Chambers's Edinburgh Journal, No. 457, page 29 by Various Authors
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ugh to discover and correct its own varying dimensions as different temperatures are brought to bear upon its material.
The first successful attempt to accomplish this useful purpose was made by George Graham in 1715. He replaced the solid weight at the bottom of the rod by a glass jar containing mercury. The rod he formed of steel of the usual length; and because mercury expands five times more than steel, he fixed the height of the column of mercury in the jar at only 6-1/2 inches. In this arrangement he found that additional heat carried up the mercury in the jar, as much as it carried down the jar by the elongation of the rod. Consequently, the motion of the one perfectly compensated the motion of the other, and the effective centre of the weight always remained at the same precise distance from the top of the rod. By the application of this compensating pendulum, clocks are now constructed that do not vary to the extent of a tenth of a second in a day.
Soon after the invention of Graham's mercurial pendulum, John Harrison--the same clever mechanician who received L.20,000 from government for making a chronometer that went to Jamaica in one year and returned in another with an accumulated error of only 1 minute and 54 seconds--hit upon another means of gaining the same end. He brought a steel rod down from the point of suspension, turned it up into a copper rod of less length; and from the top of this hung the weight. He fixed the lengths of the steel and copper rods, which expand unequally, in such a way that the steel carried the copper down exactly as much as the copper carried the weight up; and thus the centre of the weight was still kept at the same distance from the real point of suspension. Harrison's pendulum is generally seen in somewhat the form of a gridiron, because many parallel bars of copper and steel are used in its construction, for the sake of rendering it firm and unyielding in all its parts.
MAGIC IN INDIA.
A correspondent in India tells us that a military