402 Major-General Roy's Account of the 



ham's property, would correfpond with thofe above-mentioned, 

 which he had been dire(5led by the Pvoyal Sociey, with fo much 

 care and pains, to provide; yet, that nothing of this fort 

 mipht remain doubtful, it w^as judged right, in fetthng the 

 abfoUite length of the bafe, which I meafured near London 

 in 1 785, as has been mentioned in the introduction to this 

 Paper, that the two fcales fl"iould be actually compared. Hav- 

 ing accordingly obtained an order from the Frefident, for ad' 

 miffion into the Society's Apartments, I went there in the 

 afternoon of the 13th of Augufc, and laid both fcales takers 

 out of their cafes on the table of the meeting- room, with 

 thermometers along-fide of them, that they might acquire 

 the fame temperature. On the forenoon of the 1 5th of Au- 

 gufl the comparifon was made, with the afli fiance of Mr. 

 Ramsden, who for that purpofe carried along with him hisi 

 curious beam-compaffes, whofe micromcter-fcrew fhews very 

 perceptibly a motion of ^^V-^th part of an inch. Thus the 

 extent of three feet, being carefully taken from the Society's 

 ftandard, and applied to my fcale, it was found to reach exadtly 

 to 0^6 inches, the temperature being 65°. In like manner, the 

 beam-compafles being applied to the length of the Exchequer 

 yard, the extent was now found by the micrometer to over- 

 reach that yard by -roV-c-o-^^^j or nearly -,J-^-^th. parts of an 

 inch. 



Having thus fhewn that my fcale is accurately of the fame 

 length with the Society's ftandard, it remains to point out the 

 ufe that was made of it, for afcertaining the lengths of the 

 deal rods, intended for the operation on Hounflow- Heath, In 

 the firfl place, Mr. Ramsden prepared a beam-compafs, fuf- 

 ficient to take in twenty feet, trufled in all refpeds like the 



mea^ 



