60 OUTLINES OP NATURAL PHILOSOPHY, 



from two planes, which are given in position 

 with respect to that superficies. 



a. That the determinations thus afforded may be the 

 simplest possible, the two planes ought to be at 

 right angles to one another ; and if the superficies 

 is one having a centre, the planes should pass 

 through that centre. 



b. In the case of the earth, the plane of the Equator 

 having its position fixed by the diurnal motion, is 

 naturally pointed out as one of the fixed planes, 

 to which the positions of places in the earth's sur- 

 face are to be referred. 



The position of every place, relatively to the equa- 

 tor, is determined by finding its latitude as above 

 defined. 



c. The other circle to which the position of places on 

 the earth's surface is to be referred, must necessa- 

 rily be a Meridian (a) ; but as none of the meri- 

 dians is distinguished from another, by any cir- 

 cumstance in the diurnal motion, of which they all 

 partake alike, the particular meridian that is to be 

 fixed on for the determination of geographical po- 

 sitions, is a matter of arbitrary arrangement. 



d. When a meridian is chosen for a first meridian, or 

 that to which all positions are to be referred, it is 

 not by directly measuring the distance from it that 

 such a reference is made, but by measuring the 

 angle which the plane of the meridian passing 



through 



