Page:Graphic methods for presenting facts (1914).djvu/221

 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 any 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.

''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

Curves like those seen in Fig. 163 may be sketched in free-hand, or they may be much more conveniently drawn by using the irregular or so-called "French curves" which may be obtained in any store selling drafting instruments or artists' materials. As it frequently happens that an irregular curve available does not exactly fit the dots through which the curve line is to be drawn, care must be taken to