278 NICHOLLS'S SEAMANSHIP AND NAUTICAL KNOWLEDGE

Construction. — In fig. 20 (1) make ^#=10 feet to represent the
distance between the shackles

(2) Bisect AB at C and erect the perpendicular C D.

(3) "With centres A and B and radius 6 feet describe two arcs cutting
each other at D. Join AD and BD.

(4) From any convenient scale, not necessarily the one used above,
make ££=20 units =20 cwts.

(5) Draw GF and OH parallel to BD and AD respectively.

The lengths of DF and DH measured from the same scale give the
tension on each leg, viz., 18*1 cwts.

Calculation —In triangle ACD given AC =5 feet, AD=6 feet,
angle 0=90°. Find angle ADC (6).

Nat. a>sec9==~==l*2 •'• 6=56° 26/- *
AC 5
Triangle ~GFD is isosceles, and if IE be drawn perpendicular to GD
then it will bisect it at E and DE will represent half the load, that is
10 cwts.
In triangle DFE, given DE=W cwts., L 6=56° 26', /.#=90°.
Find DF. DF^DE Sec 6=10 sec. 56° 26'=10X 1-81 =18-1 cwts.
The tension on each leg is therefore 18*1 cwts.
In this example we were given the length of the legs of the sling
together with the load, and were asked to find the tension on the legs,
but the question could be reversed by asking us to find the length of the
legs when the tension on them is given, as in the following example.
Example. — It is required to lift a beam weighing 4 tons by means of
a chain sling attached to a ring, the test working load of the chain
being 3 tons. Find the minimum length of chain for each leg of the
sling when the spread between the shackles is 16 feet.
Construction. — In fig. 21 (1) draw a vertical line DC and make DO
equal to 4 units, from any convenient scale, to represent 4: tons, the
weight of the beam.
'(2) With centres D and G and radius 3 units=3 tons, describe arcs
cutting each other at F and H. Join DF and DH and produse the lines
to represent the legs of the sling.
(3) Through D drawZF at right angles to CD, and make DJ and
DF each equal to 8 feet from any convenient scale.
* Tfce values of natural sines, tangents, secants, etc., are given in Nvnt's
Vauttcal Tables.