^fO Mcijor-Gencrnl Roy's Account of the 



inches to twenty feet, the operation of re-meafurement was 

 then begun, by placing the rods in coincidence with each 

 other (which was now found <:o be eafily and -.tccurately effeifted 

 by a few repeated ilrokes with a wooden wedge only) until the 

 fifteen rod lengths were meafured off, and a fine line, cor- 

 refpondiiig with the ivory on the fifteenth, was drawn on the 

 brafs Aide. This line was found to be o.-^*/_^^\., or near half an 

 inch beyond that which terminated the 300 feet the preceding 

 evening. Hence it is evident, that the dew imbibed only in 

 one night, or a fpace of time not exceeding fourteen hours, 

 occafioned fuch an expanfion in the deal rods, as in the whole 

 bafe would have amounted to 45.484 inches. 



It is fufficiently obvious, that this laft mentioned experi- 

 ment was more accurate, in the proportion of about fifteen to 

 one, than a.:^y comparifon we could at that time have made with 3 

 the ftandard. But fince immediately after it was finifhed, the-^ 

 fun fhone out very bright, it is by no means certain, how foon 

 the rods would again have contra6led to their former length, or 

 near it, had they been expofed to his rays. Repeated compa- 

 rifons for afcertaining fa6ls of this fort, at very ihort interims, 

 are abfolutely incompatible with the nature of fuch tedious and 

 troublefome operations as the meafurement of long bafes : and 

 here, indeed, lies the great objedion to the ufe of deal rods, 

 that at no time can we be certain how foon, after a comparifon 

 has been made, they may alter their length in a proportion, and 

 fometlmes too even ia a fenfe, different from what was ex- 

 peded. 



Defcription 



