Condensing Engines. - Rule. Set 3.5 on c to 10 on D; then D is a line of diameters for cylinders, and c the corresponding number of horses' power; thus,

H. Pr.

3½

4

5

6

8

10

12

16

20

25

30

40

50

on c.

C. D.

10 in.

10¾

12

13.i

15½

17

18|

21½

21

26|

29½

33¾

37¾

on D.

The same is effected on the common rule by setting 5 on c to 12 on D.

Non-condensing Engines.-Rule. Set the pressure of steam in pounds per square inch on B to 4 upon a; and against the cylinder's diameter on D is the number of horses' power upon c.

Required the power of an engine, when the cylinder is 20 inches diameter and steam 30 pounds per square inch.

Set 30 on B to 4 on A; and against 20 on D is 30 horses' power on c.

The same is effected on the common rule by setting the force of the steam on b to 250 on a.