Mabch 22, 1907] 



SCIENCE 



447 



escapement wheel, the deviation of the cir- 

 cular arc described by an ordinary pendu- 

 lum from the theoretical cycloidal are was, 

 of necessity, taken into account. With the 

 introduction of the anchor or recoil escape- 

 ment. Fig. 5, invented by Dr. Hooke in 

 about 1675, the long swing of the pendu- 

 lum was obviated and the cycloidal cheeks 

 were found to be more detrimental than 

 advantageous. 



The expansion of metals by heat has been 

 known ever since the middle of the seven- 

 teenth century, and early in the eighteenth 

 George Graham set himself the task of 

 making a clock pendulum such that the 

 distance between the center of oscillation 

 and the center of suspension would be inde- 

 pendent of the temperature. I quote from 

 his paper 'A Contrivance to avoid Irregu- 

 larities in a Clock's Motion by the Action 

 of Heat and Cold upon the Pendulum,' 

 communicated to the Royal Society in 

 1726: 



Whereas several who have heen curious in meas- 

 uring time have taken notice that the vibrations 

 of a pendulum are slower in summer than in 

 winter, and have very justly supposed this altera- 

 tion has proceeded from a change of length in the 

 pendulum itself, by the influence of heat and cold 

 upon it, in the different seasons of the year; with 

 a view, therefore, of correcting, in some degree, 

 this defect of the pendulum, I made several trials, 

 about the year 1715, to discover whether there 

 was any considerable diflference of expansion be- 

 tween brass, steel, iron, copper, silver, etc., when 

 exposed to the same degree of heat as nearly as I 

 could determine, conceiving it would not be very 

 difficult, by making use of two sorts of metals, 

 differing considerably in their degrees of expansion 

 and contraction, to remedy, in great measure, the 

 irregularities to which common pendulums are 

 subject. But although it is easily discoverable 

 that all these metals suffer a sensible alteration 

 of their dimensions by heat and cold, yet I found 

 their differences in quantity from one another 

 were so small, as gave me no hopes of succeeding 

 this way, and made me leave off prosecuting this 

 affair any further at that time. In the beginning 

 of December, 1721, having occasion for an exact 

 level, besides other materials I made trial of. 



quicksilver was one; which, although I found it 

 was by no means proper for a level, yet the ex- 

 traordinary degree of expansion that I observed 

 in it when placed near the fire, beyond what I had 

 conceived to be in so dense a fluid, immediately 

 suggested to me the use that might be made of it 

 by applying it to a pendulum. In a few days 

 after I made the experiment, but with much too 

 long a column of quicksilver, the clock going 

 slower with an increase of cold, contrary to the 

 common pendulum; however, it was a great con- 

 firmation of the advantage to be expected from it, 

 since it was easy to shorten the column in any 

 degree required. 



As his first jar was too long, so his second 

 was too short, but by June 9, 1722, he was 

 ready to test the running of his mercury 

 pendulum clock with one regulated by an 

 uncompensated pendulum. He says: 



For the first year I wrote down every day the 

 difference between the two clocks, with the height 

 of the thermometer, not omitting the transits of 

 the stars as often as it was clear. The result of 

 all the observations was this, that the irregularity 

 of the clock with the quicksilver pendulum, com- 

 pared with the transits of the stars, exceeded not, 

 when greatest, a sixth part of that of the other 

 clock with the common pendulum; but for the 

 greatest part of the year, not above an eighth or 

 ninth part, and even this quantity would have 

 been lessened had the pillar of mercury been a 

 little shorter, for it differed a little the contrary 

 way from the other clock, going faster with heat, 

 and slower with cold; but I made no alteration 

 in length to avoid an interruption of the observa- 

 tions. 



A few years afterward John Harrison 

 brought out his gridiron pendulum com- 

 posed of four brass and five steel rods, so 

 constructed that the expansion of the steel 

 rods tended to lower the pendulum bob 

 while the expansion of the brass ones tend- 

 ed to raise it. 



Another form of compensated pendulum 

 that has found favor is the zinc-tube pen- 

 dulum, in which the zinc tube surrounds a 

 rod of steel and is itself surrounded by a 

 tube of steel. Here the zinc tube fulfils 

 the purpose of the brass rods in the grid- 

 iron pendulum. At present at least one 



