/j%6 The Account of a 
Hill, we found the depression of the ground at Chanctonhury 
Ring, vary from 1' 41" to 2' 30". The observations, however, 
on which the tables are founded, were made in close cloudy 
days, or toward the evenings, when the tremulous motion in 
the air is commonly the least. 
It has been conjectured, that the variations in terrestrial re- 
fraction, depend on the changes in the atmosphere indicated 
by the barometer and thermometer: this however, cannot be 
the case when the rays of light pass near the earth’s surface 
for any considerable distance. Mr. De la Lande, in his As- 
tronomy (Art. Terrest . Ref.), remarks, that the mountains in 
Corsica are sometimes seen from the coasts of Genoa and 
Provence, but at other hours on the same days, they totally 
disappear, or are lost as it were in the sea. And the late Ge- 
neral Roy frequently mentioned an instance of extraordinary 
refraction, which himself and Colonel Calderwood observed 
on Hounslow Heath, when they were tracing out the base. 
Their levelling telescope at King’s Arbour was directed to- 
wards Hampton Poor House, where a flagstaff was erected at 
that end of the base ; this for a long time they endeavoured 
in vain to discover, till at last, very unexpectedly, it suddenly 
started up into view, and so high it seemed to be lifted, that 
the surface of the ground where it stood, became visible. This 
will appear the more extraordinary, when it is considered, that 
a right line drawn from the eye at King’s Arbour to the other 
end of the base, would pass 8 or 9 feet below the surface of the 
intermediate ground near the Duke of St. Alban’s Park. 
The following is still more singular. s< I observed,” says Mr. 
Dalby, “ what seemed to me a very uncommon effect of ter- 
“ restrial refraction, in April, 1793, as I went from Freshwater 
