NAVIGATION. 87 



positions, to use this latitude in determining 

 local time from an observation of altitude, and 

 thence by chronometer to determine the longitude. 

 But, except any case in which the observed body is 

 on the prime vertical at the instant of observation 

 (and for every such case, the old ordinary method 

 is virtually equivalent to Sumner's method), that 

 method is not, and Sumner's method (to be ex- 

 plained later) is, the simple and direct interpretation 

 of what you learn as to the ship's place from an 

 observation of altitude (see Art. 5 above). 



51. SUMNER'S METHOD OF INTERPRETING 

 AN OBSERVATION OF ALTITUDE. The Greenwich 

 time of the instant of observation is to be cal- 

 culated according to the known error of the 

 chronometer or the mean of the errors of several 

 chronometers, w r hen there are several on board. 

 Now, what is the inference to be made from the 

 fact that the altitude of the sun's centre above a 

 true horizontal plane through the ship was so and 

 so say 40 at such and such a time, say on the 

 2;th of August 1874, at IH. 2IM. 23$. P.M. mean 

 Greenwich time ? It is simply this, that the 



