In pendulums of ordinary construction, this relation is by no means an unvarying one—changes of temperature alter the bulk of all kinds of bodies. A metal rod runs up and down under increase and diminution of heat, as certainly as the thread of mercury in the tube of the thermometer does. A hot day, therefore, lengthens the metallic suspending-rod of a pendulum, and carries the centre of its weight to a greater distance from the point of suspension. By this means, the period of each vibration is of necessity lengthened. An increase of temperature to the extent of ten of Fahrenheit's degrees, will make a second's pendulum with a brass rod lose five vibrations in a day. All substances do not, however, suffer the same amount of expansion under like increments of heat. If the rod of the pendulum be made of varnished or black-leaded wood, an addition of ten degrees of heat will not cause it to lose more than one vibration in a day. But even this small irregularity is too vast for the purposes of precise science, and accordingly ingenuity has been taxed to the utmost to find some means of removing the source of inaccuracy, to invent some plan whereby the pendulum may be made sensitive enough 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 6½ 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 friend of his, on returning to England, and finding all astir there about mesmerism, writes to him that he had often had much cause to regret that, during his long residence of more than twenty-eight years in India, he was ignorant of the very name or existence of mesmerism; as he could recall to mind many instances of what he then deemed to be native superstitions, on which he now looked very differently, believing them to be the direct effects of mesmeric influence. These instances are daily and hourly exhibited in Indian dwellings, though either passing without notice, or ascribed to other causes. Children in India, especially European children, seldom go to sleep without being subjected to some such influence, either by the ayahs or the attendant bearers; and our military friend says, that he has himself repeatedly, in a few seconds, been the means of tranquillising a fractious, teething child, and throwing it into a profound sleep by the mere exercise of the will, quite ignorant that he was thus using, though in one of its simplest forms, a power at which he laughed heartily when displayed around him in some of its more hidden ramifications. We give the following in his own words:—
I shall now relate a circumstance, proving that the natives of India apply mesmeric power to the removal of diseases with the utmost success. I had in my establishment at Lucknow a chuprassie,[2] who was a martyr to the most deplorable chronic rheumatism. His hands, wrists, knees, and all his joints, were so greatly enlarged, and in a state so painful, that his duties had gradually become merely nominal. One day, he hobbled up, and begged my permission to remain at home for a few days, for the purpose of being cured of his agonising disease. I said: 'Certainly; get cured of your complaint, and let me see you when you return.' In a very few days, perhaps in four or five, to my great astonishment he returned, smiling and joyous, with his limbs as pliant and supple as my own.
'What!' said I, 'are you come back already?'
'Yes, sir, by your favour, I am perfectly cured.'