this Incile or Sluice or no, and what the Slope <lb/>
is, certain Rules and In&longs;truments have been <lb/>
invented, which are of excellent U&longs;e. </s> <s>Ignorant <lb/>
Workmen try their Slope by laying a Ball in <lb/>
the Trench, and if this Ball rowls forwards <lb/>
they think the Slope is right for their Water. <lb/>
</s> <s>The In&longs;truments of dexterous Arti&longs;ts are the <lb/>
Square, Level, Plumb-line, and, in a Word, all <lb/>
&longs;uch as are terminated with a right Angle. <lb/>
</s> <s>This Art is a little more ab&longs;tru&longs;e; but how<lb/>
ever I &longs;hall open no more of it than is nece&longs;<lb/>
<arrow.to.target n="marg51"/><lb/>
&longs;ary for the Purpo&longs;e in Hand. </s> <s>The Practice <lb/>
is performed by means of the Sight and of the <lb/>
Object, which we &longs;hall call the Points. </s> <s>If the <lb/>
Place through which we are to convey our <lb/>
Water be an even Plain, there are two Ways of <lb/>
directing our Sight: For we mu&longs;t &longs;et up cer<lb/>
tain Marks or Objects, which we may place <lb/>
either nearer or at a greater Di&longs;tance from <lb/>
each other. </s> <s>The nearer the Points of the Sight <lb/>
and the Mark or Object are to each other, the <lb/>
le&longs;s the &longs;traight Line of the Direction of the <lb/>
Sight will depart from the Superficies of the <lb/>
Globe; the further tho&longs;e Points are from each <lb/>
other, the lower the Superficies of the Globe <lb/>
will fall from the Level of the Sight. </s> <s>In both <lb/>
the&longs;e you mu&longs;t ob&longs;erve to allow ten Inches <lb/>
&longs;lope for every Mile of Di&longs;tance. </s> <s>But if you <lb/>
have not a clear Plain, and &longs;ome Hill interferes, <lb/>
then again you have two Ways of Proceeding: <lb/>
One by taking the Height from the Incile or <lb/>
Sluice, on the one Side, and the Height of the <lb/>
Slope from the Head on the other. </s> <s>The Head <lb/>
I call that appointed Place to which you would <lb/>
bring the Water, in order to let it run from <lb/>
thence free, or to appropriate it to &longs;ome particular <lb/>
U&longs;es. </s> <s>We find the&longs;e Heights by taking different <lb/>
Steps of Mea&longs;urement. </s> <s>I call them Steps be<lb/>
cau&longs;e they are like tho&longs;e Steps by which we <lb/>
a&longs;cend to a Temple. </s> <s>One Line of the&longs;e Steps <lb/>
is the Ray of Sight which goes from the Be<lb/>
holder's Eye along the &longs;ame Level with his Eye; <lb/>
which is made by the Square, the Level and the <lb/>
Plumb-line; and the other Line is that which <lb/>
falls from the Beholder's Eye down to his Feet, <lb/>
in a Perpendicular. </s> <s>By means of the&longs;e Steps <lb/>
you note how much one Line exceeds the <lb/>
other, by ca&longs;ting up the Amount of their Per<lb/>
pendiculars, and &longs;o find which is the Highe&longs;t, <lb/>
that which ri&longs;es from the Sluice to the Top of <lb/>
the Eminence, or that which ri&longs;es from the <lb/>
Head. </s> <s>The other Method, is by drawing one <lb/>
Line from the Sluice to the Top of the Hill <lb/>
which interferes, and another Lime from thence <lb/>
to the Head, and by computing the Proporti<lb/>
ons of their Angles, according to the Rules of <lb/>
Geometry. </s> <s>But this Method is di&longs;&longs;icult in <lb/>
Practice, and not extremely &longs;ure, becau&longs;e in a <lb/>
large Di&longs;tance the lea&longs;t Error occa&longs;ioned by <lb/>
the Eye of the Mea&longs;urer is of very great Con&longs;e<lb/>
quence. </s> <s>But there are &longs;ome Things which <lb/>
&longs;eem to bear &longs;ome Relation to this Method, as <lb/>
we &longs;hall &longs;hew by and by, which, if we have <lb/>
occa&longs;ion to cut a Pa&longs;&longs;age through a Hill to <lb/>
bring Water to a Town, may be of great U&longs;e <lb/>
for obtaining the right Directions. </s> <s>The Prac<lb/>
tice is as follows: On the Summit of the Hill, <lb/>
in a Place where you can have a View both of <lb/>
the Sluice on one Side and of the Head on the <lb/>
other, having laid the Ground exactly level, de<lb/>
&longs;cribe a Circle ten Foot in Diameter. </s> <s>This <lb/>
Circle we &longs;hall call the Horizon. </s> <s>In the Cen<lb/>
ter of the Circle &longs;tick up a Pike exactly per<lb/>
pendicular. </s> <s>Having made this Preparation, the <lb/>
Arti&longs;t goes round the Out&longs;ide of the Circle, in <lb/>
order to find in what Part of its Circumference <lb/>
his Eye being directed to one of the Points of <lb/>
the Water which is to be conveyed, touches <lb/>
the lower Part of the Pike which &longs;tands in the <lb/>
Center. </s> <s>Having found out and marked this <lb/>
exact Place in the Circumference of his Hori<lb/>
zon, he draws a Line for this Direction from <lb/>
that Mark quite to the oppo&longs;ite Side of his Cir<lb/>
cle. </s> <s>Thus this Line will be the Diameter of <lb/>
that Circle, as it will pa&longs;s through the Center, <lb/>
and cut through both Sides of the Circumfe<lb/>
rence. </s> <s>If this Line, upon taking oppo&longs;ite Views <lb/>
leads the Eye on one Side directly to the <lb/>
Sluice, and on the other directly to the Head <lb/>
of our Water, it affords us a &longs;traight Direction <lb/>
for our Channel. </s> <s>But if the two Lines of Di<lb/>
rection do not happen to meet in this Manner, <lb/>
and the Diameter which leads to the Sluice, <lb/>
falls on one Part of the Circumference, and <lb/>
that which leads to the Head, on another; <lb/>
then from the mutual Inter&longs;ection of the&longs;e <lb/>
Lines at the Pike in the Center of the Circle, <lb/>
we &longs;hall find the Difference between the two <lb/>
Directions. </s> <s>I u&longs;e the Help of &longs;uch a Circle to <lb/>
make Platforms and draw Maps of Towns and <lb/>
Provinces, as al&longs;o for the digging &longs;ubterraneous <lb/>
Conduits, and that with very good Effect. </s> <s>But <lb/>
of that in another Place. </s> <s>Whatever Canal we <lb/>
make, whether for bringing only a &longs;maller <lb/>
Quantity of Water for Drinking, or a larger <lb/>
for Navigation, we may follow the Directions <lb/>
which we have here taught. </s> <s>But the Prepa<lb/>
ration of our Canal mu&longs;t not be the &longs;ame for <lb/>
a large Quantity of Water, as for a &longs;mall. </s> <s>We <lb/>
&longs;hall fir&longs;t go on with the Subject which we <lb/>
</s></p>