Earth Frank Press, 1982

 In 1865 no less a figure than the Astronomer Royal, Sir George Airy, came forward with an explanation for this discrepancy that contained the basis of the principle of isostasy discussed in chapter 17. Airy proposed that the enormously heavy mountains are not supported by a strong, rigid crust below, but that they "Float" in a "sea" of denser rock. Stated otherwise, the excess mass of the mountains above sea level is compensated by a deficiency of mass in an underlying root.

This root provides the buoyant support, in the manner of all floating bodies, just as a ship with a deep hull is buoyed up. The plumb bob " Feels " both the excess mass on top and the deficiency of mass below, hence the reduced deflection. The resolution of the Indian puzzle not only led to the concept of isostasy but also introduced gravity surveying as a method for detecting mass variations in the interior by their corresponding gravity variations.

Earth Frank Press, 1982  

Some 150 years ago during the great land survey of India, a curious discrepancy was uncovered by British surveyors. The distance between Kaliana, some 100 Kilometers (60 miles) south of the Himalaya range, and kalianpur, 600 kilometers (375 miles) further south, was determined in two precise ways- by measurement over the surface and by reference to astronomical observations- and the results disagreed by some 150 meters (500 feet) in 600 kilometers. This may seem a small amount, but it was an intolerable surveying error even by nineteenth-century standards. The astronomical method of measuring distance uses the angles of stars with respect to the vertical, which is defined by a plumb line (a weight suspended on a string). To account for the difference, it was proposed that the plumb line was tilted toward the Himalayas because of the gravitational attraction of the mountains on the plumb bob, causing an error in the distance measurement. When the effect was actually calculated, it was found that the mountains should have introduced an even larger error - one of about 450 meters ( 1500 feet ) - thus compounding the puzzle.