412 THE PRINCIPLES OF SCIENCE. 



which any error due to the wrong position of the zero 

 of the division will be reversed. As the axis of the 

 needle may not be exactly horizontal, it is now reversed 

 in the same manner as the transit instrument, the end of 

 the axis which formerly pointed east being made to point 

 west, and a new set of readings is taken. 



Finally, error may arise from the axis not passing 

 accurately through the centre of gravity of the bar, and 

 this error can only be detected and eliminated on re- 

 versing the magnetic poles of the bar by the application 

 of a strong magnet. The error is thus made to act in 

 opposite directions. To ensure all possible accuracy each 

 reversal ought to be combined with each other reversal, 

 so that the needle will be observed in eight different 

 positions by sixteen different readings, the mean of the 

 whole of which will give the required inclination free 

 from all eliminable errors k . 



There are certain cases of experiment in which a 

 disturbing cause can with much ease be made to act in 

 opposite directions, in alternate 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 much as it retards the other 1 . Again, in 

 trigonometrical surveys the apparent height of a point 



k Quetelet, ' Sur la Physique du Globe,' p. 174. Jamin, 'Cours do 

 Physique,' vol. i. p. 504. 



1 Hcrschel, On Sound, ' Encyclopaedia Metropolitans/ p. 748. 



