of Edinburgh , Session 1867 - 68 . 
341 
These Principles applied to the higher Surveying . 
Requirements 1 and 2 are comparatively simple to most geo- 
desists ; for these gentlemen purposely choose out a plain already 
nearly level, and insert at either end of the line they select therein, 
a defining mark of their own, say a block of stone, — plugged near 
the middle of its upper surface with a brass bolt, and that again 
with a platinum stud, and the platinum further marked with a micro- 
scopic black dot in its centre. Then the horizontal distance from the 
minute dot at one end of the line, say from 3 to 8 miles long, to 
the similar minute dot at the other end of that line, is the practical 
problem to be solved by the measurers, through employment of 
what we have indicated under head No. 3. 
But then comes the grand difficulty to them ; and it would be 
fearful to tell the number of tens of thousands of pounds which 
the Ordnance Survey of this country has expended, both directly 
and indirectly, on various methods of base-line measuring instru- 
ments, from General Roy’s wooden bars, glass bars, and steel 
chains, up to Gen. Colby’s compound compensation bars, with their 
microscopic and virtual, not actual, contacts. This last description 
of apparatus is now allowed to be very near perfection when pro- 
perly worked ; but to work it as it should be, requires amounts 
of officers, non-commissioned officers, men, tents, camp-equipage, 
measuring bars, microscopes, bar-comparing stations, lining theo- 
dolites, adjusting screw tripods, wooden stands, ground-clearing 
implements, sun-warding means, &c., &c., — which I believe even 
still bring up the cost of a base line, either on the Indian or the 
British Trigonometrical Survey, to several thousand pounds. Yet, 
whatever the expense, it is cheerfully incurred there ; because the 
accurate length of the line is required to be known, and it cannot 
be ascertained at a less cost of labour, instruments, and money. 
How the Principles should he applied to Pyramid-base Mensuration. 
Next we have to apply the same acknowledged principles to the 
mensuration of the Great Pyramid, or rather, as at present the case 
is presented to us, the four sides of its base. 
Under head No. 1 , defining points exist there already in the 
shape of the outer corners of the sockets cut anciently into the 
