717 



B IRI LOOT. 



nOROLOOY. 



to expand upward*, and another pair of tteel to expand downwards : 

 nine in all. The foreign artuU use tine, which require* only five ban, 



Fif. 10. 



Gridiron Pendulum. 



lea workmanship, and only one nice fitting. One or two flat brass 

 horizontal bauds are attached to 1 and 5 to keep the zinc rods in 

 their places. 



In Graham's pendulum, a glass jar, partly filled with mercury, is 

 supported in a sort of steel stirrup. The pendulum rod passes through 

 the top of the stirrup, and is held by a nut and adjusting screw at D, 

 fg, 21. The height of the mercury in the jar is about 67 inches ; but 

 this will vary somewhat with the diameter of the jar, the substance of 

 the rod and frame, and perhaps the variable expansion of the steel 

 rod. The compensation can be altered, and finally perfected, by the 

 astronomer. 



Mr. Dent has recently made many improvements in the mercurial 

 pendulum. One consists in the use of a cast iron instead of glass 

 cistern for the mercury ; another in the attachment of the cistern 

 directly to the pendulum-rod ; a third, in the prolongation of the 

 rod nearly to the bottom of the cistern; and a fourth, in giving 

 impulse to the pendulum at or nearly at the centre of percussion. 

 If an escapement could be contrived which gave its impulse to the 

 pendulum at the middle poiut of its vibration, and was wholly detached 

 from it at all other times, such an escapement would be perfect ; and 

 escapements are almost to be considered good or bad as they approach 

 this character. Uuyghens proposed a most ingenious contrivance, 

 namely, that the upper part of the pendulum, which he made of 

 two parallel strings, should wrap and unwrap on two cheeks, which 

 being shaped as cycloids, caused the bob itself to describe a 

 cycloid. Now it is a property of this curve that all arcs are de- 

 scribed in the same time, so that Huyghens's construction was 

 perfect in principle ; it had, however, many difficulties in practice. 

 Instead of suspending the pendulum by a perfectly flexible string, 

 or on a knife-edge, when the motion must be in a circle, the top of the 

 rod may be made to end in a flat spring, which has certain advantages. 

 Mr. Frodsham has found that a spring of a particular strength 

 rendered the oscillations of a pendulum of a certain weight isochro- 

 nous, and that a considerable alteration in the length of the spring did 

 not afiect this quality of isuchronism. This may perhaps be explained 

 by supposing the lower part of the spring not. to have acted when 

 it was longest, but to have always preserved it* rectilinear form. 

 There is an investigation of the effect of atmospheric pressure on the 

 rate of a transit clock at the Armagh observatory, in the ' Mem. Ast. 

 Soc.,' vol. v., p. 125. The author, Dr. Robinson, araunifs that the 

 variations of a clock from a constant rate are expiessed by the sum of 

 two terms, one depending on the temperature, the other on the pressure 

 of the atmosphere shown by the barometer. The isochronUni of the 

 spring is supposed, or that the effect of any change in the arc depending 

 ou the above two causes is already expressed in the terms. When the 



error in the compensation for temperature is determined, this can 

 readily be rectified by altering the quantity of mercury in the jar. As 



a 



Mercurial Pendulum. 



the effect of time is generally to produce a falling off in the arc, a small 

 addition to the clock weight might be made from time to time, so as 

 to bring back the pendulum to its primitive arc, until the clock is 

 cleaned, and its action restored that way. Where the clock is much 

 exposed to variations of temperature, it should be enclosed in a second 

 covering or closet. 



To bring a clock to time, first make it nearly right by the adjusting 

 screw D, but let it have a losing rate, which must be determined by 

 observation after the interval of one or more days. Suppose it in 

 losing 3' a day. Put a weight, which has been carefully ascertained, 

 say 200 grains, upon the plate which covers the jar (E e in the mer- 

 curial pendulum, in the gridiron pendulum anywhere near B 6), and 

 find the fresh rate of the clock by observation. Let it now gain 10' 

 a day. Then, as 200 grains cause a gain of 18" a day, 15'4 grains will 

 alter it ! per day, and, replacing the 200 grains with a weight of 46-2 

 grains, will bring the clock to time. The final adjustment of the 

 compensation can be best accomplished when the clock has gone several 

 months, and wjien the gain or loss in two of the warmest months in 

 the year is compared with the gain or loss in two of the coldest. 



For the mathematical and physical principles which govern the 

 action of the pendulum, we refer to PENDULUM. 



An ailronumical clock, such as those used in observatories, is the 

 best example of a pendulum clock, comprising all the most refined 

 means of adjusting the isochronous movements of the pendulum to the 

 measurement of small intervals of time. Turret or c/iurcA clocks, 

 however, are commercially of more importance, and of these we must 

 now say something. 



Turret-clocks differ from other machines employed for measuring 

 time, not only in their greatly superior size, but also in the arrange- 

 ment of their parts, and in the circumstance that they are usually 

 make to strike the hours, and often the quarters also, upon large bells. 

 They are also occasionally connected with machinery for chiming 

 whole tunes at certain intervals upon a set of bells which, when 

 mounted in a church tower, are so hung that, by disconnecting the 

 hammers of the chimes and striking apparatus, they may also be rung 

 in the ordinary manner by means of ropes. One of the peculiarities of 

 a turret-clock consists in the circumstance that it is frequently required 

 to indicate the time upon as many as four different dials, on the four 



