606 ON THE DEFLECTION OF THE PLUMB-LINEAT ARTHUR’S SEAT, ETC. 
stations on Arthur’s Seat, is 5-316, with a probable error of +-054 (or one hundredth 
part) due to the probable errors of the astronomical amplitudes. If S be the probable 
error of the assumed mean density of Arthur’s Seat, the probable error of this deter- 
mination of the mean density of the earth is 
±V^3-725^"+-003. 
Remarks. 
In the original paper as read at the Meeting of the Royal Society on 21st 
February, the mean density was given as 5' 14 with a probable error of +-07. In a 
subsequent revision of the calculations, the astronomical amplitudes and their pro- 
bable errors were determined as herein explained. These amplitudes exceed those 
previously used by 0"’02, 0"-01, 0"-03, tending to increase the density. The attrac- 
tion due to the ground within 100 feet round each of the stations, originally omitted, 
is now included, also tending to increase the density. 
Explanation of the Plates. 
PLATE XXXII. 
Is the contoured plan of Arthur’s Seat, on the scale of six inches to a mile: this is 
part of one of the sheets of the plan of Edinburghshire which has been 
published on that scale. The zenith sector stations and the lines of 
sections are marked on this plan. The contours furnish sufficient data to 
make a model. 
PLATE XXXIII. 
Contains geological sections taken on the three lines which are drawn on the plan, 
and also a table of the specific gravity of the rocks. 
These plates have been engraved and electrotyped at the Ordnance Survey Office, 
and form part of the series of plates made to illustrate the account of the Trigono- 
metrical Survey of Great Britain which is now in the press. 
