After sufficient observations had been made throughout the whole range of the horizontal scale, smooth curves were drawn which would most nearly represent the various dots plotted. In drawing curves of this kind, care should be taken to have each portion of the curve as nearly as possible at the center of gravity of the dots in any vertical section of the chart. Accuracy is not necessarily obtained by having the same number of dots on either side of the curve. If there are only three dots at some vertical line, it may be that two of these dots

CORRELATION

would be close together and their combined weight must be considered as compared with one dot which may be some distance away. As a simple rule, consider the dots in anj' vertical section of the chart as though arranged on a see-saw, as used by children, and then shift the point through which the curve line is to be drawn so that the see-saw will just balance evenly.

80,000

2 3 4 5 B 7 8 9 ,10 i: 12

Velocity of circulating- "water -=ft. per sec. '~^1\ Geo. A. Orrok, in Journal American Society of Mechanical Engineers

Fig. 163. Relation of the Rate of Heat Transmission to the Velocity of the Circulating Water in Surface Condensers

Correlation charts of this type have sometimes been called "shot-gun diagrams " The investigator makes a dot for each observation recorded, and then judges from the arrangement of the dots whether there is any general law expressing a relation between the two variables studied