210 On Terrestrial Gravilation: 
various other phenomena. Sir R. Phillips attempts to show 
that one of these effects is the cause of the other ; and calls this 
advancing human knowledge a step further—I suppose he means 
bagkwards; therefore I will leave him to pursue the course he 
has chosen. 
I am, sir, yours, &c. 
Tuomas TREDGOLD. 
XXXIII. On Sir Ricuarp Puitips’s supposed Discovery of 
the Cause of the Phenomena of Terrestrial Gravitation. 
To Mr. Tilloch. 
Sir, — To appreciate the success with which Sir Richard Phil= 
lips has defended his discovery of the cause of the phcenomena 
of terrestrial gravitation, it might, perhaps, be sufficient to re- 
mark, that he has left untouched Mr. Tredgold’s fundamental 
objection ; viz. that as neither the resistance of the atmosphere, 
nor the motions of the earth on its axis, or in its orbit, have se- 
parately any tendency to deflect towards its surface, bodies pro- 
jected upwards, it follows that their conjoint action can have no’ 
such tendency. Permit me, however, by way of commentary, to 
add, that it has long since been demonstrated, that whether a 
body be projected by a single impulse, or by many simultaneous 
impulses in different directions, the progressive motion commu- 
nicated must be reetilinear. The combination, therefore, of the 
two-fold motion of the earth with any other impulse, can, in 
projecting a body, impress on it no other than rectilinear pro- 
gressive motion; nor can these forces, after the instant of their 
joint impulse, in any way modify the direction then impressed. 
There remaius, therefore, of Sir Richard Phillips’s forces only the 
agency of the atmosphere to deflect the projectile from a right 
line into such a curve as must return to the earth. Now the 
effect of atmospheric resistance would be that of simple retarda- 
tion, did uot the rapidity of the rotary motion of the parts of 
the atmosphere augment with their altitude. A consequence 
of this circumstance is, indeed, a continual deflection of the pro- 
jectile from its initial direction: but whatever deflective force 
may be assigned to this cause, it could never make a body de- 
scribe a curve returning to, or even approaching the earth’s sur- 
face; for the very obviotts reason that the direction of its action 
must always be parallel to tangents of that surface. 
This being so, the theory of Sir Richard Phillips does not 
precisely correspond with his description of it as “a theory 
which substitutes the known motions of Nature as operative 
causes of certain physical phenomena in place of an assumed 
principle 
