58 INTRODUCTION. 



compare it with that which may be calculated from 

 the latitude and longitude of the ship at the time, 

 aided by the determination of the time which has 

 resulted from solar altitudes of above ten degrees. 

 He must likewise not neglect to notice the time by 

 the chronometer when the sun has reached the 

 altitudes of 0° 30', 1°, IF, 2°, in which, as in greater 

 solar altitudes, he must not be satisfied with a 

 single observation, but repeat the determination of 

 the altitudes every 5 or 2i minutes the sight vanes, 

 being previously adjusted accordingly. The ob- 

 servation is susceptible of still greater accuracy if 

 the observer is on shore, and can determine the 

 true time by the corresponding altitudes. 



When the sun is near the horizon, let his hori- 

 zontal and vertical diameter be repeatedly mea- 

 sured : at every measurement note the time by the 

 time-keeper, and let an assistant take, at the same 

 time, the sun's altitude. These observations can- 

 not be made on the moon, except when it is at the 

 full. 



All these observations must be made with the 

 best sextants, and with good chronometers, and 

 the angles read off with all possible exactness : in 

 fixing the time, even parts of seconds are to be 

 taken into the account. It is also essential, accu- 

 rately to observe the height of the barometer and 

 of a thermometer freely suspended. 



The observation of the rising and setting of the 

 sun and moon may serve for the examination of a 



