DEFINITIONS AND MECHANICAL PRINCIPLES 23 



fore. When the distances along the paper are propor- 

 tional to the ground passed over, the amount of work 

 may be obtained easily. The Giddings dynamometer, as 

 illustrated in Fig. 12, is made in this way. It also has 

 elliptical springs. 



Still another method is made use of in another type of 

 dynamometer, in which the in-and-out movement of the 

 pull head is made to rotate the reel. This method is not 



FIG. 13 THE PLANIMETER USED TO FIND THE AVERAGE DRAFT FROM 



DYNAMOMETER RECORDS. THERE ARE SEVERAL TYPES 



OF THIS INSTRUMENT 



so satisfactory because distances along the paper are not 

 proportional to anything. If the draft remains constant, 

 there is no rotation of the reel at all. Various devices are 

 provided dynamometers to add the draft for stated 

 distances, and in this way obtain the work done. A tape 

 line 100 feet long is sometimes used to rotate the reel of 

 the dynamometer. 



To obtain the mean draft a line is drawn through the 

 graph of the pen point, eliminating the sharp points. 

 Then the diagram may be divided into any number of 

 equal parts and the sum of the draft at the center of 

 these divisions divided by the number of divisions. The 

 quotient will be the mean draft. 



An instrument called the planimeter (Fig. 13) will de- 



