£88 The Account of a 
not properly driven, till in the afternoon, when we found that 
the curve appearance was lost, and the ebullition in the air 
had subsided. 
The new raised earth about the gun at King's Arbour, pre- 
vented a very accurate measurement of the height of the in- 
strument above the point of commencement of the base ; and 
therefore two opportunities only presented themselves for de- 
termining the actual terrestrial refraction ; namely, at the 
ends of the base of verification. From the depression taken 
at Beacon Hill, the refraction was 38" ; but the elevation of 
Beacon Hill, observed at the lower end, near Old Sarum, gives 
50". These deductions, perhaps, cannot be deemed very con- 
clusive ; because, as they depend on the difference in the ver- 
tical heights of the ends of the base, every 2 inches of error in 
that difference will produce an error of about 1" in the com- 
puted refraction. We shall close this section with the data 
whence those refractions were .obtained. 
At Beacon Hill, the top of the flagstaff near Old Sarum was 
depressed 42' 6". 
At the other end of the base, near Old Sarum, the top of 
the flagstaff at Beacon Hill was elevated 38' 42". 
The axis of the telescope at Beacon Hill was 15 inches 
above, and the top of the flagstaff 91 inches above the point 
where the mensuration began. Near Old Sarum it was 28 
inches higher, and the top of that flagstaff 95 inches above 
where the base terminated. This end (see Sect. 111.) is 429,48 
feet lower than the other. Lastly, the value of the base is 6 ' 
of a degree, very nearly. 
