VOL. LXXXIV.] PHILOSOPHICAL TRANSACTIONS. 341 



vantage, as it is not necessary, in the form I have given to this apparatus, to 

 stop the clock, in order to adjust it. These pieces of wood are mortised into a 

 transverse piece of deal at the top and at the bottom firmly. Before attempting 

 to make a very perfect machine on these principles, I resolved to try how far 

 this frame of wood might serve to connect the points i and e, and procured the 

 apparatus for altering the point i, screwed on to one of these perpendicular 

 pieces of wood on one side, and to the other on the other side. The pendulum 

 itself serves as a plummet to place them perpendicular. In Mr. Whitehurst's 

 machine the screw went through a piece of brass, and rested on it, fixed to the 

 top of the clock-case. But in my construction of it, when the length of the 

 rod IB is adjusted, the clock has nothing to do with the clock-case, excepting 

 with that part of the wooden frame which connects the point i with the point e. 

 If I had been, or were to construct a machine for this purpose ab origine, in- 

 stead of these 2 pieces of fir, I should employ a solid piece of brass, and make 

 two cylindric cavities into it, parallel to each other, and in these cavities place 2 

 glass tubes, about 2 inches diameter, perpendicularly upwards, which may be 

 done by various means; and, while in this situation, having heated them gra 

 dually to the heat of melted lead, I should pour in melted lead, so as to fix them 

 in their places when it cooled. The apparatus for fixing the point i, and that 

 for fixing the tube at f, being also of brass, in heat they would always expand, 

 and in cold contract, equally; so that the glass tubes would keep always at an 

 equal distance from each other, and equally perpendicular. Glass is not only 

 very little apt to contract and expand by heat, but is free from any such dispo- 

 sition from moisture or dryness, which is not the case with wood. 



Having added the apparatus I have described to Mr. Whitehurst's machine, I 

 set it a going, expecting in the situation I placed it, only some approach towards 

 accuracy in the length of the pendulum. I fixed beside it a transit which be- 

 longed to Mr. Ludlam, the principal parts of which were made by Mr. Ramsden, 

 the object-glass was a 4-feet focus achromatic by Dollond. I found my meridian 

 mark at about -I of a mile distance. I also borrowed, from my friend Mr. 

 Stevens, a clock with a gridiron pendulum, made by Graham, for his father Dr. 

 Stevens, in order to compare them together when I had no observations. There 

 were several trivial circumstances, which baffled the experiments for some time, 

 not worth relating, one only excepted; which was, that the curvature of the 

 wire, acquired by its being wound round a pirn, was not entirely unfolded for 

 some months, so that the clock went slower and slower during that time. At 

 length this difficulty was overcome; I then began to observe with Graham's clock, 

 in order to adjust the length of the pendulum, but found irregularities frequently 

 take place. I then adjusted it by observation, and soon found that Graham's 

 clock went much more irregularly than my own. I adjusted it by turning the 



