ADMIT OF CHANGES IN THEIR TEMPERATURE. 73 



compenfating rod added to it, in the fame manner as that con- 

 flicted by Mr. Crofthwaite or by Mr. Pine: the clock itfelf 

 (hould alfo be fattened to the ftone, particularly if Mr. 

 Crofthwaite's pendulum is ufed. 



The block of ftone may be farther fecured from change of Defence of the 

 temperature, by being furrounded on every fide by brick-work, ch ° n e f^ em _ 

 except where the pendulum and clock are fixed, and having perature, &c 

 dry (aw-duft rammed in between it and the brick-work. 

 It is probable that a block thus fitted up, would vary little 

 from changes in the heat of the air; and in thofe fituations 

 where large blocks of ftone are found naturally, (his might 

 be done at a fmall expence : where a large block of ftone 

 could not be procured, a piece of dry ftraight-grained red 

 deal balk, or folid mahogany, might be ufed to good effect, if 

 well painted, and inclofed as before directed : and for common 

 ufe, it is probable that a large glafs tube, rilled with fenien 

 Jycopodii, if to be had, or fine dry faw-dufl, well rlofed at 

 either end, and covered with oiled filk, would form a pendu- 

 lum-rod little affected by heat or cold ; but if metal is pre- 

 ferred, then a tube of metal, fitted up as lad directed, would 

 be preferable, to the fmall wires now ufed for tin's purpofe. 



The advantages to be expected from the principal method Advantages. 

 above-mentioned, of fitting up pendulums, is, that it affords 

 an unlimited mode of approximation to perfect compenfatibn ; 

 and that, as it recjuires no great refinement of workmanfhip 

 in its construction, it can be ufed in many fituations where it 

 would be impofl/ible to have a tubular compenfating, or com- 

 mon gridiron pendulum made; which, notwithftanding the 

 late improvements, are extremely difficult to conftruct with 

 accuracy, as may be feen in the paper publifhed in your Jour- 

 nal for December laft, where many of the impediments to their 

 perfection are pointed out, and more frill remain to be no- 

 ticed, of which the following rleferve fome'attenlion. 



The metals of which gridiron pendulums are compofed, Imperfections of 

 are both of them mixed metals, and of courfe every different com P ou nd pen- 

 parcel of them made, muft vary in fome degree in the rela- The metaisVe 

 tive quantities of their component ingredients, and from thence mixed > and not 

 in their degree of expanfion by heat. It may appear ftrange compofition ^ 

 to call fteel a mixed metal ; but when it is confidered that 

 charcoal forms a confiderable part of it, the juftice of cfti- 

 paling it as fuch will be evident; and in this metal the pro- 

 portion 



