Meofurement of a Bafe on Hounflow-Heath. 4 59, 
rod over- (hot the center of the pipe terminating the bafe- 
towards the fouth-eafi: by 17.875 inches, or 1.49 foot. , Hence,, 
when the feveral equations for expanfions are reflectively taken 
into the account, we find, that the alteration of the deal rods, 
from the humidity of the air, which, by comparifon with the 
ftandard, was apparently moft coniiderable in the firft and; 
fecond feflions of the bafe, has now wholly vanifhed ; that b 
td fiiy, the total amount of it has been over-rated by 20.964; 
inches* and this is the contradictory circuraftance that has. 
been formerly alluded to, 
1 have already fuggefted what appear to me. to have been 
the only, three, poffible caufes of this difference, found between 
the effimated and. real expanfion of the deal rods ; and as we 
are to abandon that meafurement. entirely, it is of little or no- 
importance now to endeavour to aifcover, were it poffible,; 
whence it may have arifien. If any error was actually com- , 
and Dr. Usher, Profeflbr of .Aftrooomy in the College of Dublin. This laft 
gentleman was fo obliging as to obferve, with the moft fcrupulous attention, 
throughout the whole operation with the glafs rods, that the coincidence of the 
fecond with the firft remained undifturbedj while that of the third with the fecond'.. 
was completing. 
* 1370 deal 
rods 
27400 ft. 
In.. 
4 3.389 for 1° of the brafsfcale from 62° to 63°. 
4 24.223 eftimated expanfion from humidity. 
4^27.612 equation, of the 1370 deal rods. 
3370 glafs 
rods r: 
27400 ft. 
^ 4 20.336 for 6° of the brafs fcale from 62° to 68°. 
4- 5-989 obferved expanfion of glafs Ifrom columns I ith 
— 1.802 obferved contraff ion of ditto J and 12th. 
>. — 17.875 fpace by which the 1370th rod over-fhot the pipe. 
4 6.648 equation of the 1 370 glafs rods’. . 
% 
20.964 over-rated in the total expanfion of the deal rods. 
mitted, 
