194 CALCULATION OF DATA 



1 1 1 mm manometer fluid V 132 fA O2 Y60 min \ 



30 min X 3 ml cell suspension A 1 mm manometer fluid/ \ 1 hr / 



1 ml cell suspension Y 1 mm^ cells \ _ c ^ ^l O2 



1 1 mm^ cells /\\.7 fJ-g chlorophyll/ ~ ' hr X ftg chlorophyll 



Some of the dimensions and some of the numbers cancel, giving us 

 the numerical rate in the desired terms. Arranging all corrections and 

 conversions in this manner provides a dimensional check. If we per- 

 formed each conversion separately, we might end with a rate in terms 

 of oxygen production per square milligram of cells, or another such 

 ridiculous unit. 



Aids In calculation 



Computations are likely to be long and involved in any extensive ex- 

 periment. If an aid to calculation can be used without sacrifice of ac- 

 curacy, it certainly should be employed. The figures resulting from 

 biological experiments are subject to natural variation, and there is no 

 need to maintain precision to twelve significant figures. Since three 

 significant figures are usually adequate, slide rule accuracy is good 

 enough. 



The biologist is likely to use his slide rule for multiplication and 

 division and for finding squares, square roots, and logarithms. The trigo- 

 nometric scales probably are used only rarely. Even this limited use, how- 

 ever, makes the slide rule a valuable instrument. 



For problems requiring more significant figures, or for sequences in- 

 volving additions and subtractions which cannot be done on a slide rule, 

 the electrical calculators are a great help. These machines can add, sub- 

 tract, multiply, and divide automatically. With more or less ease, square 

 roots can be determined. A competent operator can make calculations 

 very rapidly with few mistakes. Electrical calculators are used extensively 

 in statistical computations. 



Mathematical treatments 



Physical theories are most desirably expressed as simple mathematical 

 equations. Biologists would like to present their theories in the same 



