276 THE HUMAN MOTOR 



As the subject's head remains out of the water, it must be taken 

 into account, which is a rather difficult matter, the simplest way 

 being to stop the ears with cotton wool, and only leave the nose 

 above water. The correction is then insignificant. 



204. Measurement of Weight. It is a difficult operation to 

 weigh the human body exactly. The subject must be weighed 

 nude instead of deducting from his weight with clothes, the 

 weight of the latter. He should be fasting, otherwise the weight 

 of the food and the drink will make the result incorrect. For 

 every reason it is better to take the weight on awaking in the 

 morning before any food. To compare different weights from 

 day to day they should always be taken at the same time, seven 

 o'clock in the morning, for instance. 



The balances used are sensitive to 5 grammes per 100 kilo- 

 grammes, but even this is insufficient in seme cases. The total 

 error being 10 grammes, if from one weighing to the next a 

 subject gains 100 grammes, the maximum relative error would 

 be 10%. 



Care should be taken that the subject stands centrally on the 

 platform of the scales, with his back to the graduation and that 

 he keeps quite still. 



The scales for weighing young children and babies must be 

 very sensitive, and this is easily done. Many good models are 

 on the market ; they are of the Roberval type, and sensitive to 

 one gramme per 10 to 15 kilogrammes. 



205. Dynamometric Measurements. Dynamometers are 

 springs whose deformations are proportional to the forces which 



produce them (see 11). The general type 

 is the ordinary spring balance. It comprises 

 a blade, AB, of tempered steel, of V-shape. 

 At the extremity of the arm B is fixed an iron 

 arc, n, which is prolonged and passes freely 

 through an eye formed in the extremity of the 

 F|0 other arm, A. From the latter a similar arc, 



m, passes in the same way through B, and is 



Ordinary balance. terminated by a hook, c. The arc, n carries 



a ring, o (fig. 164). 

 The balance is graduated by hanging it up by the ring o to a 



fixed point, and by suspending 



on the hook c increasing known rt^s S\ 



weights. On the sector n notches ' |T% <f 



are cut corresponding to the ^C > 



flexions of the arm A for the 



given weights. Fig. 166. 



In order that the weight shall not exceed the limit of elasticity 

 of the spring, the arc n has a stop /, against which the arm A will 

 abut if the force is too great. 



