INTRODUCTION 



served motion of the moon's perigee I find that the index which 

 the inverse square law contains does not differ from 2 by a frac- 

 tion so great as % 00,000,000. 



And this, also, from the pen of Charles Lane Poor, 

 Professor of Astronomy in Columbia University (The 

 Solar System, p. 169) : 



Le Verrier discovered a slight irregularity in the motion of 

 Mercury, which for over a half a century has been a source of 

 trouble to astronomers and has led Newcomb to question the ex- 

 actitude of the law of gravitation. The perihelion of Mercury's 

 orbit has a secular perturbation, or regular forward movement, 

 amounting to 579." 16 per century. Taking into account the dis- 

 turbing action of all the known bodies in the solar system, New- 

 comb shows the law of gravitation will account for only 537/'62 ; 

 or the perihelion of Mercury moves forward along the plane of 

 the orbit by some 41. "54 per Century in a manner that cannot be 

 accounted for. In his Astronomical Constants Newcomb dis- 

 misses many possible explanations of this anomalous motion, and 

 after careful treatment discards them all as untenable. He 

 shows that this motion cannot be due to erroneous determination 

 of the masses of the various planets, nor to hitherto undiscovered 

 planets. For a readjustment of the masses,or the introduction in- 

 to the system of new bodies sufficiently large to explain the dis- 

 crepancy, will introduce serious discordances into the motions of 

 the other planets. He seems to accept as the most probable hy- 

 pothesis, first propounded by Hall, that the gravitation of the sun 

 is not exactly as the inverse square but that the exponent of the 

 distance is 2.0000001574, instead of 2. He provisionally accepts 

 this as a working theory and introduces it into the computation of 

 his tables of planetary motion. 



THE SOURCE OF THE SUN'S HEAT 



Newton sought to explain the light and heat of the 

 sun by supposing the original chaos to have been com- 

 posed of two main kinds of matter, luminous and non- 

 luminous, and then picturing the Creator as segregating 

 the first kind into the central body that we call the sun, 

 and molding the rest into the earth, moon and planets. 

 A century later Laplace postulated incandescence for his 

 nebula and thence derived, not only our glowing luminary 

 but the planets, also, in an original state of fusion. 

 Shortly before the middle of the past century the theory 



