Weighing Moonlight

Though we speak of the noontide sun as "beating down" on our heads, it does nothing of the kind. Both sun and moon, when overhead, actually "pull up" on us, their gravitational action counteracting that of our own planet, so that our heads and all other earthly objects are physically lighter at high noon and under the moon's zenith than at other times.

This is elementary, but what Science must argue out with Dr. Einstein is whether or. not gravity is an instantaneous thing, operating at infinite speed. He says not. One proposition of Relativity is that the pull of a body's gravity travels at the speed of light--about 180,300 mi. per sec. Specifically therefore, the moon's gravity pull is felt on earth one and a third seconds after the moon has passed its zenith; the sun's gravity, 8 seconds after high noon.

A way to determine the matter would be to weigh small objects, observing whether variations in their weight occur exactly as, or shortly after, the sun or moon passes over them. The weight variations are infinitesimal. To detect them would require fabulously sensitive scales. But could ic be done, Science would have a check on Einstein far more clean-cut and conclusive than ether-drift experiments and eclipse observations.

There lay the significance of an announcement last week by Physicist Ralph C. Hartsough of Columbia that he had perfected a set of mirror-scales capable of weighing, distinctly and faithfully, down to one 280-billionth of an ounce. Gossamer quartz filaments balance the scales, the slightest titillation of which is reflected from their gold-mirrored surfaces by a ray of light. The ray is split by two half-mirrors, being reunited on the scale-mirrors, where any disparity between the wavelengths of the reunited portions is clearly seen as shadow bands. Thus, when the object weighed (1/29,000 oz. of quartz filament) is lightened by the pull of sun or moon, the scale tips, the wavelength no longer matches (having traveled different distances), and the shadow-bands reflect weight variations caused by gravity within small fractions of a second. To eliminate error, the ray used was a green one, of uniform wavelength, passed from a mercury lamp through an interferometer (light-filter).

The vibration of traffic in Manhattan, even at dead of night, is sufficient to disturb so delicate an instrument. The test of Einstein's theory will be made "somewhere in Illinois." Thereafter, Prof. Hartsough will enlarge his scales and attempt to weigh molecules and atoms; and will consider commercial offers from people eager to try his instruments (he has made three) in locating oil and minerals.*

-The Hartsough Mirror-Scales are said to be 100 times as sensitive as the Eoetvos Balance, an instrument now in common use which determines the gravitational pull of sections of the earth's crust. Oil-bearing crust, being light, exerts slightly less gravity pull; ore-bearing crust, slightly more.