49 



PHYSIOLOGICAL PHYSICS. 



[Chap. XL. 



exerted. The dial plate also requires previous gradua- 

 tion. Thus the zero mark is placed where the hand 

 points when no pressure is exerted. A pressure of 

 1 pound, or 1 kilogramme, is then applied, and a 

 mark placed where the indicator points, and so on. 

 The pressure in pounds or kilogrammes exerted by 

 the hand can then be speedily ascertained. 



In both forms of the instrument the elasticity of 

 the steel restores the arcs to their former position, 

 when the force no longer acts. 



Quetelet states that the pressure of both hands 

 of a man equals, on the average, 70 kilogrs., and that 

 the pressure of a woman's hands is a third less. 



Representation of forces. - - Forces are 

 graphically represented by straight lines. A force of 

 1 pound, or 1 kilogr., is represented by a line of a defi- 

 nite length, and a force of 2, 3, 4, etc., pounds or 

 kilogrs., by a line 2, 3, or 4, etc., times that length. The 

 direction in which the force is acting is indicated by 

 a barb on the line. 



Resultant force. Let o (Fig. 200) be a particle 



under the influence of 

 two forces, one, OB, 

 urging it in the direc- 

 tion of B, and the 

 other, OA, urging it in 

 the direction A. It is 

 evident that the par- 

 ticle cannot proceed 

 along either path, but 

 will choose a path 

 which is a compromise 

 between the two. It 

 will move upwards. 

 Let a third force, re- 

 presented by the weight, be applied to o, and let this 

 third force be adjusted so that o remains in its original 



Fig. 200. Resultant Force. 



