536 



from the equator must be 40, and that such, therefore, must be the latitude 

 of the place. 



This method of ascertaining the latitude is, perhaps, the most easily practi- 

 cable. The observations may be performed daily, at noon, when the sun is 

 visible : and in all almanacs, the distance of the centre of the sun from the 

 equator, which is called the sun's declination, is registered. The instrument by 

 which the observations are executed on land are, usually, a quadrant furnished 

 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. 



In astronomical observatories methods of observation have been applied sus- 

 ceptible of much greater accuracy. Stars upon the meridian can thereby be used 

 with great advantage. The distance of these stars from the pole are accurate- 

 ly known, and the astronomer selects for his observation those conspicuous 

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

 tial meridian between his zenith and these stars. And from the magnitude of the 

 arc and the distance of the star of the celestial pole, he discovers the dis- 

 tance of the zenith from the pole and thence the latitude. 



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

 tween the zenith and the star being very small, is capable of more exact meas- 

 urement, for reasons connected with the structure of the astronomical instru- 

 ment, than could be attained .in the measurement of greater angles. 



In observations made at sea, it is not practicable, however, to use the plumb 

 line, and indeed, even for the purposes of geographers it is not always con- 

 venient. An admirable instrument has been invented equally applicable to 

 observations by land or by water, called Hadley's sextant, by means of which 

 the observations 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 meas- 

 urement 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 distance of any ob- 

 ject from the zenith, we observe its distance from the horizon, which will an- 

 swer 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 fro'n 

 the zenith. 



At sea we have generally, indeed almost always, a well-defined horizon. 

 If the mariner desires to measure the altitude of an object, he has only io 

 measure the distance of the object from the horizon in a direction perpendicular 

 to it, and this he is enabled to do with a little practice, with admirable facility 

 and precision, with Hadley's sextant. 



Let us see, then, how the mariner is thus enabled daily to determine the lati- 

 tude of hi.s ship. 



i As noon approaches, the sky being sufficiently clear to render the disk of 

 J the sun visible, he applies the instrument and measures the altitude of the 



