Meafurement of a Bafi on Hounflo w-fleath . 407 
ihew, 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 much accuracy was expected, no pains 
were fpared, nor the moil trivial circumftances neglected. 
From the trouble and uncertainty attending the frequent ufe 
of plummets, efpecially in windy weather, inflead of mea» 
luring level or bafe lines, as has hitherto been cuHomary (in 
which cafe it would have been neceffary to make ufe of the 
plummet, or fome inch contrivance, at every Hep of afcent or 
defcent) it was judged to be a better method to meafure hy- 
pothenufes, and, having obtained the relative heights of the 
Nations 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 Houn flow- Heath fhould be obtained 
by meafuring a line through the air, drawn parallel to 
the common furface from flation to flation, in equal diflances 
of 200 yards or 600 feet each, as reprefented in the figure at 
the top of tab. XVIII. 
For this purpofe, two kinds of Hands wereufed; onewhofe 
height was fixed, to be placed at the beginning and end of 
eacii 2co yards 5 and the others, whole heights were movea- 
ble, that their furfaces might be brought more eafdy to coin- 
cide with the line palling through the air from one fixed 
Hand to the other. The fixed Hands in their firfl Hate, repre- 
fented by that towards the left-hand in the plate for the deal 
rods, were only two feet feveii inches in height ; but when the 
glafs rods were afterwards ufed, they had an additional piece 
of ten inches- faflened to the top (as in the left-hand Hand of 
tab. XIX.) which made their total height above the Heath, 
including tne platform on which they Hood, three feet and a 
hair, i hey are tripods of white deal, whofe legs extend about 
\ ol , LXXVe H h h three 
