Applications of the Pendulum.—The most important application that has been made of the pendulum is to the measure ment of time. It is said that Galileo; while a young man, having had his at tention drawn to the oscillation of a lamp suspended from the roof of a church in Pisa, perceived that, although their ex tent was gradually diminished, they con tinued to be made in equal times, and thence conceived the idea of employing a pendulum as a means of measuring smell intervals of time in astronomical observa tions. But though a pendulous body, by the isoehronism of its oscillations, fur nishes a means of dividing time into equal portions, it could obviously be of no greet use until a method was devised of continuing the motion, and register= ing the number of oscillations. The ap plication of clock-work to this purpose has been claimed for various individuals, but is generally and deservedly ascribed to Huygens ; and the invention one of the most important that ever was made in reference to practical astronomy, dates from the year 1656.
Huygens' researches on the subject of the oscillations of the pendulum are con tained in his admirable work entitled .Horologium Oscillatorium. He soon found that the oscillations in circular arcs of different amplitudes are not equal, the wider requiring rather a longer time thari the narrower ; and, with a view to reme dy this defect, be undertook to investi gate the nature of the curve in which the oscillations would be performed in equal times, whatever might be the extent of the aro described. The curve possessing this remarkable property was found to be the cycloid.
In clocks of the best construction the arc of vibration is very small ; and the pendulum is made very heavy, in order that, by possessing a great momentum, it may be less affected by the imperfec tions of the machinery.
Compensation Pendulum—The value of the pendulum as a regulator of time pieces depends on the isochronism of its oscillations ; which, in its turn, depends on the invariability of the distance be tween the points of suspension and oscil lation. But, as every known substance expands with heat and contracts with cold, the length of the pendulum will vary with every alteration of tempera ture, and the rate of the clock conse quently undergo a corresponding change. To counteract this variation, numerous contrivances have been employed. The principle is, however, the same in all ; and consists in combining two substan ces, whose rates of expansion are une equal, in such a manner that the ex pansion of the one counteracts that of the other, and keeps the centre of oscil lation of the compound body always at the same distance from the axis of sus pension. A brief description of the two compensation pendulums in most com mon use—the Mercurial Pendulum and the Gridiron Pendulum—will sufficiently explain the means by which compensa tion is obtained.
Mercurial Pendulum—This was the invention of Mr. George Graham, a cele brated watchmaker, who subjected it to the test of experiment in the year 1721. The rod of the pendulum is made of steel, and may be either a fiat bar or a cylinder. The bob or weight is formed by a cylindrical glass vessel, about 8 inches in length and 2 inches in diameter, which is filled with mercury to the depth of about 64. inches. The cylinder is sup ported and embraced by a stirrup, formed also of steel, through the the top of which the lower extremity of the rod passes, and to which it is firmly fixed by a nut and screw on the end of the rod. Now the effect of an increase of temperature on this apparatus is evidently as follows : In the first, place, the rod expands, and the distance between the axis of suspen sion and the bottom of the stirrup is in creased. In the second place, by the ex pansion of the mercury in the cylinder, its column is lengthened, and the dis tance of its centre of gravity from the bottom of the stirrup consequently in creased. But, as the expansion of mer cury is about sixteen times greater than that of steel, the height of the mercurial column may be so adjusted by trial that the expansion of the rod and stirrup shall be exactly compensated by that of the mercury, and the centre of oscilla tion of the whole suffer no change. This pendulum is, perhaps, the most perfect of all compensators ; but, as its adjust ments are attended with considerable dif ficulty, it is seldem used excepting in as tronomical observatories.
Gridiron P endul um.—This was contrived by Mr. Harrison, the inventor of the chronome ter. It consists of a frame of nine parallel bars of steel and brass, arranged and connected as in the annexed figure. The bars marked a arc of steel ; the four marked b are of brass ; the centre rod, of steel, is fixed at top to the cross bar connect ing the two middle brass rods, but slides freely through the two lower bars, and bears the bob B. The remaining rods are fastened to the cross pieces at both ends, and the uppermost cross piece is attached to the axis of suspen sion. It is as easy to see, from the mere inspection of the figure, that the expansion of the steel rods tends to lengthen the pendulum, while that of the brass rods tends to shorten it ; conse quently, if the two expansions exactly counteract each other, the length of the pendulum will remain unchanged. The relative lengths of the brass and steel bars are determined by the ex pansions of the two metals, which are found by experiment to be, in general, nearly as 100 to 61. lf, then, the lengths of all the five steel bars added together be 100 inches, the sum of the lengths of the four brass bars ought to be 61 inches. When the compensation is found on trial not to be perfect, an adjustment is made by shifting one or more of the cross pie ces higher on the bars.