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LIX. On a new Compensation Pendulian. 

 Herapath, Esq. 



TT has long been an object with astronomers 

 ■*■ and navigators to possess a cheap and cor- 

 rect pendulum, which shall have the power of 

 correcting its expansion or contraction from 

 heat and cold. The principle adopted in the 

 construction of such pendulums has been, that 

 of an expansion upwards by one metal just equal 

 in amount to the downward expansion of the 

 pendulum rod. The most approved instruments 

 of this sort are, what has been termed the Grid- 

 iron pendulum, Mons. Thiout's, and the mer- 

 curial one. These are so well known to scientific 

 persons, that it is unnecessary to describe them 

 here, but merely remark that neither of them 

 have been extensively introduced. I conceive 

 this non-introduction to arise, from the com- 

 plexity of the gridiron pendulum, from the dif- 

 ficulty of making heat and cold act equally on 

 the two rods of Mons. Thiout's (the one being 

 outside the clock-case while the other is inside), 

 and from the expense of the mercurial pendu- 

 lum. Considering it possible to make a com- 

 pact pendulum rod which shall not possess the 

 disadvantages of the others, and that such an 

 one would still be a desideratum, I shall pro- 

 ceed at once to the description, after having 

 given the data upon which the calculations are 

 founded. 



The average expansion of iron, as taken from 

 the experimentsof Lavoisier and Laplace, Bor- 

 da, Smeaton, Troughton, and Dulong and Petit 

 between the temperatures of 32° and 212° F., 

 for every 1-000000000 is -001 24.869, or for 1° 

 Fahr. is -000006937. The expansion of steel 

 spring for 1° Fahr. is -00000761 according to 

 Muschenbroek ; and that of hammered zinc for 

 1° Fahr. is -00001672 according to Smeaton. 



If the pendulum be constructed as usual of 

 three inches of watch spring, and the remain- 

 der of iron wire, the expansion for 1 ° of Fahr. 

 will be 3 inches steel -000022830 



36-139290 iron -000250600 



-000273i30 in. 



BjJ 



