produced entirely by the attraction which one body (the sun or the moon) exerts separately upon every separate particle on the earth. Upon these I have already spoken, (see page 175); and there will be no need for me to detain you further at present, because you will have been sufficiently aware that there is general conformity between the results which I obtained upon Newton's theory and the results obtained by actual observation. With regard to the numerical agreement, I shall make some remarks presently, when I speak of the mass of the moon.
I will now speak of the ellipticity of the earth; and this, it will be found, is a case in which it is necessary to consider every particle as attracting every particle. First of all you will remember that when the hoop, Figure 23, is put in motion round the vertical spindle, it changes its form. Now in order to explain this, there is a term commonly used which I believe I have not in these lectures hitherto uttered; the reason is, that I do not like it; I allude to the term "centrifugal force." In order to explain why this hoop expands horizontally when it is whirled round the vertical axis, I must recall to your minds the first law of motion. The first law of motion as applied to the hoop is this: if the part a of the hoop is put in motion horizontally, it would go on in a horizontal straight line if it could. No matter what may be the nature of the force which puts a in motion, it has no tendency to move in a circle; and if it were set free, as a stone from a sling, it would immediately fly off in a straight line. And by motion in a straight line, it would go further and further still from the central bar. In order to keep it at the same distance from the central bar, a restraining force is necessary. The term "centrifugal
