File:Differential Screw Leveler, Modern Machine-Shop Practice, Vol. I (of 2), by Joshua Rose, Figure 417, 1887 - illo0135e.png

English: Differential foot screws used to level astronomical instruments.
"The principle of the differential screw may be employed to effect very fine adjustments in place of using a very fine thread, which would soon wear out or wear loose. Thus in Fig. 417 is shown the differential foot screws employed to level astronomical instruments. c d is a foot of the instrument to be leveled. It is threaded to receive screw a, which is in turn threaded to receive the screw b, whose foot rests in the recess or cup in e f. Suppose the pitch of screw a is 30 per inch, and that of b is 40, and we have as follows. If a and b are turned together the foot c d is moved the amount due to the pitch of a. If b is turned within a the foot is moved the amount due to the pitch of b. If a is turned the friction of the foot of b will hold b stationary, and the motion of c d will equal the difference between the pitches of the threads of a and b. Thus one revolution of a forward causes it to descend through c d 1⁄30 inch (its pitch), tending to raise c d 1⁄30 inch. But while doing this it has screwed down upon the thread of b 1⁄40 inch (the pitch of b) and this tends to lower c d, hence c d is moved 1⁄120 inch, because 1⁄30 - 1⁄40 = 1⁄120."