5I shall now describe how water is to be conveyed to houses and cities, for which purpose levelling is necessary. This is performed either with the dioptra, the level (libra aquaria), or the chorobates. The latter instrument is however the best, inasmuch as the dioptra and level are often found to be incorrect. The chorobates is a rod about twenty feet in length, having two legs at its extremities of equal length and dimensions, and fastened to the ends of the rod at right angles with it; between the rod and the legs are cross pieces fastened with tenons, whereon vertical lines are correctly marked, through which correspondent plumb lines hang down from the rod. When the rod is set, these will coincide with the lines marked, and shew that the instrument stands level.
2But if the wind obstructs the operation, and the lines are put in motion, so that one cannot judge by them, let a channel be cut on the top of the rod five feet long, one inch wide, and half an inch high, and let water be poured into it; if the water touch each extremity of the channel equally, it is known to be level. When the chorobates is thus adjusted level, the declivity may be ascertained.
3Perhaps some one who may have read the works of Archimedes will say that a true level cannot be obtained by means of water, because that author says, that water is not level, but takes the form of a spheroid, whose centre is the same as that of the earth. Whether the water have a plane or spheroidal surface, the two ends of the channel on the rod right and left, when the rod is level, will nevertheless sustain an equal height of water. If it be inclined towards one side, that end which is highest will not suffer the water to reach to the edge of the channel on the rule. Hence it follows, that though water poured in may have a swelling and curve in the middle, yet its extremities to the right and left will be level. The figure of the chorobates will be given at the end of the book. If there be much fall, the water will be easily conducted, but if there be intervals of uneven ground, use must be made of substructions.