DEFINITIONS.] 



NAVIGATION. 



1049 



That the surface of the ocean is globular may be inferred 

 from the evidence of the senses. On whatever part of it 

 an observer be placed, or however high above it he be 

 raised, he sees around him an expanse in which no defect 

 from sphericity can be discovered anywhere within the 

 compass of his vision. He sees a receding vessel, in the 

 distance, gradually disappearing behind the rotundity ; 

 he first loses sight of the hull, then more and more of the 

 upper works, till at length even the most elevated parts 

 sink below the circle that limits his field of view. These 

 effects being observed, at whatever spot on the surface 

 the spectator may be, lead to the natural impression that 

 it is a portion of a globe that is everywhere spread 

 around him. This impression is confirmed by the un- 

 questionable fact that, on the assumption of the surface 

 being uniformly spherical, and under the sole guidance 

 of rules and directions based exclusively on that assutup 

 tion, ships have actually circumnavigated the earth, and 

 that by routes so various, as to leave on the mind noi 

 the slightest doubt that the inference, as to the genera 

 figure of the ocean's surface, drawn from observation o: 

 its appearance to the eye, is correct. 



Again : an eclipse of the moon is caused by the pas 

 sage of that body through the shadow cast by the earth, 

 the earth being then between the sun and moon. As 

 soon as the moon enters the shadow, the small parl 

 of her face thus obscured always presents a circular 

 boundary ; as she advances, and the obscuration in- 

 creases, the boundary of the shadow enlarges, but it con- 

 tinues circular ; and whenever the centres of the sun, 

 earth, and moon are so nearly in the same straight line 

 a- to render the eclipse annular, as it is called, then the 

 shadow projected on the moon's surface is observed to be 

 a complete circle. These appearances being uniform, 

 whatever part of the earth's surface is exposed to the 

 sun's rays, we cannot resist the conclusion that the 

 planet we inhabit does not differ in external figure from 

 a perfect sphere, except in a very slight degree. 



The earth rotates turning once round every day ; it 

 revolves invariably about the same diameter, and com- 

 pletes each revolution invariably in the same time. That 

 the rising and setting of the sun and stars are appearances 

 really due to the diurnal rotation of the earth, and not tr 

 the motions of the celestial bodies themselves, is a truth 

 ix it so obvious to the senses as the spherical figure of the 

 earth at least, till very lately, no contrivance had 

 been thought of to render this rotation visible to our 

 eyes. But there are means now of showing that the 

 earth turns round ; so that we may have the same visible 

 proof of its diurnal revolution that we have of its 

 general figure. This will be explained when we come to 

 treat of NAUTICAL ASTRONOMY. 



That the rotation is daily performed in exactly the same 

 invariable period of time without the difference of a 

 single second is proved by innumerable observations. 

 Age after age, the same spot on the earth, after the 

 lapse of the same interval of time, invariably returns to 

 the same fixed star : which could not be the case if there 

 were the slightest irregularity in the diurnal rotation of 

 the earth. Nor could such be the case, if the diameter 

 round which the rotation is performed, were shifted. 



Admitting, then, from facts such as these, that the 

 earth is a sphere or that, at least, it differs from a 

 sphere so little as for the purposes of navigation to be of 

 no moment that it revolves uniformly once in twenty- 

 four hours and that the diameter about which it turns 

 is invariable, we may proceed to the following defini- 

 tions : 



DEFINITIONS. 



1. Arts. The diameter about which the earth per- 

 forms its daily revolution is called the axis of the earth : 

 the revolution about this axis is from west to east. 



2. Poles. The extremities of this diameter are called 

 the poles of the earth. The extremities of any diameter 

 of a sphere are also called poles the poles, namely, of 

 that great circle of the sphere the circumference of which 

 is at every point of it, at the same distance from each, 

 this distance being a quadrant, or 90 : these same 

 points are also spoken of as the poles of every imaU circle 



VOL. I. 



parallel to the great circle just mentioned ; but from the 

 circumference of any small circle, the poles are, of course, 

 at unequal distances. 



