Meafuremcnt of a Bafe on Hounllow-Heath. 403 



jneafuiing rodi;, but fomethiog deeper, and fitted as ufual with 

 proper points and micrometer. The ftandard rod being now 

 confl:ru6led was laid on the fliop board, ftrongly framed for 

 the purpofe, and nearly level. To one fide of it, at the diftance 

 of about twenty feet two inches from center to center, two 

 ftronp' bell-metal cocks were firmlv fcrewed. Thefe cocks 

 were about 2| inches in length, three-eighths in thicknefs, 

 and rofe above the ftem nearly two inches, fo as to be on the 

 fame plane with the furface of the meafuring rods, when 

 placed upon it. 



A large plank, cut from the New-England m.afl:, upwards of 

 thirty feet long, nine or ten inches broad, and about three 

 inches thick, being fet edge-wife in the fame room, on part of 

 *the ftands now ready for the operation, was, in that poiition, 

 planed perfediy fmooth and ftraight. A filver wire being then 

 flretched very tight, along the middle of the plank, from one 

 end to the other, fix fpaces of forty inches each w^ere marked 

 off by the fide of the wire, at which points feven brafs pins, 

 about one-tenth of an inch in diameter, were driven into the 

 wood, and their tops polifhed with the ftone. During the whole 

 of this operation, and that which followed, the thermometer, 

 lying by the iide of the brafs fcale, continued fleadily at or very 

 near 63". 



A fine dot being now made on one of the extreme pins, and 

 the filver wire being ftretched over the dot, and as near as pof- 

 fible over the middle of the other pins, in which pofition it 

 was made faft ; the extent of forty inches, taken with the 

 utmoft care from the brafs fcale, was then marked off, by 

 placing one point of the beam-compafles in the dot, and with 

 the other defcribing a fliort faint arc on the furface of the fe- 



cond 



