THE LATITUDES AND LONGITUDES. 53? 



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 altitud.) be taken from 

 9(P, 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 practice of this prob- 

 lem, which do not affect its general spirit, and the introduction of which here 

 would be unsuitable to the object of these discourses ; 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 oceau 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, east 



or west on the globe. 



) * 



TO DETERMINE THE LONGITUDE. 



In expiessing 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 Trom 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 df the ' 

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

 times at which these two points would cross the meridian of the place whose 

 I longitude is to be determined. 



To comprehend fully the spirit of the celebrated problem of finding the lon- 

 gitude, we must imagine the globe of the earth turning on its axis, having around < 

 it the starry firmament. Let us suppose A B to be the northern hemisphere of ! 

 the globe, p being the pole, and let F E represent the firmament. Let P 4>e a < 

 place whose zenith is the point on the firmament marked by Z. If we suppose 

 the globe to turn upon its axis in the direction of Q P N, P will, by its ro- i 

 tation, be carried to the right of Z, and the same point Z will become succes- 

 sively the zenith of the points R Q ; and, in fact, every point in the circumfer- 

 ence of the earth will successively come under the point Z, which will be, 

 therefore, in regular succession, their zenith points. In twenty-four hours, or, 

 more accurately, in twenty-three hours and fifty-six minute.s, the globe will 

 make its complete revolution ; therefore three hundred and sixty degrees of the 

 I earth will successively pass under the same point of the firmament. 

 < By knowing exactly the time of rotation of the earth, and having ascertained 

 \ that its diurnal motion is uniform, we can ascertain by simple arithmetic what 

 ; extent of its surface will pass, in a given time, under any point of the firma- 

 | ment. Thus if we say in round numbers that the whole circumference corre- 

 : sponds to twenty-four hours, it will follow that fifteen degrees will move under 

 ( the point Z each hour, or one degree in four minutes. 



j If we suppose Z to represent the place of the sun, then it will be noon, or 

 ) twelve o'clock, at the place which is immediately under Z ; that is, at P. If 

 I R be fifteen degrees west of P, then it will arrive under Z one hour aft 

 ) consequently, when it is noon at P it is eleven o'clock at a place fifteen d< 



