LATITUDES AND LONGITUDES. 



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 to 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. 



10. Let us see, then, how the mariner is thus enabled daily to 

 determine the latitude of his ship. 



As noon approaches, the sky being sufficiently clear to render 

 the disc of the sun visible, he applies the instrument and finds the 

 altitudes of the lower and upper limbs of the sun from the verge 

 of the horizon. The mean of these will be the altitude of the 

 sun's centre. If this altitude be taken from 90, the remainder 

 will be the distance of the sun's centre from the zenith. He 

 finds in his almanac the distance of the centre of the sun on that 

 day from the equator, and hence he at once, as already explained, 

 obtains the distance of his zenith from the equator; that is the 

 latitude of the ship. 



There are several minute circumstances observed in the prac- 

 tice of this problem, which do not affect its general spirit, and 

 the introduction of which here would be unsuitable to our object ; 

 we therefore omit them. 



Thus we see that, whether by sea or by land whether in the 

 observatory of the astronomer, traversing the sands of the desert 

 or the forests of America, or voyaging over the trackless and 

 unimpressible surface of the ocean we are in every case by 

 science supplied with suitable and practicable means by which we 

 can ascertain the distance of the place where we are, north or 

 south, on the globe. 



TO DETERMINE THE LONGITUDE. 



11. In expressing and determining the latitude of a place, we 

 have fixed points and lines on the firmament to refer to such as 

 the celestial pole and equator ; and to find it, nothing more is 

 necessary than to ascertain the position of the zenith of the place 

 with reference to these. But with respect to the longitude, the 

 case is very different ; it is impossible even to express the 

 longitude without involving a reference to two places at least 

 that of which we wish to determine the longitude, and that 

 which is selected as the starting point from which all longitudes 

 are to be measured. If we could observe in the firmament the 

 two points which at the same time form the zeniths of the two 

 places, then the difference of their longitudes could be found by 

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