The Law of Gravitation

law of gravitation hold for ordinary masses and distances, as well as for the huge bulk of planets and celestial distances? Is this attraction affected by the nature of the intervening medium? Does it depend on the temperature or any other variable which influences the ordinary properties of matter? Is it dependent only on

the quantity of matter, and not on the nature of the attracting substances? The answer to all of these questions have been sought by many great physicists. In order to realize the difficulty experienced in measuring the attraction, say, of two grains of water one centimeter apart, it might be borne in mind, that this force has been found to be about

ON THE PROGRESS OF SCIENCE. Ill

one-fifteen-millionth of a dyne, and that it takes several hundred dynes of force to break an extremely fine spider's thread. The first attempt at accurately measuring the attraction between two bodies was made by Cavendish in 1797-8. Nearly a hundred years later, in 1896, Prof. Boys repeated the measurement, but with all the refinements and improvements which a century of scientific progress could suggest. He suspended two balls of pure gold on a small horizontal beam, by means of an exceedingly fine fibre. This fibre, which was his own invention, was strung out of molten quartz. It was found to be surpassingly

strong and constant in its elasticity, and when the weight sus-

pended by it was twisted, the fibre always exerted a perfectly defi-

nite, although very minute, force, trying to untwist. It was

against this slight untwisting force of an almost invisible fibre that he balanced the small attractions to be measured. If the system was hung with the gold balls, say east and west respectively of the centre of the horizontal beam, and a ball of lead was brought close up to the south side of the east gold ball, and another lead sphere were placed to the north side of the west gold ball, the attractions of each lead ball for the nearest gold ball, being stronger than for would cause the beam to rotate through a the more distant one,

small angle— the east ball rotating toward the south. Now this but by having a tiny plane mirror attached angle was very small,

to the beam, and observing with a distant telescope how much the

scale was rotated, Boys was able to calculate reflectionof a distant which the quartz fibre was just how large was the angle through turned. Then he knew exactly how hard the quartz was trying to and untwist, and he knew that this force balanced the attractions was therefore equal to it. Such was the delicacy of the measurements, that the reflec-

as seen in the telesc ipe, spun through many tion of the scale thousands of scale divisions when the fibre turned through the

slightest fraction of a degree. Numerous corrections were applied to the results, to take ac-

count of the disturbances of air currents and those due to vibra- Many separate measurements were taken, tions of the floor. value stated above and the various results agreed in giving us the

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112 IMPROVEMENT ERA.

for the attraction. Now we know the attraction, in ordinary units of force, which exists between any two bodies. Other physicists have repeated this and similar experiments, with different attracting substances, and have confirmed the astronomical conclusion that the force between given substances does not depend on the kind of matter they contain. Still other physicists have varied the experiment by placing various substances between the attracting balls and noting the effect. This has been tried with lead, zinc, mercury, water, alcohol, glycerine and air as the intervening medium, and no change in the attraction of the balls has ever been detected. Hence it bas been concluded that gravitation, whether or not it be in a state of stress in an infinite ether, acts through all space far and near, between all masses large and small, and that two bodies will attract each other with a supreme disregard for whatever may occupy the space between them. Chester Snow, B. S.

The Highest Bridge in the World.

Another unique distinction is to be awarded western America. At present the