Motions of the Earth. — 205 
the title I was led to expect. I cannot conceive what could in- 
duce you to suppose, that the orbicular and rotary motions of 
the earth, are the cause of that great principal attraction, of 
which you justly observe, the Newtonians and all the modern 
schuols of philosophy have acknowledged themselves ignorant. 
‘| think it is very easy to show, that these motions, which are 
themselves only effects, cannot be the cause of what in every 
point of view appears to be a first principle. If you were to 
attempt an illustration of your theory by actual experiment, I 
am persuaded you would discover its fallacy. 
A circular plane surface, ten or twelve inches in diameter, 
lying in the plane of our horizon, with grooves cut in its upper 
surface on lines drawn from the centre to the circumference, 
might have a rotary motion given to it, and if globules of mer- 
cury were put into the grooves, the centrifugal force would by 
them be exhibited, and you would find that no orbicular or any 
other motion, that you could communicate, would be able to 
bring all the globules of mercury at the same time to or towards 
the centre, which, if your doctrine was true, would undoubtedly 
be effected by giving it a circular motion, similar to the motion 
of our earth in its orbit. 
I should very much like that you would try this, or some other 
experiment, by way of illustration, before you apply your “ prin- 
ciples to the phenomena of a system of bodies moving within 
the gaseous medium of universal space.” 
\ 5 ke 
\ 
~ 
> 
— 
Let the circle OR represent the orbit of our earth; S the 
sun in the centre; E the earth; PE, a line drawn from the 
centre of the earth through the point of projection; TG a tan- 
gent of the earth; AD a diagonal of the rectangle DPA, the 
longer sides of which are to the shorter, as the orbicular motion 
18 
