Trigonometrical Survey. 587 
44 Gate, in the Isle of Wight, towards the Needles. Soon after 
44 you leave Freshwater Gate, you get on a straight and easy 
44 ascent, which extends 2 or 3 miles ; a mile, or perhaps a mile 
44 and an half beyond this to the westward, is a rising ground, 
44 or hill; and it is to be remarked, that its top and the afore- 
44 said straight ascent, are nearly in the same plane : now in 
44 walking towards this hill, I observed that its top (the only 
44 part visible) seemed to dance up and down in a very extra- 
44 ordinary manner ; which unusual appearance however, evi- 
44 dently arose from unequal refraction, and the up-and-down 
44 motion in walking ; but when the eye was brought to about 
44 2 feet from the ground, the top of the hill appeared totally 
44 detached, or lifted up from the lower part, for the sky was 
44 seen under it. This phenomenon I repeatedly observed. 
44 There was much dew, and the sun rather warm for the sea- 
44 son, consequently a great evaporation took place at that 
44 time/" Here, and also on Hounslow Heath, the rays of 
light passed near the earth's surface a great way before they 
arrived at the eye ; and it is more than probable, that moist 
vapours were the principal cause of the very unusual refrac- 
tions : the truth of which conjecture seems to be verified by 
the following circumstance. In measuring the base on Houn- 
slow Heath, we had driven into the ground, at the distance of 
100 feet from each other, about 30 pickets, so that their heads 
appeared through the boning telescope to be in a right line ; 
this was done in the afternoon. The following morning proved 
uncommonly dewy, and the sun shone bright ; when having 
occasion to replace the telescope, we remarked that the heads 
of the pickets exhibited a curve, concave upwards, the farther- 
most pickets rising the highest ; and we concluded they were 
