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XlV. On Pendulums. By Ez* Walker, Esq* 



JL he mechanism of pendulums is a subject which has of 

 late years attracted much attention ; but whether any real 

 improvements have been made since the days or" Harrison, 

 is a question on which there are various opinions. Rods 

 of' zinc, pewter, lead, and other soft metals have been 

 substituted for those of brass, to reduce the gridiron pen*- 

 dulum to a more simple form : but it has been found by 

 experience that some of those soft metals, when under the 

 pressure of the weight of the lens, do hot long retain ftife 

 same power of expansion and contraction. 



The late Mr. James Bullock, a very ingenious clock- 

 maker, and a man of much experience, told me, that brass 

 and steel were the only metals he could rely on, in the 

 construction of compound pendulums. 



The gridiron pendulum is constructed on the supposition, 

 that it is kept invariably of the same length by rods of 

 different metalsj which have their lengths duly propor- 

 tioned to their expansions and contractions ; but late 

 writers have advanced several objections to this mode 

 of compensation. The principal of these objections are: 

 1st, The length of the pendulum may be increased by 

 its weight. 2dly, Where the rods pass through the con- 

 necting bars there is some friction, which causes theiri 

 to move by starts, and not according to the increase and 

 decrease of heat: and, 3dly, The difficulty of exactly ad- 

 justing the lengths of the rods. But there is another 

 source of error in this pendulum, which has not, I believe, 

 been attended to by writers on this subject. 



Suppose that the distance between the centre of the lens" 

 and the point of suspension were hot to suffer any change 

 by the vicissitudes of heat and cold, still the length of the 

 pendulum might vary. For as the ends of the compensation 

 rods are connected by cross pieces, and as all these, except 

 one, are put in motion bv everv variation in the tempera- 

 ture of the air, some moving in a direction contrary to the 

 others ; therefore it is evident, that the motions of these 

 cross bars must alter the distance between the point of 

 suspension and the centre of oscillation, unless their 

 weights be adjusted according to their motions to and from 

 the point of suspension. 



Suppose the cross piece which connects the two extreme 

 rods of the gridiron pendulum be fixed to the centre of 

 the lens, and that the expansions of steel and brass be as 



Vol. 36. No. 148. August 1810. F three 



