On reaching the end of D^1 it enters the second, or intermediate, set of vanes. The drum here is of a greater diameter, and the blades are longer and set somewhat farther apart, to give a freer passage to the now partly expanded steam, which has lost pressure but gained velocity. The process of movement is repeated through this stage; and again in D^3, the low-pressure drum. The steam then escapes to the condenser through B, having by this time expanded very many times; and it is found advisable, for reasons explained in connection with compound steam-engines, to have a separate turbine in an independent casing for the extreme stages of expansion. The vanes are made of brass. In the turbines of the _Carmania_, the huge Cunard liner, 1,115,000 vanes are used. The largest diameter of the drums is 11 feet, and each low-pressure turbine weighs 350 tons. BALANCING OF THRUST. The push exerted by the steam on the blades not only turns the drum, but presses it in the direction in which the steam flows. This end thrust is counterbalanced by means of the "dummy" pistons, P^1, P^2, P^3. Each dummy consists of a number of discs revolving between rings projecting from the casing, the distance between discs and rings being so small that but little steam can pass. In the high-pressure compartment the steam pushes P^1 to the left with the same pressure as it pushes the blades of D^1 to the right. After completing the first stage it fills the passage C, which communicates with the second piston, P^2, and the pressure on that piston negatives the thrust on D^2. Similarly, the passage E causes the steam to press equally on P^3 and the vanes of D^3. So that the bearings in which the shaft revolves have but little thrust to take. This form of compensation is necessary in marine as well as in stationary turbines. In the former the dummy pistons are so proportioned that the forward thrust given by them and the screw combined is almost equal to the thrust aft of the moving vanes. [Illustration: One of the turbine drums of the _Carmania_. Note the rows of vanes. The drum is here being tested for perfect balance on two absolutely level supports.] ADVANTAGES OF THE MARINE TURBINE.