The Great Arc by John Keay Read Free Book Online

the circle’s calibration, and horizontally round a larger horizontal circle so that angles in a plane can be read in the same way. Plummets, spirit levels and adjustment screws are incorporated for the alignment and levelling of the instrument, and micrometers and microscopes for reading the calibration. Additionally, the whole thing has to be rock stable and its engineering, optics and calibration of the highest precision. In fact there were probably only two or three instruments in the world sufficiently sophisticated and dependableto have served Lambton’s purpose. Luckily he had discovered one, almost identical to that used by William Roy, which had just been built by William Cary, a noted English manufacturer. But it had to be shipped from England, a considerable risk in itself for an instrument weighing half a ton and about the size of a small tractor. And unfortunately the ship chosen was unaccountably overdue.
    It had still not arrived when Lambton marked out and cleared his Madras base-line. The site chosen was a stretch of level ground between St Thomas’s Mount, a prominent upthrust of rock where the ‘doubting’ apostle was supposed to have once lived in a cave, and another hill seven and a half miles to the south. Situated on the south-east edge of the modern city, the Mount has since been overtaken by development, but the other end of the base-line is still predominantly farmland and scrub as in Lambton’s day. Having cleared and levelled the ground and aligned the chosen extremities, Lambton commenced measurement with Dinwiddie’s hundred-foot chain.
    By now he had received from England a second chain, but this was reserved as a standard against which Dinwiddie’s was frequently checked for any stretching from wear or expansion. Expansion and contraction due to temperature change was a major problem. William Roy of the Ordnance Survey, while measuring his first base-line on Hounslow Heath (now largely occupied by Heathrow Airport), had discarded both wooden rods and steel chains before opting for specially made glass tubes. Lambton in India had no such handy alternative; he had to make the best of the chains. When in use, the chain was drawn out to its full hundred feet and then supported and tensioned inside five wooden coffers, each twenty feet long, which slotted cleverly onto tripods fitted with elevating screws for levelling. Each coffer he now equipped with a thermometer which had to be read and recorded at the time of each measurement. By comparison with the other chain, which was kept ina cool vault, a scale of adjustment was worked out for the heat-induced expansion.
    But April and May are hot months in Tamil Nadu. The temperature seesawed between 80 and 120 degrees Fahrenheit. Although Lambton says nothing of the inconvenience of working in such heat, he was worried sick by the variations. After endless experiments he came to the conclusion that a one-degree change of temperature made a difference of 0.00742 of an inch in the hundred-foot length of the chain. But were the locally purchased thermometers sufficiently accurate? And might the temperature not have changed in the interval between marking the measurement and reading the thermometer? Lambton was deeply concerned; measurements and readings were to be taken only at dawn or in the early afternoon when the temperature was as near stable as it got; the thermometers were checked and rechecked, both chains measured and remeasured against a standard bar. Nothing gives a better idea of his passion for shaving tolerances to an infinitesimal minimum than this pursuit of a variable amounting to just seven thousandths of an inch.
    To complete the full seven and a half miles of the base-line required four hundred individual measurements with the chain. For each of these measurements the coffers and tripods as well as the chain itself had to be moved forward. It was a slow business even after Lambton’s men had been drilled to do it by numbers. The