LATITUDE BY THE LINE. 







nished with a telescope moving upon its centre. One radius of 

 the quadrant is placed in the direction of the plumb line, and 

 therefore points to the zenith. The telescope moves round the 

 centre until it is directed to the object whose distance from the 

 zenith is to be observed. The angle between the telescope and 

 the vertical radius of the quadrant will then be the same as the 

 distance of the object from the zenith, 



8. In astronomical observatories methods of observation have 

 been applied susceptible of much greater accuracy. Stars upon 

 the meridian can there be used with great advantage. The 

 distances of these stars from the pole are accurately known, and 

 the astronomer selects for his observation those conspicuous 

 stars which pass near to his zenith. He observes the arc of the 

 celestial meridian between his aenith and these stars. And 

 from the magnitude of the arc and the distance of the star of the 

 celestial pole, he discovers the distance of the zenith from the 

 pole and thence the latitude. 



The principal source of accuracy in this method is, that the 

 distance between the zenith and the star being very small, is 

 capable of more exact measurement, for reasons connected with 

 the structure of the astronomical instrument, than could be 

 attained in the measurement of greater angles. 



9. In observations made at sea, it is not practicable, however, to 

 use the plumb line, and indeed, even for the purposes of geogra- 

 phers it is not always convenient. An admirable instrument 

 has been invented equally applicable to observations by land or 

 by water, called Hartley's sextant, by means of which the obser- 

 vations can be made with reference to the horizon, independent 

 of the zenith, and therefore independent of the plumb line. 



It is not our purpose here to enter into a description of the 

 principles and structure of this celebrated and most useful 

 instrument. It will be sufficient for the present purpose to state 

 that it is capable of being applied to the measurement of the 

 angular distances between any two visible objects with a very 

 great degree of precision, and that it may be used with facility, 

 even when the position of the observer is subject to all the 

 unsteadiness incidental to the condition of the mariner. 



When this instrument is used, instead of observing the dis- 

 tance of any object from the zenith, we observe its distance from 

 the horizon, which will answer the same purpose, inasmuch as that 

 whenever the distance of an object from the horizon is known, 

 its distance from the zenith can be found, since the distance from 

 the zenith to the horizon being 90, if we subtract the distance of 

 the object from that, the remainder will be the distance of the 

 object from the zenith. 



105 