3. Erpuitor. The equator is that great circle of the 

 earth, the axis of which is perpendicular to the plane of 

 that circle : the poles of the equator are the poles of the 

 earth. These poles are called one of them the NORTH 

 POLE, and the other the SOUTH POLE. Whenever, in 

 navigation, we speak of the poles, the poles of the earth 

 that is, of the Equator are always to be understood. 

 4. Meridians. Every semicircle which terminates at 

 the two poles, and which is therefore perpendicular to the 

 equator, is called a meridian: it is said to be the meridian of 

 each place on the earth through which it passes. For the 

 convenience of Navigation and Geography, every civilised 

 kingdom selects one of these innumerable meridians as 

 a meridian of reference ; it is usually that which passes 

 through the national observatory, or the principal city, 

 and is called the first meridian. In this country the 

 first meridian is that of Greenwich ; in France, it is that 

 of Paris. 



The plane of every meridian (as also the plane of the 

 equator) is conceived to be extended to the heavens, and 

 to mark out, on the celestial sphere, the celestial meri- 

 dian of the place. The apparent daily motion of the 

 sun and stars is across the celestial meridians ; the path 

 of a ship, easterly or westerly, is across the terrestrial 

 meridian ; but, in general, the distinctions terrestrial 

 and celestial are dropped : a ship cannot be on a celestial 

 meridian, nor a star on a terrestrial meridian. 



5. Latitude. The latitude of a place on the surface of 

 the earth, is the distance of that place from the equator, 

 measured on the meridian of the place. Latitude is, 

 therefore, either north or south : a place cannot exceed 

 90 in latitude, this being the latitude of each pole. 



6. Parallels of Latitude. A small circle on tho globe, 

 parallel to the equator, is called a parallel of latitude : 

 every point on the circumference of such a circle has the 

 same latitude, as the parallel is everywhere equi-distant 

 from the equator. The arc of a meridian, intercepted 

 between two such parallels drawn through any two 

 places on the globe, measures the difference of latitude 

 of those places. When the places are both on the same 

 side of the equator that is, both north or both south 

 their difference of latitude is found by subtraction ; 

 when they are on opposite sides of the equator, their 

 difference of latitude is found by addition. 



7- Longitttde. The longitude of any place on the earth's 

 surface is the arc of the equator intercepted between tin; 

 meridian of that place and the Jirst meridian. It is 

 estimated, like latitude, in degrees and parts of a degree, 

 and is, of course, the measure of the angle at the pole, 

 included between the two meridians spoken of. When 

 the place is to the east of the first meridian, it has east 

 longitude ; when to the west, west longitude. 



As every place on the surface of the earth performs a 

 complete revolution in twenty-four hours, 15 of longi- 

 tude become the measure of one hour of time ; the whole 

 300 of the equator, and of every parallel to it, having 

 the measure of twenty-four hours. Longitude is thus 

 sometimes expressed in time ; a place 45 east of the 

 meridian of Greenwich is, for instance, said to be three 

 hours east of Greenwich. It is necessary to precision, 

 that the latitude of a place should be designated either 

 north or south, according as it is situated in the northern 

 or southern hemisphere ; but the distinction of longi- 

 tude into east or west is unnecessary and inconvenient. 

 "It woulo 1 add greatly to systematic regularity, and tend 

 much to avoid confusion and ambiguity in computations, 

 were this mode of expression abandoned, and longitudes 

 reckoned invariably westward from their origin round 

 the whole circle from to 360."* In the present mode 

 of reckoning, however, longitude, like latitude, is of two 

 denominations ; so that the difference of longitude of two 

 ilaces is found sometimes by subtraction and sometimes 

 >y addition : by subtraction, if both places are on the 

 same side of the first meridian ; and by addition, if they 

 ie on opposite sides : the limit of longitude, whether 



Sir John Ilerschel. 



Ca 



