Major-General Roy '$ Account of the 
the times of companion, was always carefully taken into the 
account. 
Now, from this lafl experiment, it feems probable, that we 
fhall not be very wide of the truth in fuppofing, that the 
flandard deal rod, which lay clofed up in its cheft, under the 
canopy on Hounflow- Heath, would fuffer the fame fort of 
alternate expanfion and contra&ion with the above-mentioned^ 
plank ; that is to fay, being of Riga wood, its mean expan- 
fion about the middle of the day would be of an inch. 
By this quantity then we muff augment the actual obferved 
expanfion of the meafuring rods,, in order to obtain within, 
certain probable limits (fi nee we cannot determine it accurately),; 
the equation for the expanfion ; or that fpace by which the 
apparent meafurement, given by the 1370 deal rods,, fhould 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 exprefifed in the 
following table* 
Table 
