Meafurement of a B'afe on Hounflow-Hfeath. 4-5^. 



rod ovei'-fhot the center of the pipe termlnatnig the bafe 

 towards the foiith-eall: by iJ^^JS inches, or 1.49 foot. Hence,, 

 when the feveral equations for expanfions are refpe£lively taken 

 into the account, we find, that the alteration of the deal rods 

 from the humidity of the air, which, by comparifon with the 

 flandard, was apparently moft confiderable in the firft and 

 fecond fe6lions of the bafe, has now wholly vanifhed ; that is 

 to fay, the total amount of it has been over- rated by 20.964. 

 inches*; and this is the contradictory circumflance that has. 

 been formerly alluded to. 



I have already fuggefted what appear to me to have been 

 the only three pofTible caufes of this difference,, found between 

 the eflimated and. real expaiifion of the deal rods ; and as we 

 are to. abandon that meafurement entirely, it is of little or no. 

 importance now to endeavour to difcover, were it poffible,. 

 whence it may have arifen. If any, error was actually com- 



and Dr. Usher, Profeflbr of Aftronomy in the College of D,ublin. This laft 



gentleman was fo obliging as to obferve, with the mofl fcrupulous attention, 



throughout the whole operation with the glafs rods, that the coincidence of the 



fecond with the firft remained undillurbed, while that of the third with the fecond> 



was completing. 



In., 

 * 1^70 deal") r, . r. 



rods rr I + 3-389 ^or 1 of. the brafs fcale from 62° to 63°. 

 27400 ft. J "^ 24.223 eftimatedexpanfion from humidity. 



+ 27.612 equation of the 1 370 deal rods. 



r + 20.336 for 6° of the brafs fcale from 62° to 68°. 

 1370 glafs I -f 5.989 obferved expanlion of glafs "| from columns iith. 

 rods— — 1.802 obferved contraftion of ditto J and 12th. 



27400 ft. V. — 17.875 fpace by which the 1370th rod over-fliot the pipe. 



+ 6.648 equation of the I370.glafs rods. 



ao.964 over-rated in the total expanfion of the deal rods. 



mitted, . 



