the pipe it will often draw back into the motor when first cranking it to start.

Until a motor gets well under way it is often very difficult to get good results with an under-water exhaust, and in such cases the three-way valve will prove very convenient, for by its use the exhaust can be deflected

into the air until well started, when it can be turned into the under-water connections.

Various methods of leading the under-water exhaust out from the bottom of the boat are in use, but the best method is to use an under-water exhaust head of some sort. A very good form known as the Reid is shown in Fig. 102. This is fastened to the side or bottom of the boat and connected with the exhaust pipe as illustrated in Fig. 103, a thin piece of rubber being placed between the planking and the head to make a water-tight joint. The arrangement of expansion chamber, exhaust pipe, three-way, relief, and drain cocks, and outboard head are shown in Figs. 104 and 105. In place of cooling or

expanding the exhaust gases in order to reduce the noise a system may be used by which the rapidly moving volume of gas is broken up into innumerable small jets before reaching the air or by so retarding a portion of the gas that the exhaust reaches the air in a continuous or nearly uniform stream or jet. The fact that a steady stream of exhaust gas makes less noise and commotion

than alternating jets renders the exhaust of a multiplecylinder motor far easier to silence than that from a single-cylinder engine. In order to break up the volume of exhaust gases various devices are used. In the older forms of mufflers the exhaust was led into a chamber or casing filled with stones, pebbles, or coke (Fig. 106) which served to break up the gas into many small jets before it reached the air. Another form, shown in

Fig. 107, consists in a casing and a perforated pipe leading from the exhaust port of the engine. The gas, leaving this pipe through the numerous small holes, loses much of its pressure and speed and divides into many jets. As some of these are far nearer the opening to the air

Fig. 106.-Muffler with Pebbles

than others, the gases finally leave the silencer in a more or less continuous stream. If a second pipe of larger diameter is placed in the muffler, as illustrated in Fig. 108, still better results are obtained. Such mufflers are used considerably, but usually are rather

Fig. 107. Muffler with Perforated Inlet

inefficient and often create considerable back pressure. Other forms of silencers consist of hollow chambers provided with segments or plates perforated by small holes and set alternately in the chamber. This style of muffler is shown in Fig. 109, and if properly designed and

of ample volume is quite effectual and creates little

back pressure.

The well-known Yankee mufflers are constructed with a combination of these two systems. Sectional views are shown in Figs. 110 and III. The gases entering at A

Fig. 108.-Muffler with Perforated Pipes

partly pass through the perforations at B and are partly deflected by the partition C and the interior of the pipe D, and then expand in the chamber E. From this chamber they pass through the opening in the plate at F and hence through perforations in the outlet pipe at G and

HAC

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Fig. 109.-Baffle-plate Muffler

into the air through the tail piece H. In the Ejector muffler a very different system is used. This is illustrated in Fig. 112. It consists of three expansion chambers, A, B, C, which are separated by conical plates, D, E, F, perforated at top and bottom and arranged in two sets.

The central tube G, leading through the muffler, is of varying diameter and a portion of the gases from the exhaust passes directly into the central chamber B and hence through the second set of cones D, E, F (2), before the gas which enters the first chamber A has

Fig. 110.-"Yankee" Auto Muffler

passed through the first series of cones D, E, F (1). A small portion of the gas is also led straight through the central pipe G to the outlet at a very high velocity. This creates a partial vacuum in the third chamber C,

Fig. III. -"Yankee" Marine Muffler

and the gas moves rapidly from the second chamber B to fill the partial vacuum in the chamber C. The forward movement of the gas through the first and second chambers A, B, to the third C, causes a sudden expansion. which removes the heat from the gases and reduces the