the point of the screw J from touching the knife-edge when pressure is applied by the screw by means of the hand-wheel K.

The arm CL is graduated to represent cwts. and fractions of a cwt., and carries a heavy sliding-weight M, moved by a small friction-roller. When the centre of

the weight points to O, it is just counterpoised by the weight E, and when it is at the mark 40, it exerts a pressure of 40 cwts. on the centre of the test-bar. The movement of the lever c is limited by the check-bracket P.

The horizontal distance from the knife-edge D to the line passing through the centre of the screw J, along which the pressure is exerted, is 12 inches, and the distance from 0 to 40 of the scale is 35 inches, which gives an advantage of 20 as regards leverage. The weight M weighs 2 cwts., and this multiplied by 20 equals 40 cwts.

To test a bar of cast-iron it is placed in the position shown in Fig. 72A. The centre of the weight M pointing to 0, the screw is made tight and the sliding weight moved a little away from zero, which causes the arm c to dip. The screw is again tightened, which produces a pull on the centre of the bar and restores equilibrium in the lever c. The weight is again moved along the scale and the screw again tightened. The amount of deflection is now measured by means of a rule and calipers, and the process repeated with frequent measurements until the bar breaks, when the breaking weight is noted.

Punching and drilling.-Punching holes in iron and steel plates weakens the metal more than drilling, but if a smaller hole than required be punched and afterwards rimered out to the proper size, the strength appears to be about the same as by drilling alone. The effect of punching is to cause a compressed ring round the punched hole, and rimering removes the metal which is thus forced into a state of tension.

The tensile strength of good wrought iron varies from

20 to 27 tons per square inch, and that of steel from 27 tons upwards per square inch, varying with the amount of carbon. The tenacity of both iron and steel is considerably increased by cold-rolling, and by wiredrawing. The following table will show the effect of different proportions of carbon on iron :—

The tensile strength of cast-iron varies from 4 to 14 tons per square inch of section, but the average of good cast-iron is about 7 tons. The crushing strength is about four times that of the tensile strength, ranging from 25 to 50 tons per square inch. The safe working stress however is 7 tons for compression, and 11⁄2 tons for tension. With regard to transverse stress, a bar of cast iron 3 feet long by 2 inches deep by 1 inch wide, is required by engineers to bear a load at its centre of 25 to 30 cwts. and to give inch deflection without breaking.

QUESTIONS.

1. If you wished to produce a large steel casting, would you use the open-hearth process or the Bessemer-process in the production of the steel? Give your reason for the selection.

2. What methods have been adopted for the prevention of blow-holes in steel castings?

3. How do you account for the formation of blow-holes in cast-steel?

4. Why is steel cast under great pressure, as in Sir Joseph Whitworth's process, alleged to be superior to the same kind of steel cast not under pressure?

5. Steel-castings are sometimes found to be useless when cold; to what causes may the defect be attributed?

6. What is the nature of silicon-ferro-manganese; for what is it used; and how do you account for its efficacy?

What

7. How are compound armour-plates prepared? advantages are gained by the peculiar mode of construction. 8. Describe the construction, mode of use, and advantages of Gjers' soaking-pits.

9. What kind of tests would you apply to a sample of wrought iron and steel respectively to determine their value as constructive materials?

10. The tensile strength of iron is increased by the addition of carbon. To what extent is this the case?

11. What effect is produced on the tensile strength of a bar of wrought iron by "cold-hammering" or "cold-rolling" when the strength is tested in the usual manner and when it is tested by concussive action? ""strain'

12. Define the terms "tensile-strength," "stress," and "limit of elasticity."

13. Describe with sketch some simple form of tensile testing-machine.

14. What is the average ultimate tenacity of good cast-iron, good wrought iron, mild steel, and high quality cast-steel? 15. Show with the aid of a sketch the shape and dimensions of a piece of steel used in determining its tenacity.

16. What influence have the dimensions and shape of the test-piece on the result of the test?

17. Describe the method used in various tensile testingmachines for holding the test-piece, and state which method is the most suitable.

18. What are the usual tests applied to cast-iron? Why do they differ from those required for steel?

19. Sketch and describe some form of transverse testingmachine.

20. How would you conduct a transverse test with a sample of cast-iron, and what sized test-piece would you employ?