of the Royal Society of Edinburgh. 849 



I may mention here, that in Fig. 2. the pendulum is sup- 

 posed to be in the middle of an oscillation, with a tooth of the 

 scape- wheel pressing on the pallet D. In Fig. 3. the oscillation 

 has been completed to the left ; the pallet D has allowed the 

 tooth to escape past it, and a tooth of the opposite side of the 

 wheel has been arrested by the pallet E. 



In Fig. 4. the pendulum has swung to the right ; the tooth 

 which rested on E has in its turn escaped, and another, on the 

 opposite side of the wheel, has dropped on D. 



The next peculiarity in this clock which merits attention, is 

 the material of which the pendulum rod and ball have been made. 

 Marble has been adopted for this purpose, in consequence of 

 a suggestion made to me by Dr BREWSTER, and since repeat- 

 ed by him in the Philosophical Transactions for 1830, page 

 94, where, in speaking of Mr MITSCHERLICH, he says, " This 

 eminent philosopher has found, by direct experiment, that heat 

 expands a romb of calcareous spar in the direction of its axis, and 

 contracts it in directions at right angles to that axis." Dr 

 BREWSTER adds, in a note : " It follows from this fact, that mas- 

 sive carbonate of lime, in which the axes of the molecules have 

 every possible direction, should neither contract nor expand by 

 heat, and would therefore form an invariable pendulum*." 



In constructing this pendulum, care has been taken to ex- 

 clude every thing which could interfere with its principle, and 

 the whole of the pendulous portion, from the point of suspen- 

 sion downward, is continuously of marble, without the interven- 

 tion of metal, and even the convenience of an adjusting screw 

 at the bottom has been sacrificed to insure this. A method of 



* Some experiments made since this paper has been read, seem to shew, that a 

 rod of Carrara marble, on being measured at 32 and at 21 1 Fahrenheit, will be 

 found to have expanded 5555. A rod of Lucullite marble, 3 5 . 



xx 2 



