INSTRUMENT FOR TELLING T^E HOUR IN THE DARK. gO| 



If platina was used instead of steel, and steel instead of Platina & steel 

 einc, a pendulum might be made equally good, and more ^' "*'• 

 compact ; but not at so small an expense. 



The above dimensions are to be understood in the finished The diraen- 

 state of Iheii^diameters and lengths, proper for a second a°econd^^ . 

 pendulum. dulum. 



I have constructed a pendulum on this principle, which Answered on 

 has been in use some months, and I have the satisfaction to *"*'* 

 find it has answered my expectations ; the temperature of 

 the room was from 58 to 34 degrees, and no variation of the 

 clock, when compared with ray other clock, which, from 

 many years trial, I know to be a good one, in a room where 

 the thermometer does not vary more than four degrees. 



The difficulty, or rather irn possibility of making a good Inconven^encB 

 pendulum, where a compound metal as brass is employed, "netal''"^" 

 prises from the circumstance, that neither brass jior any 

 compound metal can be made uniform, not even for one 

 foot in length ; and then if drawn into wire the parts acquire! 

 a longitudinal grain, which adds to the variation of the ex- 

 pansive powers. To avoid this, zinc has been substituted, *ri<J of zinc, 

 with no more certainty of success ; for if a pendulum is 

 fnade in the summer, and the steel pine fill the holes well, 

 and it is exposed to a severe frost, and put to the clock for 

 only a few years, the invisible fissures will become visible in 

 the performance of the best clock, and a visible separation 

 render the pendulum useless ; as I have witnessed. , 



I am willing to furnish the Society with any farther infor- 

 piation in -my power upon the subject. 



Woolwich, April 27, I8O9. ADAM REID. 



VIII. 

 Method of ascertaining the Hour in the Nighty by an Appa- 

 ratus connected with a common Watch : bi/ Mr. G. Spark, 

 of Elgin, Murrayshircy Scotland*, 

 SIR, 



jOY Mr. John Newton, watchmaker, I have forwarded Invention for 

 an invention for knowrng the hour in the dark, by feeling, knowing the 



• Trans, of the Soc. of Arts, vol. XX VIII, p. 234. The silver me- 

 dal was voted to Mr. SpArk for this invention. 



I thinlj: 



