OBSERVATIONS EOR TIME AND LONGITUDE. 
159 
of a star, is extremely short and simple, and therefore best suited to the 
ordinary traveller. As the declination of a star may, for the purposes of 
this observation, be considered constant, there is no necessity to compute 
the equation of equal altitudes, which must always be done in the case 
of the solar observation. The number of minutes by which the index is 
to be advanced or put back must depend on the rapidity with which the 
heavenly body is changing its altitude; it has here been mentioned as 
20' to illustrate the manner in which the observation is taken; but no 
general rule can be given for this; it is a matter in which the observer 
must use his own discretion. The same side of the roof of the artificial 
horizon must always be used for both sets of observations. 
