318 Scientific Intelligence. 
third its amount. If it could be supposed possible that the 
moon’s mean motion and the earth’s axial rotati 
together, the observations of the moon and of Mercury’s transits 
and 1875 was really less than it was between 1720 and 1800. | 
The discordance between the observed and the theoretical 
of the perihelion of Mercury is greater by 43 seconds than the 
theoretical motion computed from the best attainable values of 
t f planets. In speculating upon the possible 
causes of this excess of motion, Professor Newcomb says, “ 1he 
ons. 
“In the first place, on any probable hypothesis of the relation of 
mass to reflecting power, it is impossible that a planet or group of 
planets of sufficient mass to produce the observed motion of the 
perihelion of Mercury could exist without being very conspicuous 
objects during total eclipses of the sun, if at no other time. e 
cannot, indeed, assign an exact value to the mass unless we know 
the mean distance. But the less we suppose the mean distance, 
and therefore the greater we suppose the liability that the planet 
should be lost in the sun’s rays, the greater the mass required 
e the 
f the 
perihelion. But observations do not indicate any such exce® 
If, therefore, the group exists its plane must be very nearly coll 
cident with the orbit of Mercury. But here we meet with two 
difficulties : ae 
“If the mean plane of the group were at any epoch coincide 
with that of Mercury, it could not remain so permanently, Pvt 
the planes of the different orbits would, in time, group themse ves 
