be dealt with singly as they arise. 



In what concerns the motion'r of the planets it is believed that the 

 existing tables satisfy all requirements. 



The case is quite different with the satellites. The want of new 

 tables of the moon is one of the most urgent iu our exact astronomy, 

 and one of the most difficult to .supply. This difficulty arises from 

 the two facts that the problem of the moon's motion is the most 

 complicated of mathematical astronomy, while the amount of Jabor 

 involved in carrying out any solution of it is greater than in the case 

 of any other heavenly body. The promotion of the necessary pre- 

 liminary research and the construction of new tables of the moon's 

 motion, therefore, seem to me of the first order of importance. 



Tables of the satellites of Jupiter are also greatlj^ needed. It is 

 believed that one, or perhaps two, competeiit astronomers are ready 

 and desirous to undertake the preparation of such tables, but are 

 deterred by the amount of labor involved and the difficulty of the 

 problem. 



Most interesting problems are altorded by the satellites of Saturn 

 and Mars. It is believed, however, that the.se can be adequately 

 dealt with by individuals, and therefore need no promotion by the 

 Carnegie Institution. The same remark will apply to the satellite 

 of Neptune. The writer is engaged in a discussion of the obser- 

 vation of this body, but needs no additional a.ssistauce. 



A question of transcendent importance in astronomy is wdiether 

 the force of gravitation varies exactly as the inverse square. There 

 is now strong reason to su.spect that such is not the case. If a devia- 

 tion can be fully established, its explanation ma}' open up a new 

 field in physics. The work bearing on this question, which is not in 

 regular progress, and yet requires most urgently to be done, is a 



