VOL. LXXV.] PHILOSOPHICAL TRANSACTIONS. 25 



nioLis optician Mr. James Short, Gen. R. purchased a finely divided brass scale, 

 of the length of 42 inches, with a Vernier's division of 1 00 at one end, and one 

 of 50 at the other, by which the 1000th part of an inch is very perceptible. It 

 was originally the property of the late Mr. Graham, the celebrated watch-maker; 

 has 'the name of Jonathan Sisson engraved on it; but is known to have been 

 divided by the late Mr. Bird, who then worked with Sisson. The brass standard 

 scale of the r. s. about 42 inches long, which contains on it the length of the 

 standard yard from the Tower, that from the Exchequer, and also the French 

 half-toise, together with the duplicate of the said scale, sent to Paris for the use 

 of the Royal Academy of Sciences, were both made by Mr. Jonathan Sisson, 

 under Mr. Graham's immediate direction. Now, though there seemed to be 

 every reason to suppose that the scale above-mentioned, originally Mr, Graham's 

 property, would correspond with those above-mentioned, which he had been 

 directed by the r. s., with so much care and pains, to provide; yet, that nothing 

 of this sort might remain doubtful, it was judged right, that the two scales 

 should be actually compared. Accordingly the extent of 3 feet, being carefully 

 taken from the Society's standard, and applied to Gen. R.'s scale, it was found to 

 reach exactly to 36 inches, the temperature being 65°. In like manner, the 

 beam compasses being applied to the length of the Exchequer yard, the extent 

 was now found by the micrometer to over-reach that yard by t^Vo-o-j or nearly 

 j^7__ parts of an inch. Having thus shown that his scale was accurately of the 

 same length with the Society's standard, he next points out the use that was 

 made of it, for ascertaining the lengths of the deal rods intended for the opera- 

 tion on Hounslow-heath; which was, by the assistance of Mr. Ramsden, to 

 measure and set off the true length of those rods. Other preparatory matters 

 are then described, such as, the stands for the measuring rods, the boning tele- 

 scope and rods, the cup and tripod for preserving the point measured to during 

 each night, marks for terminating in a permanent manner the extremities of the 

 base, clearing the ground, &c. 



Next, about the middle of June 1784, we enter on the sought measurement 

 of the base with the chain, and determination of the relative heights of the sta- 

 tions by means of the telescopic spirit level, a business which was completed the 

 22d of the same month, extending from the south-east extremity near Hampton 

 Poor-house, to the north-west extremity near the Magpye public-house at King's 

 Arbour. This measurement gave IQ hypothenusal distances of 6oo feet each, and 

 one of 404.55, making in the whole 11804.55 feet, the mean temperature being 

 62^°. Now, when the length of the chain, in its original state, was ascertained 

 by the dots on the brass pins in the New-England plank, it was found, in the 

 then temperature of 74°, to exceed the 100 feet by near -j- of an inch, or 0.245 

 inch. Therefore in the temperature of 63°, being that in which the lengths of 



VOL. XVI. E 



