It is generally better to use India ink in making the bars if a good result is to be assured. The liquid drawing ink sold at most stationery stores is

COMPARISONS INVOLVING TIME

INCREASE IN STUDENTS IN] CO-OPERATIVE COURSE ENGINEERING COLLEGE

INCREASE 1913 OVER ..1907 I

lassv.

available in many different colors. Some grades of the drawing ink are water-proof after drj'ing. On elaborate charts the water-proof cjuality should always be used to make certain that a few rain drops, or handling with moist hands, will not ruin the finished w^ork.

In the discussion of Fig. 51 and Fig. 52 it was mentioned that a line could be drawn through the tops of the vertical bars to give a curve. If the curve were actually drawn, the bars themselves would be omitted. In Fig. 53, instead of using bars, we have lines which may be considered as curves for each of the several items compared. In this case it would be impossible to use bars for each of the items shown because the bars would cover each other. The bars are entirely omitted and lines are simply drawn from the 100 per cent point in 1897 to the various points for different items in 1907. It is certain that the prices (that of pine lumber, for instance, shown by the upper curve) did not have the uniform rate of increase which the straight hue from 1897 to 1907 would indicate. We are considering here, however, the changes over the period as a whole, and we can for simplicity draw a straight line and neglect all the fluctuations of intervening years. The general scheme of Fig. 53 is convenient, as the neglect of detail brings the main information out clearly. Fig. 53 has, unfortunately, been drawn