STATISTICAL METHODS. 



The Determination of Graduated Variates. 

 Methods of Measurement. 



Straight lines on a plane surface are easily meas- 

 ured by means of a measuring-scale of some sort. The meas- 

 urement should always be metric because 

 this is the universal scientific system. Vari- 

 ous kinds of scales may be obtained of 

 optical companies and hardware dealers, 

 such as steel measuring tapes, graduated to 

 millimetres (about $1.00), and steel rules 

 (6 cm. to 15 cm.) graduated to ^ of a milli- 

 metre. Steel "spring-bow" dividers with 

 milled-head screw are useful for getting 

 distances which may be laid off on a scale. 

 Tortuous lines, e.g., the contour of the 

 serrated margin of a leaf or the outer 

 margin of the wing of a sphinx moth, may 

 be measured by a map-measurer ("Entfer- 

 nungsmesser," Fig. 1), supplied at artist's 

 and engineer's supply stores at about $3.50. 

 Distances through solid bodies 

 or cavities are measured by calipers of 

 some sort. Calipers for measuring diameters 

 of solid bodies are made in various styles. 

 Micrometer screw calipers (" speeded") 

 reading to one-huudredths of a millimetre 

 and sold by dealers in physical apparatus for 

 about $5.00 are excellent for determining diameters of bones, 

 birds' eggs, gastropod shells, etc. Leg calipers for rougher 

 work can be obtained for from 30 cents to $4.00. The 

 micrometer " caliper-square," available for inside or outside 

 measurements and measuring to hundredths of a millimetre, 

 is a useful instrument.* 



The area of plane surfaces, as, e.g , of a wing or leaf, 

 is easily determined by means of a sheet of colloidin scratched 

 in millimetre squares. By rubbing in a little carmine the 



* Many of the instruments described in this section are made by the 

 Starrett Co., Athol, Mass., and by B rown and Sharpe, Providence, tool 

 cutters. 



FIG. 1. 



