Rule. - Invert the slide, and the operation is the same as direct proportion.

1. I know that six men are capable of performing a certain given portion of work in eight days, but I want the same performed in three; how many men must there be employed?

Set 6 upon c to 8 upon a; and against 3 upon c is 16 upon a.

2. The lever of a safety-valve is 20 inches in length, and 5 inches between the fixed end and centre of the valve; what weight must there be placed on the end of the lever to equipoise a force or pressure of 40 lbs. tending to raise the valve?

Set 5 upon c to 40 upon A; and against 20 upon c is 10 upon A.

3. If 8 yards of cloth, 1 yard in width, be a sufficient quantity, how much will be required of that which is only 7-8ths in width, to effect the same purpose?

Set 1.5 upon c to 8.75 upon a; and against .875 upon c is 15 yards upon a.