ggO INVENTION OP CHRONOMETERS, &C, 



The balance The alterations to which the length of. the pendulum is liable 



are morea? by the different degrees of heat and cold, affect the going of 



fected by tern- clocks with that sort of regulator ; and watches, with a balance, 



the^pendulum ^'^ ^^'^^ more subject to irregularity from that source ; because 



not only th(! balance expands^ or contracts, according to the 



rise or fall of the thermometer, but the regulating spring itself, 



\Aihile it suffers similar changes, becomes weaker or strorger ; 



so that, from these causes, a timepiece must go slower or faster 



in too great a proportion 'to be overlooked or neglec:ed. 



Graham fust Graham* is the first who thought of applying two metals of 



this in the pen- different expansibility, to correct the errors proceeding from the 



dulum by two variation of temperature in a pendulum ; but as he seemed to 



have had in view to effect it immediately, without the aid of 



mechanism, he was obliged to fix on steel and mercury, these 



being the metals which offered to him the greatest difference 



Harrison's of dilatation and contraction. Harrison, by multiplying the 



^ulum" ^^"' l^a^rs, increased the total length of the two metals acting on 



one another, without exceeding the Hmits of the pendulum ; and 



thereby produced a sufficient compensation with brass and 



steel in the compound, or gridiron pendulum, which has been 



His expansion almost universally adopted ever since. This contrivance could 



laiice spring ' ^^°^ ^® easily applied to balances; but Harrison, following still 



the principle of the different expansibility of metals, applied it 



in a manner which had not been thought of before, and made 



it act on the spiral spring, in order to produce the desired 



compensation in the regulator. This method is described 



— composed of as follows:* " The thermometer kirb is composed of two 



Sefent metals " *'"" plates of brass and steel rivetted together in several" 



ri vetted toge- " places, which, by the greater expansion of brass than steel 



'^^' " by heat, and contraction by cold, becomes convex on the 



" brass side in hot weather, and convex on the steel side in 



" cold weather; whence, one end being fixed, the other end 



" obtains a motion corresponding with the changes of heat 



" and cold, and the two pins at the end, between which the 



" balance spring passes, and which it touches alternately as 



" the spring bends and unbends itself, will shorten or lengthen 



" the spring, as the change of heat and cold would otherwise 



* Fhilosophical Tiansactions, 17<26. 



* Principles of Mr. Harrison's Timekeeper, p, xii. notes. 



" require 



