500

THE NEW ZEALAND GAZETTE.

[No. 16

15. A six-cylinder four-stroke cycle Diesel engine develops 1,000 brake
    horse-power at 120 revolutions per minute. The cylinder
    diameter is 24 in., the length of the stroke 36 in., and the
    mechanical efficiency of the engine 80 per cent. Find the
    mean indicated cylinder pressure.

FIRST CLASS.
First Paper.

Do not copy the questions, but be careful to work the problems on the
appropriate numbered pages of the answer-book.
Memoranda to be used in working the problems will be found on the first
page of the answer-book.
Do not deface or make any marks whatever on this paper.

The following six questions are for both Ordinary and Motor
Examination:

1. The high-pressure cylinder of a triple-expansion engine is 18 in.
   diameter, the stroke of the piston 3 ft., and the length of the
   connecting-rod 7 ft. If the boiler pressure is 180 lb. per
   square inch and the back pressure on the piston 70 lb. per
   square inch, what is the thrust in the connecting-rod in pounds
   and the turning-moment on the crank-shaft due to the high-
   pressure unit in foot-pounds when the crank has moved
   30 degrees from the top dead centre.

2. The travel of an engine slide valve is 8 in., the steam lap 1³⁄₄ in., the
   lead ¹⁄₈ in., and the stroke of the piston 4 ft. Find, by
   calculation, how far the piston has moved from one end of its
   stroke when cut-off takes place, neglecting the obliquity of the
   connecting-rod and of the eccentric rod.

3. A hollow shaft transmits 5,000 horse-power at 90 revolutions
   per minute, the internal diameter being half the external
   diameter. If the ratio of maximum to mean turning-moment
   is 1⋅5, find the external diameter of the shaft, allowing a safe
   working-stress in the material of 8,000 lb. per square inch.

4. An officer of a vessel which is steaming due west at a speed of 15
   knots walks from the starboard to the port side of the bridge,
   a distance of 60 ft., in 15 seconds. Find (a) the direction of
   his motion; (b) the speed at which he is travelling relatively
   to the surrounding ocean.

5. A block of wood of triangular cross-section 5 ft. long, 12 in. wide
   at base, and 18 in. high floats base upwards in fresh water.
   If the block weighs 104 lb. what is the distance between its
   centre of gravity and the centre of buoyancy ?

6. The internal resistance of a dynamo is 0⋅5 of an ohm, the resistance
   of the external circuit 10 ohms, and the voltage across the
   terminals 200. Find (a) the output of the dynamo in electrical
   horse-power, and (b) the power required to drive it, taking into
   account its internal resistance and neglecting all other losses.

Second Paper.

The following three questions are for both Ordinary and Motor
Examination:

7. The speed of a vessel of 5,000 tons displacement is uniformly
   increased from 10 to 12 knots in 15 minutes. Find the
   additional propeller thrust in pounds required to increase the
   vessel's speed, neglecting the augmentation of resistance to
   its motion as the speed increases.

8. The solid intermediate shaft of a steamer is 12 in. diameter, the
   shaft flanges 24 in. diameter, and the radius of the flange
   fillets 3 in. Find, from purely theoretical considerations, the
   thickness of the flanges, if the maximum stress allowed in them
   is similar to that allowed in the body of the shaft.

9. An engine weighing 60 tons is raised 2 in. by means of 4 screw-
   jacks, the load being equally distributed. The screws have
   three threads per inch, and the effective length of the toggle
   bars, one of which is provided with each jack, is 4 ft. What is
   the efficiency of the lifting-gear, if a force of 90 pounds is
   applied at the end of each toggle bar in order to raise the
   engine.