Meafzirement of a Bafe on Hounflow-Heath, 407 



^ew, how much one end of the bafe was really higher than 

 the other; and to convince the world, that in an operation of 

 this fort, where fo rtinch accuracy was expelled, no pains 

 were fpared, nor the moll: trivial circumflances neglected. 



From the trouble and uncertainty attending the frequent ufc 

 of plummets, efpecially in windy weather, inftead of mea- 

 furing level or bafe lines, as has hitherto been cuftomary (in 

 which cafe it would have been neceflary to make ufe of the 

 plummet, or fome fuch contrivance, at every f^ep of afcent or 

 defcent) it was judged to be a bettf^r method to meafure hy- 

 pothenufes, and, having obtained the relative heights of the 

 ftations by the accurate application of the telefcopic fpirit- 

 level, to compute the bafe lines. Thus it was propofed, that 

 'the length of the bafe on Hounflow-Heath fhoald be obtained 

 by meafuring a line through the air, drawn parallel to 

 .. ,the common furface from ftation to ftatlon, in equal diftances 

 of 200 yards or 600 feet each, as repreiented in the figure at 

 the top of tab. XVill. 



For this purpofe, two kinds of ftands were ufed ; one whofe 

 height was fixed, to be placed at the beginning and end of 

 each 200 yards ; and the others, whofe heights were movea- 

 ble, that their furfaces might be brought more eafily to coinr 

 clde with the line paffing through the air from one fixed 

 Hand to the other. The fixed ftands in their firft flate, repre- 

 fented by that towards the left-hand in the plate for the deal 

 rods, were only two feet {^w&w inches in height ; but when the 

 glafs rods were afterwards ufed, they had an additional pl^ce 

 of ten inches faftened to the top (as in the left-hand ftand of 

 tab. XIX.) which made their total height above the Heath, 

 including the platform on which they flood, three feet and a 

 half. They are tripods of white deal, whofe legs extend about 

 Vol. LXXV. H h h three 



