4^6 Major-General Roy's Acccunt of the 



the times of comparlfon, was always carefully taken into the 

 account. 



Now, from this laft experiment, it feems probable, that we- 

 Hiall not be very wide of the truth in fuppofing, that the 

 ftandard deal rod, which lay clofed up in its cheft, inider the 

 canopy on Hounflow- Heath, would fuffer the fame ibrt of 

 alternate expanfion and contratlion with the above-mentioned 

 plank ; that is to fay, being of Riga wood, its mean expan- 

 lion about the middle of the day would be ^-j^VW ^^ ^^^ inch.. 

 By this quantity then we mufl augment the a€lual obferved' 

 expanfion of the meafuring rods, in order to obtain within 

 certain probable limits (lince we cannot determine it accurately); 

 the equation for the expanfion ; or that fpace by which thei' 

 apparent meafurement, given by the 1370 deal rods, (hould be. 

 augmented in order to obtain the true length of the bafe ; or. 

 that which would have been given by unalterable rods, of the 

 fame original length with thofe of deal, as expreffed in tha 

 following table. 



- 





Table 



