THE TIDES.] 



ASTRONOMY. 



983 



an attractive external influence were uniformly diffused 

 round its equatorial regions, and penetrating only to a 

 small depth below the surface. If, however, the external 

 attractive influence, instead of being thus diffused as a 

 ring round the earth, were confined to a limited place, 

 the bulging out of the waters would of course be confined 

 to the place immediately under the attracting body; and 

 if the force of attraction extended to the centre of the 

 earth, the elevation of the waters would evidently.be due, 

 not to the force acting on their surface, but only to the 

 difference of the forces acting on the surface and on the 

 centre. The waters would, as it were, be drawn away 

 from the earth by the influence of this difference. 



If the attractive force of the external body extended 

 to tlie opposite part of the earth's surface-, then the force 

 on the centre being greater than that on the remote sur- 

 face, the earth would, as it were, be drawn away from 

 the waters at the further surface, and there would in 

 consequence be also a bulging out there ; and as the 

 difference of the forces on the centre and near surface 

 must be very nearly the same as the difference of the 

 forces on the centre and remote surface, the waters will 

 be heaped up to nearly the same extent at both places. 

 The application of these general considerations to the 

 moon and the tides, will 

 sufficiently show that the 

 bulging out of the waters 

 by the attraction of the 

 moon M (Fig. 148), is 

 nearly equal at the two 

 places under the moon 

 marked in on tlie earth's 

 surface, diametrically 

 opposite to each other, 

 canning the waters of the 

 entire globe to assume a 

 spheroidal form, the ele- 

 vations at m necessitating 

 a subsidence or depres- 

 sion at C and D. As 

 different portions of the 

 earth's aqueous surface 

 are brought under the di- 

 rect influence of the moon, 

 those portions in like man- 

 ner bulge out, producing a continuous succession of tides. 



Fig 143. 



M 



If the moon alone acted, and always moved directly 

 over the equator, the interval between two consecutive 

 high tides would be just half a lunar day. In like man- 

 ner, did the sun alone act on the waters, its motion being 

 supposed to be directly over the equator, the interval 

 would be just half a solar day : but the combined action 

 of both these bodies causes, of course, the intervals to 

 vary. 



In the actual state of the earth, whose surface presents 

 both land and water, the phenomena of the tides cannot 

 be expected to be in strict agreement with what they 

 would be if no solid matter existed on it. Headlands, 

 coasts, the shallowness and contraction of channels, <fcc., 

 continually obstruct the free motion of the tide-wave ; 

 these obstructions often greatly delay the time of high 

 water, and cause a much greater rise than would take 

 place if the advance were unimpeded. Such local influ- 

 ences, .indeed, greatly modify the results of pure theory, 

 both as to the time and height of high water at different 

 ports. Special observations, therefore, at each place are 

 necessary to supply the proper data for predicting the 

 time of high water, and to determine what is called the 

 "establishment of the port." The height of the tide, 

 too, often depends upon the set and force of the wind ; 

 but, "of all the causes of difference in the height of the 

 tides, local situation is the most influential. In some 

 places, the tide-wave, rushing up a narrow channel, is 

 suddenly raised to an extraordinary height. At Anna- 

 polis, for instance, in the Bay of Fundy, in Nova Scotia, 

 it is said to rise 120 feet. Even at Bristol, the differ- 

 ence of high and low water occasionally amounts to fifty 

 feet." Herxhel. 



The same influences which produce the tides of the 

 ocean, operate also on the surrounding atmosphere of 

 tlie earth ; and produce tides in U. It is popularly sup- 

 posed, too, that the weather is subjected to the moon's 

 influence ; but careful and long-continued observation, 

 by competent persons, has shown this supposition to be 

 fallacious. A change in the moon is imagined to be at- 

 tended with an immediate change in the weather ; it 

 being overlooked that the moon changes her position, and 

 passes through her several phases, by imperceptible 

 gradations ; which is incompatible with a sudden change 

 in the weather. The predictions frequently given in 

 alm.-inacks, in reference to this point, will generally be 

 found to prove fallacious. 



CHAPTER XI. 



PRACTICAL ASTRONOMY. 



WE have now reached perhaps we are somewhat )>o- 

 yond the point where the astronomer's labours must 

 ' have ceased but for the intervention of instruments, by 

 which he has been enabled to sweep the heavens, and re- 

 veal Home of their hidden mysteries. We propose, at 

 this stage of our labours, to give some description of 

 these instruments, of the principles on which they are 

 constructed, of the manner in which they tend to the 

 augmentation of our power of sight, and the most con- 

 vi-nii-iit, mi'thods of mounting them, with directions for 

 th'-ir practical application. 



The apparent size of an object, as perceived by its 



image on the retina, depends on the distance between 



the eye and the object. Thus, for instance, if the object 



be M (Fig. 149), and the eye be placed at o, the right 



Fig. 149. 



A 



line which joins the extreme points, A and B, will be 

 een under the angle A o 1>. If the eye be placed at 

 tli-- point /, or at half the distance at which it was pre- 

 viously situated, the line A B will, in the same manner, 



be viewed under the angle A o' B, which will be the 

 double of the preceding, supposing that the line AB is 

 small in comparison with tho distance of the object M 

 from the eye of the observer. In the same manner, the 

 angle A B will be three or four times greater, as the dis- 

 tance of the object is one-third or one-fourth of the 

 primitive distance. The angle A o B is, therefore, the 

 measure of its apparent size (which is different from its 

 real, the latter being invariable), and is called the tiininl 

 angle, or the apparent diameter of the object. The 

 surface of the body becomes four, nine, and sixteen 

 tinius greater, as the distance is decreased to one-half, 

 one-th'rd, and one-fourth of the primitive distance. In 

 other words, the diameter of an object varies inversely 

 as the distance of the eye from it, and its apparent area 

 varies inversely as the square of this distance. The 

 power of the eyesight is limited ; and it is generally held, 

 that when the diameter of an object is less than a minute, 

 the object ceases to be visible. But, the brightness of tho 

 image performs as important a part in this respect as its 

 size., as is easily proved by the visibilitj'of tlie fixed stars 

 and of the planets, none of the former of which can sub- 

 tend an angle as great as a second. Where the colour 

 or brightness of an object, however, differs less from tlie 

 ground on which it is placed, the above rule holds true. 



