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( 4 ) object and No. 1's ivory strip, with his body in Position I (Vide Fig. IV.) (b) He has then to maintain the coincidence by gradually bending forwards into Positions II and III (Vide Figs V and VI.) (c) In this position whenever he obtains an exact coincidence, he calls "on." NOTE I:—When the object gets beyond Position III (Vide Fig VI), he will rapidly advance, so as to get the coincidence again with his body in Position I (Vide Fig. IV) and bend forward as before to Positions II and III (Vide Figs. V and VI) NOTE II:—Should No. 1 be reading his drum, No. 2 should seize the opportunity of getting into Position I, so as to lose no time. NOTE III:—Now suppose that the object is moving from right to left, No. 2 will assume the forward Position (III) (Vide Fig. VI), first gradually changing to central and Backward Positions (II and I Vide Figs V and IV), and will retire in places where he advanced when taking ranges to an object moving from left to right. 19.—Observer No. 1's duties. No. 1 will act exactly in the same manner as if taking the range of a stationary object, care being taken to read the range at once when No. 2 calls "on." Calculation of Ranges. 20.—To take a range with an "ordinary" 25 yard base or the joint length of the two cords, the figure indicated on the drum D is the actual length of distance in yards. 21.—To take a range with a "half base," i.e., 12½ yards or the length of one cord, No. 1 must remember to divide figures indicated on the drum by 2. NOTE:—Half bases (12½ yards or one cord) give ranges in half-yards; e.g, working with a "half base" or one cord, the drum reads 1,500. The range, therefore is 1,500 yards divided by 2 or 1,500/2 = 750 yards. 22.—To take a range with a "double base," i.e., 50 yards or the joint length of four cords, No. 1 must remember to multiply the figures indicated on the drum D by 2.