The interest shown by the Greeks in the heights of mountains was rooted in their desire to understand the nature of the earth. There were two sides to it. First, it was commonly accepted by the fourth century BC, if not by the fifth, that the earth was a sphere; yet, since mountains very obviously projected above the general surface, it could not be a perfect sphere. By finding the heights of mountains it could be shown that relative to the diameter of the earth, whether the circumference was taken as 252,000 or 180,000 stades, they were insignificant and did not detract from the basic sphericity. The second consideration was the depth of the atmosphere: what height did clouds reach? The widespread acceptance that the highest peaks always rose above the clouds gave fuel to this debate. The earlier attempts to measure mountains, like those to measure the earth, were limited by the lack of adequate equipment, and philosophers had to be content with the roundest of round figures. But as surveying instruments improved, a more accurate assessment became possible. It is salutary to reflect that a selection of relatively modern atlases can ascribe to a particular mountain an equal number of different heights, which vary by several metres.