OBSERVATIONS FOR LATITUDE. 
147 
This observation may be taken, at the same place, at considerable intervals 
between the times of the two stars’ meridian passage, and indeed days 
have sometimes been allowed to elapse before the second set of altitudes 
has been taken; the results, nevertheless, being quite satisfactory. When 
possible, however, it is better that the two observations should be taken 
consecutively, so as to ensure similar conditions of weather and refraction. 
Latitude by Double Altitude. 
When clouds prevent the altitude of the sun being observed at or 
near enough to noon to obtain the meridian altitude, or when the sun 
on the meridian is too high for observation in artificial horizon, the 
method known as double altitude may be very useful, except when the 
declination approximates to the latitude , in which case this method should 
never be used . This consists in observing the altitude of the sun (or star) 
at two times differing not less than one hour from each other. The 
latitude can be calculated from these with great exactness. The error 
of the watch on local time is only required approximately. 
