scale and read the information from the horizontal scale, or the reverse. If charts are always plotted with the independent variable as the horizontal scale, there need be no question in the reader's mind as to how he should interpret the chart. The rule for scale arrangement is not always followed, and a so' or/- piEfj-/!- few examples are shown here to indicate the

(JImple" 3soTo 3Si) difficulty of interpretation which the reader

„ „ ^ , , may have iust because a rather simple prin-

MelTOpolUan Sewage Commission J J t 1

of New York, HJ12 (;.jp]g of curvc plotting has been neglected. Fig. 8i. Number of Bac- ^^ p- gj ^j^^ j^^j^ ^j ^^^ ^ ^ ^

tena per Cubic Centi- 111 1 c

meter of Hudson River been plotted downward from the top of the

Water at New York at ^^art so that the reader may get the impres- Different Depths below . „ . . 1 xi-o. ,ithe Surface ^^*^^ 01 measurements taken at dmerent distances below the surface of the water. In making the tests which are represented in Fig. 81, different depths below the surfaces were selected and the bacteria determined from the water samples taken at these depths. The depth is here the independent variable, and bacteria per cubic centimeter the dependent variable. The decision as to which is the independent variable and which is the dependent variable rests entirely on how the problem is approached. Numerous samples could have been taken at different depths, and then a curve plotted to determine the depth at which certain numbers of bacteria per cubic centimeter were found. In