Alberti, Leone Battista, Architecture, 1755

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              this Incile or Sluice or no, and what the Slope
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              is, certain Rules and Inſtruments have been
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              invented, which are of excellent Uſe. </s>
              <s>Ignorant
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              Workmen try their Slope by laying a Ball in
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              the Trench, and if this Ball rowls forwards
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              they think the Slope is right for their Water.
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              </s>
              <s>The Inſtruments of dexterous Artiſts are the
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              Square, Level, Plumb-line, and, in a Word, all
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              ſuch as are terminated with a right Angle.
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              </s>
              <s>This Art is a little more abſtruſe; but how­
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              ever I ſhall open no more of it than is neceſ­
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                <arrow.to.target n="marg51"/>
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              ſary for the Purpoſe in Hand. </s>
              <s>The Practice
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              is performed by means of the Sight and of the
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              Object, which we ſhall call the Points. </s>
              <s>If the
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              Place through which we are to convey our
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              Water be an even Plain, there are two Ways of
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              directing our Sight: For we muſt ſet up cer­
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              tain Marks or Objects, which we may place
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              either nearer or at a greater Diſtance from
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              each other. </s>
              <s>The nearer the Points of the Sight
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              and the Mark or Object are to each other, the
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              leſs the ſtraight Line of the Direction of the
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              Sight will depart from the Superficies of the
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              Globe; the further thoſe Points are from each
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              other, the lower the Superficies of the Globe
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              will fall from the Level of the Sight. </s>
              <s>In both
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              theſe you muſt obſerve to allow ten Inches
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              ſlope for every Mile of Diſtance. </s>
              <s>But if you
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              have not a clear Plain, and ſome Hill interferes,
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              then again you have two Ways of Proceeding:
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              One by taking the Height from the Incile or
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              Sluice, on the one Side, and the Height of the
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              Slope from the Head on the other. </s>
              <s>The Head
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              I call that appointed Place to which you would
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              bring the Water, in order to let it run from
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              thence free, or to appropriate it to ſome particular
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              Uſes. </s>
              <s>We find theſe Heights by taking different
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              Steps of Meaſurement. </s>
              <s>I call them Steps be­
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              cauſe they are like thoſe Steps by which we
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              aſcend to a Temple. </s>
              <s>One Line of theſe Steps
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              is the Ray of Sight which goes from the Be­
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              holder's Eye along the ſame Level with his Eye;
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              which is made by the Square, the Level and the
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              Plumb-line; and the other Line is that which
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              falls from the Beholder's Eye down to his Feet,
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              in a Perpendicular. </s>
              <s>By means of theſe Steps
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              you note how much one Line exceeds the
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              other, by caſting up the Amount of their Per­
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              pendiculars, and ſo find which is the Higheſt,
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              that which riſes from the Sluice to the Top of
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              the Eminence, or that which riſes from the
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              Head. </s>
              <s>The other Method, is by drawing one
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              Line from the Sluice to the Top of the Hill
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              which interferes, and another Lime from thence
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              to the Head, and by computing the Proporti­
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              ons of their Angles, according to the Rules of
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              Geometry. </s>
              <s>But this Method is diſſicult in
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              Practice, and not extremely ſure, becauſe in a
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              large Diſtance the leaſt Error occaſioned by
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              the Eye of the Meaſurer is of very great Conſe­
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              quence. </s>
              <s>But there are ſome Things which
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              ſeem to bear ſome Relation to this Method, as
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              we ſhall ſhew by and by, which, if we have
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              occaſion to cut a Paſſage through a Hill to
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              bring Water to a Town, may be of great Uſe
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              for obtaining the right Directions. </s>
              <s>The Prac­
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              tice is as follows: On the Summit of the Hill,
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              in a Place where you can have a View both of
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              the Sluice on one Side and of the Head on the
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              other, having laid the Ground exactly level, de­
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              ſcribe a Circle ten Foot in Diameter. </s>
              <s>This
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              Circle we ſhall call the Horizon. </s>
              <s>In the Cen­
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              ter of the Circle ſtick up a Pike exactly per­
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              pendicular. </s>
              <s>Having made this Preparation, the
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              Artiſt goes round the Outſide of the Circle, in
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              order to find in what Part of its Circumference
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              his Eye being directed to one of the Points of
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              the Water which is to be conveyed, touches
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              the lower Part of the Pike which ſtands in the
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              Center. </s>
              <s>Having found out and marked this
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              exact Place in the Circumference of his Hori­
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              zon, he draws a Line for this Direction from
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              that Mark quite to the oppoſite Side of his Cir­
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              cle. </s>
              <s>Thus this Line will be the Diameter of
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              that Circle, as it will paſs through the Center,
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              and cut through both Sides of the Circumfe­
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              rence. </s>
              <s>If this Line, upon taking oppoſite Views
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              leads the Eye on one Side directly to the
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              Sluice, and on the other directly to the Head
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              of our Water, it affords us a ſtraight Direction
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              for our Channel. </s>
              <s>But if the two Lines of Di­
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              rection do not happen to meet in this Manner,
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              and the Diameter which leads to the Sluice,
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              falls on one Part of the Circumference, and
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              that which leads to the Head, on another;
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              then from the mutual Interſection of theſe
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              Lines at the Pike in the Center of the Circle,
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              we ſhall find the Difference between the two
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              Directions. </s>
              <s>I uſe the Help of ſuch a Circle to
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              make Platforms and draw Maps of Towns and
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              Provinces, as alſo for the digging ſubterraneous
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              Conduits, and that with very good Effect. </s>
              <s>But
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              of that in another Place. </s>
              <s>Whatever Canal we
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              make, whether for bringing only a ſmaller
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              Quantity of Water for Drinking, or a larger
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              for Navigation, we may follow the Directions
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              which we have here taught. </s>
              <s>But the Prepa­
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              ration of our Canal muſt not be the ſame for
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              a large Quantity of Water, as for a ſmall. </s>
              <s>We
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              ſhall firſt go on with the Subject which we
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              </s>
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