4^0 Major-General Roy’s Account of the 
inches to twenty feet, the operation of re -measurement was 
then begun, by placing the rods in coincidence with each 
other (which was now found to be eafily and accurately effected 
by a few repeated ftrokes with a wooden wedge only) until the 
fifteen rod lengths were meafured off, and a fine line, cor- 
refponding with the ivory on the fifteenth, was drawn on the 
brafs Hide. This line was found to be o.yt 9 JL., 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, 
oceafioned 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 any comparifon we could at that time have made with 
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 contracted to their former length, or 
near it, had they been expofed to his rays. Repeated corrrpa- 
i-ifons for afcertaining facts of this fort, at very fhort 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 objection 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 
fometimes too even in a fenfe, different from what was ex-- 
peCted. 
Defcription 
