LESSONS IN GEOGRAPHY. 



14* 



X' 



P." 



LESSONS IN GEOGRAPHY. XVIII. 



LATITUDE AND LONGITUDE-FIRST MERIDIAN, ETC. 



IN the preceding view of the determination of tho position of 

 a given point, we have not considered all the possible positions 

 of a point p round tho point A, tho origin of the co-ordinate 

 axes. If these axes were produced in the preceding figures so 

 as to assume the appearance represented in Fig. 10, then, with 

 the same given distances or rectangular co-ordinates, there might 

 be four different positions of the point P with 

 reference to the rectangular axes X x' and 

 Y Y', or tho north and south straight line Y Y', " 

 and the east and west straight line X x'. To 

 prevent confusion, therefore, and to fix the 

 exact position of the point in question, it 

 might be agreed upon that every distance *"" Y' 

 measured from the origin A, along the portion 

 of the axis A x, should be called east, and Fi - 1- 



every distance measured from tho origin A 

 along tho portion of the axis A x' should be called tcest ; in like 

 manner, that every distance measured upwards from the axis x x' 

 should be called north, and every distance measured downwards 

 from tho axis x x' should bo called south. Moreover, it might 

 be agreed upon that every distance measured upwards from the 

 axis x x' should be called north latitude, and every distance 

 measured doicmcards from the axis x x' should bo called south 

 latitude; and in like manner, that every distance measured 

 from tho point A to the right or eastward, should be called east 

 longitude, and every distance measured from tho point A to the 

 left or westward, should be called west longitude. Now this 

 supposition is that which has actually been agreed upon ; so 

 that in a map of the world upon Mercator's projection, as it is 

 called, the straight lino Y Y' would represent the first meridian 

 or that of Oreemcich; and the straight lino x x' would represent 

 the equator; also, the point A would represent the point of 

 intersection of this meridian with the equator, which arc, in fact, 

 the two rectangular axes to which all points on the earth's 

 surface are referred, in order that their true positions (geo- 

 graphical positions) may bo determined. Accordingly, the point 

 P (Fig. 10) would be described as the position of a place in north 

 latitude and east longitude; the point P', that of a place in 

 north latitude and west longitude ; the point p", that of a place 

 in south latitude and west longitude ; and the point P'", that of 

 a place in smith latitude and east longitude. 



If the straight lines x x', and Y Y', which we have supposed 

 to represent tho first meridian and the equator, were to become 

 circumferences of circles of the same size, they would then more 



truly represent tho actual rectangular axea which are employed 

 on the surface of the globe. The aspect they would thai 

 assume will be understood by reference to Fig. 6, where the 

 circle PMKTPNQH may represent the first meridian, and the 

 circle E o Q it tho equator, their point of intersection B being 

 their origin oa rectangular axes. We still call these rectangular 

 axes, because the planes of the circles cut each other at right 

 angles, and the spherical angles P E o, p E B on each side of the 

 meridian are right angles, whether taken from the upper or 

 north pole p, or from the lower or sooth pole p ; so that we 

 still have four right angles round the origin E ; bat these are 

 now spherical right angles, that is, angles formed by the quad* 

 rants or fourth parts of the circumferences of these great circle* 

 of the sphere. In order to have a proper view of the rectangular 

 axes on the sphere, we should require to be looking at the edge 

 or circumference of tho circle P E P Q, and not at its plane or 

 surface as in the figure ; then we should see the edge or circum- 

 ference of the circle a E o Q cutting the former at right "fV ; 

 and both exhibiting at a distance the same appearance ae the 

 lines x x 7 and Y Y' in Fig. 10. In this view, the point E being 

 the origin of the axes, all points on the surface of the sphere 

 included between the semicircle E M p s Q and the semicircle 

 E o Q, ore said to be in north latitude and east longitude ; the 

 longitude being measured from s along the eastern half of 

 the equator E o Q, and the latitude from the same part of the 

 equator on a meridian passing through any of the point* in 

 question and the two poles PP. All point* on the surface 

 of the sphere included between the semicircle E M p 8 Q and 

 the semicircle E R Q, are said to be in north latitude and wett 

 longitude; the longitude being measured from E along the 

 western half of the equator E B Q, and the latitude from the 

 same part of the equator on a meridian passing through any of 

 the points in question and the two poles p p. All points on the 

 surface of the sphere included between the semicircle E T p x Q 

 and the semicircle E R Q, are said to be in south latitude and 

 west longitude ; the longitude being measured from E along the 

 western half of the equator E B Q, and the latitude from the 

 same part of the equator on a meridian passing through any 

 of the points in question and the two poles P p. Lastly, all 

 points on the surface of the sphere included between the semi- 

 circle E T P N Q and the semicircle E o Q, ore said to be in 

 smith latitude and east longitude ; the longitude being measured 

 from E along the eastern half of the equator E o Q, and the 

 latitude from the same port of the equator on a meridian 

 passing through any of the points in question and the two 

 poles P P. 



Tho student who reads the above explanation of latitude and 

 longitude on the rounded surface of the earth for the first time, 

 may have a difficulty in realising the appearance of the sphere 

 from the diagram made on a flat surface, but he must endeavour 

 by his natural ideas of perspective to obtain as clear a notion as 

 he can. Of course the difficulty would be entirely removed by 

 the actual inspection of a globe ; but, as it may not be possible 

 for him to have a globe by his side while he is reading this 

 lesson, we must try what we can do by means of the small 

 Map of tho World in the next page, although it must be remem- 

 bered that the surface of no solid body as a whole can be truly 

 represented on a plane surface such as the page referred to. 

 With that map before you, then, you will see the two rides of 

 the globe represented in what are called the eastern and western 

 hemispheres. In order to see the whole of only one side, or half 

 of the globe, the eyo must be supposed to be at on infinite d- 

 and still possessing the power of sight ; accordingly, htw 

 sitch sitjhts directly opposite to each other will enable yon to wee 

 the whole of the globe. This is the reason that two circles are 

 necessary to represent the globe, because only one-half can be 

 seen at a time. If these two circles could be pasted along their 

 edges or circumferences, back to back, so that their north and 

 south poles coincided, and then inflated till they assumed the 

 form of a globe, they would then form a pretty correct ipie- 

 sentation of the earth's surface. The equator, which yon know 

 is a circle equally distant from the two poles, is represented on 

 the Map of the World by a straight line drawn across the middle 

 of both hemispheres, marked by the word eiptator, and with 

 degrees from to 180 east, and from to 180 west, reckoned 

 from the/irs< meridian. 



In tho map referred to, these degrees are marked only at the 

 ! distance of every 20 degrees, on account of its smallnesa ; in 



