Herschel, John F. W. (John Frederick William), Sir, 1792-1871 · London : Longman, Rees Orme · leaf 99
by such a line, admitting its perfect flexibility, is that ofgravity, or perpendicular to the surface of still water.Its application to the purposes of astronomy is, however,so delicate, and difficult, and liable to error, unless ex-traordinary precautions are taken in its use, that it is atpresent… … now generally used, on account of its extreme mobi-lity, and not being liable to freeze,) the bubble in which,when the tube is placed horizontally, would rest indif-ferently in any part if the tube could be mathematicallystraight. But that being impossible to execute, andevery tube having some…
Astronomy
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