68 Is Light Imponderable ? 



IS LIGHT IMPONDERABLE ? 



What do we mean when we speak of any object around us, 

 and call it a ponderable substance, or one that has weight ? 

 Simply that the body in question attracts, and is attracted by 

 the mass of our earth. No doubt it also attracts, and is 

 attracted by, the moon, the sun, and other celestial bodies ; 

 but from their great distance their attraction is so much 

 diminished, that we take no account of it in relation to masses 

 of moderate dimensions. The tides result, as we all know, 

 from the attractive force exerted upon the waters of our globe 

 by the sun and moon ; and if we could put the Pacific or the 

 Atlantic ocean into a scale, the weight of those fluid masses, 

 or in other words the amount of force with which they pressed 

 towards the earth's centre, would noticeably vary, as it was 

 more or less counteracted by opposing forces exerted by the 

 sun and moon. The weight of bodies on the globe, or on any 

 other planet whose form differs from a true sphere, varies 

 according to its position. Our globe is a little flattened at the 

 poles, and thus any body taken from the equator to either pole 

 approaches a little nearer to the earth's centre, and is more 

 powerfully attracted. The projecting equator, being larger in 

 circumference than a circle near the poles, has to move faster 

 than the latter as the earth rotates, or it would be left behind, 

 which we know is not the case. Now the centrifugal force 

 arising from rotation tends to throw any body off, while the 

 gravitation attraction of the earth tends to pull it towards its 

 centre. Hence the weight of a body, measured by the force 

 with which it tends to the earth's centre, is diminished at any 

 point at which the centrifugal force is increased. Thus two 

 circumstances cause a difference in the weight of bodies moving 

 from the equator to the poles. " Owing to the elliptical 

 form of the earth, alone, and independent of the centrifugal 

 force, its attraction ought to increase the weight of a body 

 in going from the equator to the pole by almost exactly 

 3-i-oth part, which together with -a-g-g-th due to the centrifugal 

 force, makes up the whole quantity, T^th, observed."* 



Weight, then, is simply a result of attraction ; and if we 

 desire first to weigh a body at the equator and then at the 

 poles, in order to ascertain the difference, we must obtain an 

 invariable standard of comparison. Counterpoising it with a 

 weight or lump of metal clearly would not do. For, suppose 

 we had one pound of iron as the thing to be weighed, and 

 another pound of iron as the thing to weigh it by, it is obvious 



* Sir J. Hcrsehel, Outlines of Astronomy, p. 150, seventh edition. 



