382 Beport of the Great Trigonometrical Survey. TO . 4 



The apparatus employed, consisted of a set of Compensating 

 Bars and Microscopes, on the principle of those designed by Colonel 

 Colby, for the Ordnance Survey of Great Britain, which had been 

 constructed under the superintendence of Colonel Everest, by whom 

 they were brought out to India in 1832. This apparatus has been 

 employed in measuring three Base Lines on the Great Arc, two at the 

 north and south extremities of the Calcutta Meridional Series and 

 two ab the extremities of the Indus Series. The length of these bases 

 has, in each instance, been determined in terms of ten foot Standard 

 Bar A, the unit of measure of the Indian Survey. 



At the time this Standard was constructed, it was believed that 

 the length of a well made iron bar, supported by rollers at its points 

 of least flexure, might be considered invariable for any given tempera- 

 ture. But, of recent years, there has been a growing tendency to 

 doubt the invariability which has hitherto been assumed. Series of 

 comparisons made by the Ordnance Survey show there is much proba- 

 bility that the texture of an iron bar changes gradually in the course 

 of years; for the factors of expansion obtained from groups of com- 

 parisons made at intervals a few years apart, differ from each other by 

 larger quantities than are due to errors of observation. It is prefer- 

 able, therefore, to employ several Standards, constructed of different 

 metals, rather than to trust to the integrity of a single bar. 



To ascertain whether our Standard has altered in length, it 

 would be necessary to remeasure the whole, or part, of one of the 

 Base Lines which were first measured after the arrival of the Bar 

 from England. I wished to obtain some light on this subject, by 

 remeasuring certain short sections of the Calcutta Base Line, the 

 extremities of which were originally indicated by permanent marks* 

 But, on examining the positions of the section markstones, I found 

 that, though concealed from view, there had been a regular thorough- 

 fare over them, for many years, of carts and elephants, as well as foot 

 passengers; consequently, they must, in all probability, have been 

 disturbed, and they cannot be safely referred to, to decide so delicate 

 a matter as the constancy of the Standard. 



Disappointed at being baffled in my efforts to investigate this 

 matter by any simpler and shorter process than the remeasurement of 

 a whole Base Line, I determined to mark the intermediate section 

 stations of the Vizagapatam Base as permanently as the extremities, 



