the ocean and measured temperatures at various depths. While in India, Pratt was approached by Andrew Scott Waugh, Surveyor General of India who had succeeded George Everest, to examine the gravitational anomaly caused by the Himalayas on the plumbline which resulted in errors in the Great Trigonometrical Survey. Pratt estimated the deviation of the plumbline that would be expected by the mass of the mountains (based on density estimated in 1772 at Mount Schiehallion in Scotland) but the observed deviations were much less. He propounded a theory that the density of the mountains was less than that of the underlying substrate. It was also noticed that the deviations of the plumbline were greater at the foot of the mountain and reduced as one moved away from the mountain. George Airy came up with the explanation that the roots of the mountain go deep into the earth. He compared it with a heap of logs in water and suggested that when a log juts higher above the water, a greater amount of it must be submerged. Pratt on the other hand suggested that there was a lower density under mountains below sea-level (actually refined later and termed the depth of compensation) and that this is offset by the mass above sea-level. Pratt's explanation assumes a variation in density whereas Airy assumes a constant crustal density. Pratt found fault with Airy's idea as it assumed a thinner crust (10 miles) than that estimated by William Hopkins (800 to 1000 miles) and also that the crust is lighter. Pratt said that the crust cools from the solid interior and should therefore be denser. Airy did not defend his view but Samuel Haughton used the debate to claim that mathematics was a useless tool for speculation. Pratt suggested that the crust would be depressed in the cooler parts of the world and suggested that the plumb deflection was caused by anomalous high density in the oceans south of the Himalayas. Pratt also applied his knowledge of physics and mathematics to a number of applications in India on which he was consulted by engineers. He examined arches, the physics involved in the sudden movement of a mass of water such as in the Indus floods of 1858 and in the bore of the Hooghly river, and computed the iron required for cantilever bridges. Pratt took a great interest in Hindu Astronomy and supported the translation