Each of these tenthof-an-inch spaces was made to represent ten-minutes time, so that a much larger scale was obtained than in Fig. 60. As the tug-boat captains regularly kept log-books in which their work was recorded to a five-minute interval the chart was made to the same interval by splitting the ten-minute squares to represent five-minute intervals. With a scale of this size it was feasible to use colored crayons, even though some of the divisions of time were very short.

Fig. 60 was drawn by hand and shows the possibilities in preparing an illustration of this kind when only one color of printing ink is available.

The chart from which Fig. 60 is taken was made to determine how much idle time there was in the operation of three tug-boats, and to ascertain whether the boats could be so run as to reduce the amount of idle time and give better service. The black in Fig. 60 shows the idle time vividly. By looking from bar to bar, it is possible to study all the work of the three tug-boats and to determine whether, if the work were differently assigned, there would be less waiting between jobs. Frequently the tug-boats had to tow two or more lighters or two or more car-floats, simultaneously, as will be seen from the extra

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bars placed below the main bar for each tug. The number of lighters and car-floats towed simultaneously is clearly shown in the chart, as well as the time at which each was picked up, and the time at which each was delivered by the tug. In the case of car-floats, a frequent break will be noticed at the end of the towing, one-half the width of the horizontal bar being marked with black. This convention was adopted to show that the idle time was occasioned by the necessity of waiting to obtain an unoccupied float bridge into which the carfloat could be shifted.