Meofurement of a Bafe on Hounflow-Heath. 399 



being right) are of opinion, that the adjuftment hy the coin- 

 cidences of lines fliould have the preference. The firll is un- 

 doubtedly the mod expeditious method ; but feems at the fame 

 time to be liable to this very objection able circumftance, that 

 the probable errors fall all one way : whereas, in the fecond me- 

 thod, although by far the moil tedious, the errors of coinci- 

 dence falling fometimcs on one fide, and fometimes on the 

 othej', they compenfate for, or deftroy, each other; and tliere- 

 fore no error is committed. 



With the view of fatisfylng both parties, and in order to put 

 the matter, if poffible, out of doubt, it was judged proper to 

 conftrucl the rods in fuch a manner as to admit of both me- 

 thods being tried, that w^e might adhere to that which fhould 

 be found by experience to be the befl:. 



Three meafuring ro^ls were accordingly ordered to be 

 made, and alfo a ftandard rod, With which the former 

 were from time to time to be compared. Their general 

 conflruiflion will be better conceived from the plan and 

 elevation, and other reprefentations of their principal parts, 

 in tab. XVIII. than by any defcription, however parti- 

 cular, conveyed in words. It will be fufficient to fay, that 

 the ftems of the three meafuring rods are each twenty feet 

 three inches in length, reckoning from the extremities of the 

 bell-metal tippings ; very near two inches deep ; and about i J 

 inch broad. Being truffed laterally and vertically, they are thereby 

 rendered perfe£lly, or at leaft as to {qi\{q, inflexible. The ftan- 

 dard rod could only be truffed laterally ; and it is juftly repre- 

 fented 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. G g g By 



