MEASUREMENT OF ORGANISMS. 5 



scratches may be made clearer. The number of squares 

 covered by the surface is counted (fractional squares being 

 mentally summated) and the required area is at once obtained. 

 If the area has been traced on paper it may be measured by 

 the planimeter (Fig. 2). This instrument may be obtained at 



FIG. 2. 



engineer's supply shops. It consists of two steel arms hinged 

 together at one end; the other end of one arm is fixed by a 

 pin into the paper, the end of the second arm is provided with 

 a tracer. By merely tracing the periphery of the figure whose 

 area is to be determined the area may be read off from a drum 

 which moves with the second arm. This method is less 

 wearisome than the method of counting squares. 



The area of a curved surface, like that of the elytra 

 of a beetle or the shell of a clam, is not always easy to find. 

 To get the area approximately, project the curved surface on 

 a plane by making a camera drawing or photograph of its 

 outline. By means of parallel lines divide the outline draw- 

 ing into strips such that the corresponding parts of the curved 

 surface are only slightly curved across the strips, but greatly 

 curved lengthwise of the strips. Measure the length of each 

 plane strip and divide the magnitude by the magnification of 

 the drawing. Measure also, with a flexible scale, the length 

 of the corresponding strip on the curved surface. Then, the 

 area of any strip of the object is to the area of the projection 

 as the length of the strip on the object is to the length of its 

 projection. The sum of the areas of the strips will give the 

 total area of the surface. 



