Meafurement of a Bafe on Hounflow-Heath. 455 



So nice an agreement between two refults, with iiiflruments 

 fo very different, could not fail to be confidered as aftonidiiiig ; 

 and as it rarely happens, that the graduation of thermometers 

 will fo nearly correfpond with each other, as not to occafion a 

 much greater error, all were very defirous that it could have 

 been farther confirmed by continuing the operation in the fame 

 way through a more confiderable proportion of the whole 

 length. But befides the tedious nature of the double mea- 

 furement, owing to the multiplicity of {lands, platform3| 

 coffers, and other articles, that were now fucceflively to be 

 moved forward, and for which purpofe it had been found ne- 

 cellary to re-inforce the party of foldiers with fix additional 

 men ; the operation had already trained out to a much more 

 confiderable length than had been expefted ; the fummer was 

 now far advanced, and the continuance of good weather un- 

 certain ; the coffers likewife for the chain, having been con- 

 flru(£led in a hurry, were found to be defe6live : in (liort, all 

 thefe reafons contributed to induce us to give up, for the pre- 

 fent, any farther experiment with the chain, and to proceed 

 with the glafs rods alone in the completion of the meafurement. 



Accordingly, on Thurfday the 19th of Auguff, the opera- 

 tion with the glafs rods was continued for the five hypothe- 

 nufes, from the forty-fourth to the fortieth inclufive. It will 

 be remembered, that in proceeding with the deal rods, double 

 pickets had been placed in the ground, at the middle of the 

 forty-firft hypothenufe, or that point which terminated the 

 1315th rod, reckoning from the fouth-eaft, or the 155th from 

 the north-weft end of the bafe. Now, in returning to this point 

 with the glafs rods, the extremity of the 155th fell ihort of 

 the filk thread ftretched from picket to picket, juft one-tenth 

 of an inch. The expanfion of the brafs flandard fcale, and 



Vol. LXXV. O o o that 



