356 THE PRINCIPLES OF SCIENCE. [CHAP. xv. 



ought to be combined with each other reversal, so that the 

 needle will be observed in eight different positions by 

 sixteen readings, the mean of the whole of which will give 

 the required inclination free from all eliminable errors. 1 



There are certain cases in which a disturbing cause can 

 with ease be made to act in opposite directions, in alter- 

 nate observations, so that the mean of the results will be 

 free from disturbance. Thus in direct experiments upon 

 the velocity of sound in passing through the air between 

 stations two or three miles apart, the wind is a cause of 

 error. It will be well, in the first place, to choose a time 

 for the experiment when the air is very nearly at rest, and 

 the disturbance slight, but if at the same moment signal 

 sounds be made at each station and observed at the other, 

 two sounds will be passing in opposite directions through 

 the same body of air and the wind will accelerate one 

 sound almost exactly as it retards the other. Again, in 

 trigonometrical surveys the apparent height of a point will 

 be affected by atmospheric refraction and the curvature of 

 the earth. But if in the case of two points the apparent 

 elevation* of each as seen from the other be observed, the 

 corrections will be the same in amount, but reversed in 

 direction, and the mean between the two apparent dif- 

 ferences of altitude will give the true difference of level. 



In the next two chapters we really pursue the Method 

 of Eeversal into more complicated applications. 



1 Quetelet, Sur la Physique du Globe, p. 174. Jamiii, Cours di 

 Physique, vol. i. p. 504. 



