Word: convexity
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Dates: during 1880-1889
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...exerted suddenly, considerable resistance is offered even by as mobile a body as air. Birds are enabled to fly only by the resistance of the air during the downward stroke of the wings. During the upward stroke, less resistance is offered, owing to the fact that the wing is convex on the upper side, and is at the same time contracted in area, thus moving with less velocity. In fact instantaneous photographs show that it takes twice the time to complete the upstroke that is needed for the downstroke. Unless matter offered resistance, no force could be brought to bear...
...present time, the flint is entirely ground and polished on one side, but on the other has yet to undergo corrections for diffraction. Whereas it is now slightly convex on this latter side it will have to be ground flat or even a trifle concave, to bring about the desired result. The crown is already ground and is being put through the process of polishing...
...done as follows: In the first place the curves of the lens are determined by experiments with smaller models, aided, of course, by the maker's long experience. Then an iron disk, large enough to-cover the glass, is made into a concave shape exactly corresponding to the desired convexity of the lens, thus, in reality forming a species of mould. This disk, which by the way is called the "tool," is placed on the glass, and by a simple mechanical device is made to rotate upon it. When the grinding is completed by the use of this tool...