satisfactory proof of how degrees along the meridian increased in length as he headed north, they were not increasing by any consistent value; in one case they actually decreased. Similar anomalies had been found on the arcsmeasured in France and England and led cynics to suggest that the surveyors were not as infallible they pretended.
But after checking and rechecking, Lambton was sufficiently confident of his own working practices and calculations to look elsewhere for an explanation. The fault, he supposed, lay in the plummet-line, whose vertical was particularly critical in observations conducted with the zenith sector. It was known that the existence of nearby hills might distort the plummet by attraction, another knotty problem as yet unforeseen by Himalayan surveyors. But Lambton now found that even when he was well clear of hills, irregularities still occurred. The worst example of deflection had been at Bangalore, which he could only ascribe to signs of a subterranean ‘vein of dense ore’. And not without a note of triumph he announced that it was this speculation which ‘discovered to us an agent unthought of in former days, viz., a disturbing force occasioned by … diversity in the density of strata under the [earth’s] surface’.
Hence the need for a geologist and hence, in time, a whole new field of geodetic experimentation in which pendulums were used to discover variations in the direction of gravity due to the variable density of the earth’s crust. These in turn would reveal that the vertical attraction exercised by mountains was compensated for, and often more than compensated for, by the greater density of the subterranean strata which supported the mountains. As with icebergs, these invisible substrata might extend well beyond that part of a mountain which was visible above ground. Plummet-lines, instead of being attracted towards visible mountain masses, were thus just as likely to be deflected away from them and towards the denser outlying sub-strata, a contradictory and compensatory effect known as ‘isostasy’. The Himalayan surveyor was in for more surprises.
In 1818 Lambton learned that he was at last to get both his geologist and his doctor, as combined in the person of HenryVoysey. In the previous year he had submitted his third report, having in 1815 completed the Great Arc up to Bidar, about eighty miles west of Hyderabad. There he laid out his sixth base-line. The Arc, now of nearly ten degrees (or over seven hundred miles), was much ‘the longest that has ever been measured on the surface of this globe’. It had overtaken even that in Europe and, like the Anglo-French arc, ‘in grandeur and accuracy [it] must be allowed to exceed anything of the kind recorded in the history of practical science’. No longer merely a curiosity, the Arc had acquired a celebrity and a momentum of its own.
Lambton, too, was becoming something of a legend. In belated recognition of his achievement, the Survey, hitherto variously known as the ‘trigonometrical’, ‘astronomical’, or ‘mathematical’ survey of Mysore – or sometimes simply as ‘Lambton’s’ – was now officially designated as The Great Trigonometrical Survey of India. And in recognition of its having passed beyond the territories controlled from Madras it was transferred from the Madras government to the supreme government in Calcutta and to the personal attentions of the Governor-General. First intended just to cover Mysore, it had since been extended to the whole peninsula, and now in 1818 it was hoped that it might be continued north, east and west at least until lateral chains of triangles could link Bombay and Calcutta.
This meant extending the Great Arc itself still further. Lambton’s initial ‘foray’ into the more unruly territories of the Nizam of Hyderabad had gone smoothly enough. Provided he could evade the ‘gangs of plunderers which infest that part of the country when the army is not in the field’,
But after checking and rechecking, Lambton was sufficiently confident of his own working practices and calculations to look elsewhere for an explanation. The fault, he supposed, lay in the plummet-line, whose vertical was particularly critical in observations conducted with the zenith sector. It was known that the existence of nearby hills might distort the plummet by attraction, another knotty problem as yet unforeseen by Himalayan surveyors. But Lambton now found that even when he was well clear of hills, irregularities still occurred. The worst example of deflection had been at Bangalore, which he could only ascribe to signs of a subterranean ‘vein of dense ore’. And not without a note of triumph he announced that it was this speculation which ‘discovered to us an agent unthought of in former days, viz., a disturbing force occasioned by … diversity in the density of strata under the [earth’s] surface’.
Hence the need for a geologist and hence, in time, a whole new field of geodetic experimentation in which pendulums were used to discover variations in the direction of gravity due to the variable density of the earth’s crust. These in turn would reveal that the vertical attraction exercised by mountains was compensated for, and often more than compensated for, by the greater density of the subterranean strata which supported the mountains. As with icebergs, these invisible substrata might extend well beyond that part of a mountain which was visible above ground. Plummet-lines, instead of being attracted towards visible mountain masses, were thus just as likely to be deflected away from them and towards the denser outlying sub-strata, a contradictory and compensatory effect known as ‘isostasy’. The Himalayan surveyor was in for more surprises.
In 1818 Lambton learned that he was at last to get both his geologist and his doctor, as combined in the person of HenryVoysey. In the previous year he had submitted his third report, having in 1815 completed the Great Arc up to Bidar, about eighty miles west of Hyderabad. There he laid out his sixth base-line. The Arc, now of nearly ten degrees (or over seven hundred miles), was much ‘the longest that has ever been measured on the surface of this globe’. It had overtaken even that in Europe and, like the Anglo-French arc, ‘in grandeur and accuracy [it] must be allowed to exceed anything of the kind recorded in the history of practical science’. No longer merely a curiosity, the Arc had acquired a celebrity and a momentum of its own.
Lambton, too, was becoming something of a legend. In belated recognition of his achievement, the Survey, hitherto variously known as the ‘trigonometrical’, ‘astronomical’, or ‘mathematical’ survey of Mysore – or sometimes simply as ‘Lambton’s’ – was now officially designated as The Great Trigonometrical Survey of India. And in recognition of its having passed beyond the territories controlled from Madras it was transferred from the Madras government to the supreme government in Calcutta and to the personal attentions of the Governor-General. First intended just to cover Mysore, it had since been extended to the whole peninsula, and now in 1818 it was hoped that it might be continued north, east and west at least until lateral chains of triangles could link Bombay and Calcutta.
This meant extending the Great Arc itself still further. Lambton’s initial ‘foray’ into the more unruly territories of the Nizam of Hyderabad had gone smoothly enough. Provided he could evade the ‘gangs of plunderers which infest that part of the country when the army is not in the field’,
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