LATITUDES AND LONGITUDES. 



The circumstances under which the scientific traveller and 

 geegrapher makes his observations, with a view to the general 

 determination of the points of a country, are less favourable to 

 accuracy than those available to the astronomer, but still are 

 more susceptible of precision than those which can be placed at 

 the disposal of the mariner. It is, however, the business and the 

 duty of those who devote their lives to the advancement of the 

 sciences, to supply to each class of observers those instruments 

 and methods of inquiry which are capable, respectively of giving 

 results which, in the circumstances of the case, have the greatest 

 attainable accuracy. 



TO FIND THE LATITUDE. 



6. Let us suppose the globe of the earth to be represented at O, 

 and let N be its north pole, and E its equator ; let P be a place 

 upon it, whose latitude, that is, whose distance from the equator 

 is to be determined. Let n Z e represent the firmament sur- 

 rounding the globe at an indefinite distance. The point n, 

 immediately over the north pole, and which is in fact the con- 

 tinuation of the line O N, will be the place of the north pole in 

 the heavens, very near to which is a star, called the Polar star. 

 The point e, in the continuation of the line O E, will be that 

 which is directly over the equator, and will be that point in the 

 heavens representing the position of the equator j and the point Z, 

 in the continuation of the line O P, the point of the heavens 

 which is directly over the observer at the place P, will be that 

 which is called his zenith. This point is that to which a plumb 

 line would direct itself. 



Now the points n, Z, and e are the points in the firmament 

 which correspond with the points N, P, and E, upon the earth, 

 and it is evident that whatever arcs of the terrestrial meridian 

 N P E are included between these points, similar arcs of the 

 celestial meridian must be included between the points n Z e. If, 

 then, P E were 40, Z e must also be 40, just as n e is 90, while 

 N E is also 90. 



In short, the zenith of any place in the heavens is the point in 

 the firmament which corresponds with the position of the place 

 on the globe, and the distance of the zenith in the heavens of one 

 place from the zenith of another, must necessarily be the same in 

 degrees as the distance between two places on earth. Thus Z is 

 the zenith of P ; e is the zenith of E ; Z is the same number of 

 degrees from e as P is from E. This being clearly understood, it 

 is evident that if we can, by any means ascertain by observations, 

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