98 Transactions.—Miscellaneous. 
Having the area of the country, therefore, to be submitted to this process, 
the total cost may be nearly calculated. 
But the most important feature of this system of triangulation affecting 
colonial survey requirements is in its requiring to advance or spread out 
from one or two points only, from very accurately measured base lines. 
Thus in the British Islands, all parts of the country, have only been reached 
by a lapse, from the commencement, of 90 years, and what has been done 
for India has taken 75 years. There may be no inconvenience from this 
in these immemorially possessed countries, but where an unmarked wilder- 
ness is to be divided amongst an inflowing people the case is of an opposite 
nature. Here the wants of the people demand ubiquitous and immediate 
attention over the whole area of the country under the process of coloniza- 
tion. But to this subject I will afterwards refer, and next notice the degree 
of accuracy attained by great or primary triangulation. 
All first efforts are necessarily imperfect, however great the skill and assi- 
duity of the projectors, and in this most refined system of geodesy we find no 
exception. The original labours of Roy and Mudge, in England, and Lamb- 
ton, in India, have been revised in'order to bring their work up to the perfection 
attained by modern instruments and inventions, Thus, in the first meridian 
arc triangulations, between Dunose and Misterton Carr (200 miles apart), 
the distance between Arbury Hill and Corly (a side of one of the “ chain ” 
of triangles near the middle of the arc), it was found that from the Dunose 
base the length was 117,463 feet, and from the Misterton Carr base, 
117,457-1 feet, a difference of 5:9 feet, or 70 inches in somewhat more than 
22 miles—i.e., 8:2 inches per mile. In recent times bases in Ireland have 
been measured by Colby's eompensation bars, by which each 100 feet can 
be measured with an accuracy equal to half the breadth of a sharp steel 
point, on a plate of metal observed by a microscope. * In India the 
Dehhra Doon base, 7:42 miles in length, was measured in reverse order, with 
an error of only 2:896 inches, or about 4%, of an inch per mile, and the 
Bider base, about 745, miles in length, compared with the Sironj base by 
triangulation and computation, though 400 miles apart, was found to have 
a difference of 4-296 inches, or about ,5, of an inch per mile, 
Steel chains had been used in the measurement of bases but to be aban- 
doned, .and on this subject Colonel Walker says that, * taking all cireum- 
stances into consideration, the conclusion is inevitable and irresistible that 
the chain base lines are worthless for the purpose of controlling the principal 
iriangulation of this survey (of India), and more partieularly that portion 
* Markham, ** Great Trigonometrical Survey of India,” page 90. The Loch Foyle 
base measured, in this manner, nearly eight miles, or exactly 41,040-8873 feet, with an 
error of less than a quarter of an inch per mile, 
