This section is from the book "The Boy Mechanic Vol. 2 1000 Things for Boys to Do", by Popular Mechanics Co.. Also available from Amazon: The Boy Mechanic, Vol2: 1000 Things for Boys to Do.
The leader for trout is preferably of single gut, and as fine as the angler's skill will allow. The fly caster's rule is to use a leader whose breaking strain is less than the line, then, when the tackle parts, it is simply a question of putting on a new leader and the more expensive line is saved. Ready-made leaders may be purchased, or the angler can tie them up as desired. For length, a 3 or 3 1/2-ft. leader is about right for average fishing. Longer leaders are used, and while they sometimes are of advantage, the 3-ft. length is more useful. A longer leader is awkward to handle because the loop is apt to catch in the top of the rod when reeling in the line to bring the fish close to the landing net. Leaders may be had with a loop at each end, or with loops tied in, for using a cast of two or three flies. For all average casting, the two-fly cast is the best, but the expert angler uses the single-fly very often. For lake fishing, the single large fly is generally preferred. For using two flies, the leader is provided with three loops, one at the top, another at the bottom, and an extra loop tied in about 15 in. from the lower loop. In fly casting, the first, or upper, fly is known as the "dropper," and the lower one as the "tail" fly. For the single-fly cast but two loops are required.
Gut used for leaders should be carefully selected, and only those lengths which are of uniform diameter And well rounded chosen, the lengths which show flat and rough spots being discarded. Dry gut that is very brittle should be handled very little, and previous to a day's fishing the leaders must be soaked in water over night to make them pliable, then coiled in between felt pads of the leader box to keep them in fishing shape. After use, put the frayed leaders aside and dry them out between the flannel leaves of the fly book.
Gut is the product of the silkworm, and the best quality is imported from Spain. It comes in bundles, or hanks, of 1,000 strands, 10 to 20 in. long and in different thicknesses, or strengths. The heaviest are known as "Royal" and "Imperial," for salmon; "Marana," for extra-heavy bass; "Padron," for bass; "Regular," for heavy trout; "Fina," for light trout, and "Refina," for extra-light trout. The grades "Fina" and "Refina" are well suited for all average fly fishing, while the heavier sizes are useful for heavy large fishing.
To make the leaders, soak the strands of gut in warm water over night until they are soft and pliable. Select the strands for each leader of the desired thickness and length so that the finished leader will have a slight taper to one end only. By using the "Fina" gut for the upper length and tying in two lengths of "Refina" gut, a nicely tapered leader of light weight is obtained. Begin the leader by uniting the strands together to make it the correct length, three 12-in. strands being about right for average casting. The "single water knot" is the strongest and neatest to use. Make it by taking the thick end of the strand and doubling it back enough to tie in a common knot just large enough for the line to pass through and drawing it up tightly. Tie a single loose knot in the other end of the strand, about 1/8 in. in diameter and close to the end; take the next thickest strand of gut, thread the thicker end through the loose knot and tie a second square knot around the strand, as shown at A. By pulling on the two long ends the loops can be drawn up tightly, and the two knots will slide together and make a neat and very strong knot. Repeat this operation until as many strands of gut are knotted together as required to make the leader of the desired length. For making the loop at the ends, a double-bighted knot, tied as shown at B, is used. If a dropper fly is desired, do not pull the water knot tightly, but first insert a short length of gut with a common knot at the end and a loop in the other, then draw the water knot up tightly, and a short snell will be made for attaching the fly as usually.
Ill: The Single Water Knot Used in Tying Leaders: a Good Knot for Making the Loop at the End of the Leaders; an Angler's Knot Used for Attaching the Line to the Leader, and a Jam Knot for Attaching Eyed Flies, or Hooks, to the Leader or Snell