small by the amount the horizon is inclined to the tangential plane. At Darjeeling the surface of water in repose is inclined about 35'' to this plane, at Kurseong about 51'', at Siliguri about 23'', at Dehra Dun and Mussooree about 37''. For this reason all angles of elevation to Himalayan peaks measured from the plains, as Mount Everest was measured, are too small and consequently all our values of Himalayan heights are too small. Errors of this nature range from 40 to 100 feet." This then is a considerable source of error, but the most serious source of uncertainty affecting the value of heights is the refraction of the atmosphere. A ray of light from a peak to an observer's eye does not travel along a straight line but assumes a curved path concave to the earth. The ray enters the observer's eye in a direction tangential to the curve at that point, and this is the direction in which the observer sees the peak. It makes the peak appear too high. Corrections have therefore to be applied. But there is no certainty as to what should be the amount of the correction; and it is now believed that the computers of the height of Mount Everest applied too great a correction for refraction and consequently reduced its height too much. Burrard brings together in the following table the different errors to which the carefully determined height of Mount Everest is liable:-- ---------------------------------------------------+-------------------- Source of error. | Magnitude of | possible error. ---------------------------------------------------+-------------------- Variation of snow level from the mean | Unknown Errors of observation | 10 feet Adoption of erroneous height for observing station | 10 feet Deviation of gravity | 60 feet, too small Atmospheric refraction | 150 feet, too small ---------------------------------------------------+-------------------- The following table shows how the different values of the height of Mount Everest have been deduced:-- HEIGHT OF MOUNT EVEREST