For registering the height of the tide at every instant, hydrographic
services generally adopt quite a simple marigraph. The apparatus
consists in principle of a counterpoised float whose rising and
falling motion, reduced to a tenth, by means of a system of toothed
wheels, is transmitted to a pencil which moves in front of a vertical
cylinder. This cylinder itself moves around its axis by means of a
clockwork mechanism, and accomplishes one entire revolution every
twenty-four hours. By this means is obtained a curve of the tide in
which the times are taken for abscisses and the heights of the sea for
ordinates. However little such marigraphs have had to be used, great
defects have been recognized in them. When we come to change the sheet
on the cylinder (and such change should be made at least once every
fifteen days), there is an interruption in the curve. It is necessary,
besides, to perform office work of the most detailed kind in order to
refer to the same origin all these curves, which are intercrossed and
often superposed in certain parts upon the original sheet.
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