Trigonometrical Survey. 579 
SECTION NINTH. 
Heights of the Stations. Terrestrial Refractions. 
art. 1. Height of the Station at Dunnose. 
With a view to obtain the heights of the stations nearly, 
from their elevations or depressions, we determined the height 
of that at Dunnose above low water in May, 1 793, by levelling 
down to the sea shore near Shanklin, a distance of about a 
mile. Instead of a levelling telescope, we made use of the 
' transit instrument, which, on account of its very accurate spi- 
rit level, seems extremely well adapted for the purpose. Two 
circular wooden platforms were provided, broad enough for the 
feet of the transit stand ; these platforms rested on pegs driven 
into the ground, and were always made horizontal at the time 
of levelling, by means of a mahogany spar, or straight-edge, 
furnished with a spirit level. The graduated rods, of course, 
were constantly set vertical on the lowest platform, while the 
transit stood on the other. 
The ground is favourable enough down to Shanklin Chine: 
this is a large deep chasm, opening to the sea ; but the descent 
is not so sudden on the western side, which is by far the 
steepest, and to which we levelled, but a person may get up 
or down with safety. We found its. perpendicular height by 
means of several rods placed end ways against the sloping side, 
and supported in an horizontal position, and then letting fall 
a measuring tape from one rod to another : but this was the 
most troublesome and difficult part of the whole operation. 
The fall from the bottom of this chasm or opening, to the 
water's edge, was found in the usual manner. 
The whole perpendicular descent thus determined, was 792 
feet ; which, we have no reason to suppose, is more than 2 or 3 
