line between m₁ and m₂. The wheel is then moved back to the position C₁ of the crank, and again moved forward until the trammel with one leg in Q exactly reaches the pop M with its other pointed end. The point M will now occupy the position which m₂ has in the illustration, and the position of the crank will now be at F, the edge of the slide block being at the end of the stroke A. This is the front dead centre. The back dead centre B is found similarly by turning the wheel through half a revolution, and two further dead centres are also marked for the crank on the opposite side of the engine.
It may be added that the above somewhat elaborate method is necessary, because it is practically impossible to find the dead centre sufficiently accurately by merely watching the crosshead and marking the point when the latter begins to reverse its direction of motion. The motion of the crosshead near dead centre is very slow, and the crank will move through a small angle without apparently moving the crosshead.
The engine having been set on front dead centre with the crank at F (Fig. 44), by setting the wheel trammel in the pops Q and M, the front port should be open a slight amount A₁ B₁ (Fig. 43–III). This amount is the lead at the front end, and is marked with the valve trammel on the valve spindle at b₁. The distance a₁ b₁ is therefore equal to A₁ B₁. These marks on the spindle are also shown in Fig. 43–IV, and a similar mark b₂ is also
