OF THE t'NlTEU STATES. 363 



with the horizontal diaineterof the four holes of the springs, 

 in order that the motion of the pendulum and the spring may 

 go oft" as from one axis, to avoid ail friction in the toucliing 

 of tlic ])ins rand o ujjon the inclined plane p. 



The pendulum rod itself is a parallelopipedon of steel, one- 

 third of an inch broad and one-tenth of an inch thick. 'I'hc 

 compensation for temperature is made hv a mercury column 

 about seven inclies high, and i.'j inches in diameter, included 

 in a glass cylinder, which serves as the lens of the pendulum. 

 ft is therefore adjustable by exi)criment. and completely at 

 the disposal of the observer, and lor any latitude. There is 

 also a screw at the end of the bar. by which this arrange- 

 ment is suspended to it, and by which the length of the pen- 

 dulum itself is adjustable. 



This pendulum is well known by the nante of the Mercu- 

 ry Compensation Pendulum; and it is evident that it was the 

 only one adapted to my purpose, as I was uncertain in what 

 latitude the observatories would be built. On general prin- 

 ciples, any compensation of a pendulum must compensate 

 for the sum total of the effect of temperature upon the going 

 of the clock, and not merely for the expansion of the rod 

 itself It must therefore be determined by experience and 

 observation ; as a rod compensating itself exactly in a pyro- 

 meter, would not for that compensate every clock ; and these 

 clocks would not be compensated by it, on accoin>t of the 

 influence of the temperature upon the scapement springs. 



The jewelling of the larger pinion holes of a clock does 

 not appear to me to be of any advantage. The pressure 

 upon them appears too great, and on that account occasions 

 a grinding of" the pinions. Therefore only the scapements 

 are jewelled in these clocks. 'I'he other pinion holes are 

 boxed with brass taken from a piece brought to England 

 from B( ngal as a sainple. which was 2;iven by the Tioard of 

 Longitude to iMr. Hanison, the first inventor of chronome- 

 ters. At his death, Mr. Hardy bought it, and uses it with 

 the greatest economy for such purposes, 'f'he ends p. p of 

 the cross bar on the pendulum are also lined with this brass. 



