Examples.—I. The number of divisions on a vernier = 25. The value of the smallest division on the main scale = 25 yards. What is the least reading of the vernier?
Ans. 25 ÷ 25 = 1 yard.
II. The value of the smallest division on the main scale of the mercurial barometer = 1/10 of an inch. The number of divisions on the vernier = 10. What is the least count of the vernier?
Ans. 1/10 ÷ 10 = 1/100 of an inch.
Note.—The following scheme for accurately counting seconds has been found valuable to gunners who have no stop-watches; it is also used by many photographers in timing pictures. When ready to start to count the time of flight, for example, trail your gun or instrument on the target, stop traversing, and count to yourself: One one thousand, two one thousand, three one thousand, four one thousand, etc., until finished, saying one thousand after each number. The time required by the average man to say one one thousand or eight one thousand is equal to one second. With but little practice a gunner can be trained to count as high as 20 seconds accurately. In such cases stop-watches are not necessary.
(b) THE PLOTTING-ROOM.
Q. Point out or describe the following parts of the Whistler-Hearn plotting-board: The table, the azimuth-circle, azimuth graduations for primary and secondary stations, base-line arm, base-line plate, primary station, secondary station, primary arm, secondary arm, directing-gun arm, directing-gun azimuth-circle, base-line verniers, directing-gun vernier, base-line-arm verniers, azimuth-indices for primary and secondary stations, auxiliary arm, connecting-bar, clamp for arm index-clamp, gun-arm clamp, reading-opening for directing-gun azimuth-circle, index for gun azimuth-circle, speed-scale for range, speed-scale for azimuth or azimuth-travel devices, range correction-device, azimuth correction-device, micrometer, the "targ, tally dials."
A. See Figs. 59, 60, 61, and 62. These figures show by steps the "evolution of the Plotting Board."