Meafurcment of a Bafe on Hounflow-Heatft. 399 
being right) are of opinion, that the adjuftment by the coin- 
cidences of lines fliould have the preference. The firft is un- 
doubtedly the moft expeditious method ; but feems at the fame 
time to be liable to this very objeflionable circumftance, that 
the probable errors fail all one way : whereas, in the fecond me- 
thod, although by far the moft tedious, the errors of coinci- 
dence falling fometimes on one fide, and fometimes on the 
other, they compenfate for, or deftroy, each other ; and there-, 
fore no error is committed. 
With the view of fatisfying both parties, and in order to put 
the matter, if pofiible, out of doubt, it was judged proper :o 
conftrufl the rods in fuch a manner as to admit of both me- 
; thods being tried, that we might adhere to that which fliouid 
be found by experience to be the beft. 
Three meafuring rods were accordingly ordered to be 
made, and alfo a ftandard rod, with which the former 
were from time to time to be compared. Their genera' 
< conftruction will be better conceived from the plan ana 
elevation, and other reprefentations of their principal pares, 
in tab. XVHI. than by any delcription, however parti 
cular, conveyed in words. It will be fufficient to fay, that; 
the fterns of the three meafuring rods are each twenty feet 
thiee inches 111 length, reckoning from the extremities of the 
bell-metal tippings ; very near two inches deep; and about rj 
inch broad. Being truflfed laterally and vertically, they are thereby 
rendered perfe&ly, or at leaft as to fenfe, inflexible. The ftan- 
dard rod could only be trufled laterally; and it is juftly repre- 
sented by the plan of the other rods, excepting that its ftem 
is fomething ftronger, and that it has two or three inches at 
each end of extra-length, the reafons for which difFerences 
will appear hereafter. 
Vol. LXXV, 
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