700 Some Results of Gravitation . [December, 
fall towards another it is scarcely correct to say that the 
vigour of attraction constantly augments, with the idea of 
an instantaneous projection of increased increments of at- 
traction from one particle to the other. It seems more 
correct to say that the effectiveness of a fixed and constant 
vigour of attraction augments as its leverage increases, or as 
the solid angle subtended by the falling particle increases in 
dimensions. The central particle seems to send out lines of 
attraction which extend throughout space, with equality of 
possible energy at every distance. The number of these 
lines cut by a mass of unit thickness determines its gravita- 
tion towards the particle. If it approach the particle it cuts 
an increased number of lines ; if it recede it cuts a dimi- 
nished number ; and the effective attraction increases or 
diminishes accordingly. If, finally, the attracted mass ex- 
ceed the unit thickness, it may be viewed as a series of 
separate unit masses, each feeling a separate vigour of 
attraction. If it vary from the unit density the attractive 
vigour also varies in accordance, there being more or fewer 
of the lines of possible attraction cut by the matter of the 
mass. This idea of lines of attraction is given but as an 
illustration. There is no real attraction except where matter 
is encountered, and such a line encountering a small quantity 
of matter would feel a far less vigorous attraction than one 
which encountered a great quantity of matter. The aCting 
particle is, of course, forced to move in the direction in which 
the effective energy of its lines of attraction is greatest. 
Leaving this abstract deduction from the principle of 
Gravitation, we may proceed to review certain concrete re- 
sults of its aCtion, in connection with the aCtion of other 
physical forces. Returning again to the question of terres- 
trial gravity, it may be well to consider the results likely to 
arise during the condensation of a nebular mass into the 
present condition of the earth, through the aCtion of gravity. 
In this process gravity has one opposing energy to overcome 
— that of heat. And the final result must depend on the 
inter-relations of attraction and of heat dissemination. 
If the density of the original nebula approached uniformity, 
then the greatest energy of attraction towards the centre 
must have been felt at the surface. The process of con- 
densation, however, would soon produce an interior com- 
pression, the region of greatest weight per unit volume 
extending inwardly. But this compression would be resisted 
by the generally distributed heat of the interior matter. 
And as the centre was approached condensation would be 
less and less assisted by gravity as a direCt agent, since 
