Passage:
In 1802, the British East India Company initiated what would become the Great Trigonometrical Survey of India, an ambitious geodetic endeavor aimed at measuring the curvature of the Earth and creating an exact mathematical framework for mapping the subcontinent. Directed initially by William Lambton, the survey relied on the method of triangulation, which required establishing an initial baseline of immense precision against which all subsequent angular measurements were calculated. Any minute error in this foundational distance would propagate exponentially across hundreds of miles of linked triangles.
To measure initial baselines, Lambton utilized hundred-foot blistered-steel chains supported on five wooden coffers and tensioned with heavy weights. Because steel expands and contracts predictably with temperature, each link was periodically checked against a standard brass yard, and surveyors used mercury thermometers placed inside the coffers to calculate thermal expansion corrections. However, Lambton’s method suffered from two persistent operational defects. First, the mechanical friction between the articulating links caused microscopic wear that imperceptibly lengthened the chain over successive measurements. Second, because the ambient air temperature within the coffers varied unevenly along the hundred-foot length, thermometer readings often failed to capture localized thermal gradients, introducing systematic discrepancies into the baseline records.
When George Everest succeeded Lambton as superintendent in 1823, he sought a standardized system that could circumvent the uncertainties of thermal calculation and mechanical wear. While in England during the late 1820s, Everest observed trials of the bimetallic compensation apparatus invented by Major-General Thomas Colby of the Ordnance Survey. Colby’s apparatus consisted of two ten-foot bars—one of iron and one of brass—firmly riveted together at their exact centers. Because brass expands under heat at a greater rate than iron, the ends of the bars did not expand uniformly. To harness this difference, Colby attached pivoting steel tongues to the outer ends of the bars, with microscopes focused on minute dot markers positioned at the tongues' geometric centers of compensation. When temperature fluctuated, the differential expansion of the two metals tilted the pivoted tongues, keeping the distance between the focal markers precisely constant.
Upon resuming field operations in India in 1830, Everest deployed Colby’s compensation bars to measure the Calcutta baseline. The apparatus proved exceptionally reliable, eliminating the necessity of continuous temperature logging and post-hoc mathematical corrections during measurement runs. However, the system introduced substantial logistical burdens. The bimetallic bars and their protective wooden casings were exceedingly heavy, necessitating large platoons of military sappers and porters to transport and align the delicate optical tripods across rough terrain, thereby increasing the operational costs of the survey.
According to the passage, which of the following is an explicit characteristic or consequence of using Colby’s bimetallic compensation apparatus in the Great Trigonometrical Survey?
- AIt utilized articulating joint links encased in wooden coffers to minimize frictional wear during field measurements.
- BIt substantially reduced the logistical burden and physical weight of the equipment transported across rough terrain.
- It removed the requirement to log continuous temperature measurements and apply subsequent mathematical adjustments.Answer
- DIt prevented any physical expansion from occurring in either the brass or iron component bars when exposed to heat.
- EIt was designed primarily to measure and correct for plumb-line deflection caused by the gravitational attraction of the Himalayas.