368 HYDROGRAPHICAL SURVEYING [chap. xvt. 



moment will be seen at an equal altitude from any part of a 

 small circle of the earth, circumscribed round the spot where 

 the body is vertical, with a radius equal to the zenith 

 distance. 



Knowing, therefore, the altitude of the sun, and the point on 

 the earth whose zenith it is in, an observer will always be able 

 to say that he is somewhere on the circumference of that circle. 

 This alone would not give us much information, but it is seldom 

 that we do not know om* latitude to twenty or thirty miles. 

 We shall know then that we are on that part of the circum- 

 ference which includes these latitudes, and as when the sun 

 is not very high the circle will be of large diameter, the portion 

 of it within our limits may be, without much error, taken as a 

 straight line. 



In practice, then, having obtained an altitude of the sun, 

 or any other body, we assume a latitude from our dead reckon- 

 ing, and work out the longitude. By the aid of the azimuth 

 tables we obtain the bearing of the body observed, and, having 

 plotted the position obtained, we draw a line at right angles 

 to the bearing, wliich is the " position line." We now know- 

 that Ave are somewhere on that line. We must know the 

 Greenwich time, and therefore our positions will be as much 

 dependent on the clu-onometers as any ordinary longitude. 



The position line can be also obtained by Avorking Avith tAvo 

 assumed latitudes and joining the positions resulting, but the 

 aboA^e is shorter. 



Waiting until the earth has revolved a sufficient amount to 

 alter the bearing of the sun, Ave repeat the operation, and 

 obtain another line, the direction of Avhich Avill differ from the 

 former by the difference in the azimuths of the sun at the tAvo 

 observations. 



If AA-e have been motionless in the interval, the intersection 

 of these tAvo Hnes Avill give us our exact position (ahvays 

 dependent on the chronometers) ; but if Ave have moved, we 

 must so far trust our dead reckoning as to transfer the first hne 

 in the direction, and for the distance, Ave have run, AA^hen the 

 intersection of the second position of the first line Avith the 

 second line Avill be our position at the second observation. 

 Plotting H.M. ships are provided Avith a large sheet on Avhich longi- 

 tude is marked, leaving the navigator to complete the Mercator's 



Sheet. 



