The earliest time-keeping machine is the clepsydra [1], which was no doubt made in various ways. Vitruvius mentions one made as a water wheel, [2] which would need a specialized instrument of measurement. This simplest in construction is a graduated cylindrical vessel with a hole in the bottom, and this appears to have been the most commonly used: but [3] the cylindrical object itself had to be durable in the hot Greek sun, something not found easily, though they did not know, and some of the modern writers on antiquities apparently do not either that the velocity varies as the square root of the height of the water above the hole. [4]Beckmann thought William, Abbot of Hirshaw in the eleventh century, had the best claim to the invention; of whom it was said, ‘Naturale horologium ad exemplum cœlestis hemisphœrii excogitavit;’ and soon after, certain monks had the duty ‘horologium dirigere et temperare, et signa pulsare;’ which however looks rather like their directing the clock than the clock them, or as they say at sea, ‘making it ten o’clock,’ rather than learning from the clock that it is so. I do not however find any notice of such a one in books on Greek and Roman antiquities. Various other forms of clepsydra are described in them, which it is not worth while to copy here.

[5] The case is different with sand. If a column of dry sand ever so high stands over a small hole, the sand will run out no faster than if it is a very moderate height, for the same reason that a pile or cone of sand on a given base will stand up to a certain height and angle only, which depends on the amount of friction between the particles. [6] Little written evidence exists to explain why its external form is the shape that it is —if that is of any consequence, which it hardly is for the boiling of eggs, or even for the old use of limiting the length of sermons; which object might sometimes be advantageously accomplished now by a descending sounding-board, or as sounding-boards are gone out of fashion, by a pulpit bottom or ‘drop’ let down by clock work after 25 minutes.

[7], and if the candles were of wax, as they probably were, and sheltered from wind, and the wicks uniform throughout, the measure would be accurate enough for any purpose for which it was likely to be used. [8] The consumption of oil in a lamp might also be marked and used in the same way.

This seems the proper place to notice a genuine water clock, [9] invented and used by Lord Rosse for the important purpose of driving an equatorial telescope so as to keep it pointed to a star, against the earth’s rotation, for which there are various contrivances. It is described in the R.A.S. Monthly Notices, vol. xxvi., and the principle of it is that the water is let out slowly from a box by an india rubber tube carried by a float, so that its mouth is always at the same small depth below the surface of the water; and as the pressure depends only on that depth the outflow is [10] maintained and the descent of the float with it, and that has a cord attached to it, [11] for safety purposes. The great equatorial at Greenwich is driven by another kind of water clock, which will be notified farther on, as it is combined with a revolving pendulum.