van 



. "10 ATI OX NAUTICAL A8TRONMV. I>,TATIOX OF TUB BIRTH. 



CHAl'TKU III. 

 NAUTICAL ASTRONOMY. 



Ox TUB ROTATIOX OF THE E.uiTii. Tho common 

 arguments in siip|Hirt of tlio doctrine that tin- earth has 

 irual rotation aKmt one of its diameters, give to 

 that doctrinu a degree of probability, so nearly appi 

 ,:i_- to .1'. -.li,;.- c--i : .inty, tli.-it the mind readily acquiesces 

 in tlio nullity of thu phenomenon. Since tliu time ol 

 micus, tlio evidence for thu earth's rotation has 

 ntinually increasing ; but still this evidence is 

 not of t!i:it direct and positive kind that is necessary to 

 to it tlio character of demonstration. All the other 

 hitherto, discovered planets of our system revolve on 

 thi'ir axes, and, as might be expected as a consequence 

 of this revolution, those of them upon which the exami- 

 nation -can he nui.il- are seen to be flattened at thoir 

 It is pri.lial.li-, therefore, that the planet we 

 inhabit 'ilso revolves on an axis ; if so, it too may be 

 to be flattened at its poles. Whether or not 

 tins is the case can be actually ascertained by experi- 

 ments : these have been undertaken and repeated, again 

 and again, with the greatest care, and by independent 

 and widely different means : the results all show that 

 the earth i,i flattened at tlio poles. There is thus a very 

 hi^li degree of probability that the earth rotates ; and 

 is further increased by the fact that all thu pheno- 

 ini-iia of tho heavens are completely consistent with tlie 

 hv]Kitlie.->Ls of such a rotation; that it is, moreover, tho 

 simplest hypothesis upon which the celestial appearances 

 - explained : and that to attempt to account for 

 them mi any other hypothesis, involves the system of the 

 universe in such intricacy and extreme complication, that, 

 ng from all the other o|ierations of nature, we 

 in: ves to suppose that such complex 



machinery should really be tho "handiwork" of an all- 

 wise when means so immeasurably simpler 

 invented themselves, of bringing about the same ends, 

 t can scarcely be charged to the King Alphouso, as a 

 in nt of impiety, when he exclaimed, in reference to 

 tho confused astronomical systems of the ancients " If 

 I had U-cn the Almighty's counsellor when he framed 

 the universe, I would have advised him better." 



lint, notwithstanding all this, a sensible or experi- 

 mental ;;<./ that the earth rotates was still wanting: 

 its general figure, as already declared (page 104'J), can 

 be experimentally discovered: its superficial rotundity 

 can bo seen. It is very desirable that wo should have 

 the r evidence of its rotation. A profound 



r mi J'liyuical Astronomy has observed "We 

 . be content, at present, to take for 

 granted the truth of the hypothesis of the earth's rotation. 

 If it continue to explain, simply and .satisfactorily, other 

 astronomical phenomena than those already noted, the 

 J.ility of its being a true hypothesis will go on 

 increasing. 



\\ e shall never, indeed, arrive at a term when wo 

 shall Ix: able to pronounce it absolutely ;/n<iv/ to IK; true. 

 Tin; nature of tin- subject excludes 1 Mich a possibility." 

 This prediction of 1'rofessor Woodhoiise has been 

 we can IKIIV obtain sensible evidence of the 

 !i <>f the earth. 



The idea of proving this interesting phenomenon to 



.uses occurred a few years ago to M. r'oiic.-uilt, of 



Paris : it was suggested to him by accidentally obsei 



the niotinii of a weight Kiispemled by a string to a high 



,', and which, by chance, had been wet vibrating. 



Mint of our readers must remember the sensation pn.- 



I in the scientific world, in the splin . I \r ..I, by 



markablu I'ENIIULI Koiicault. 



nire propose to submit to them, without going into 



any minut.- m.itli.-matical details, what app- 



be a conclusive and satisfactory explanation of the 



manner in Inch thin oxitriuicut renders thu rotation of 



the earth a matter of personal observation ; but as we 



write chiefly for th md for those wh 



supposed to be but little habituated to scientific res. 

 we shall previously offer to their attention a few g. 

 remarks in reference to what may be called the two 

 great postulates of astronomy : these are the rotation of 

 tho earth, and the hypothesis of gravitation. 



We speak of gravitation as a physical hypothetic : it is 

 not, like a proposition in Geometry, a necessary truth ; 

 nor is it an observed fact recognisable by our senses. 

 Certain phenomena in nature are observed : they exhibit 

 a regularity of succession, and a mutual dependence, 

 that suggest tho idea of a connecting principle and a 

 governing law. The phenomena are seen, their proxi- 

 mate cause is inferrtd. The philosopher looks abroad 

 upon nature, and carefully studies the facts presented to 

 him, the order of their recurrence, and the measure of 

 their intensities ; he retires to his closet, and endeavours 

 to frame a law, of which, tho appearances he hag been 

 studying, shall be the outward the practical manifesta- 

 tion. This is an hypothetic. An hypothesis, therefore, 

 need not be more than co-extensive with the phenomena 

 actually observed ; but it is a strong continuation of the 

 soundness of an hypothesis when it is found that new 

 and unexpected phenomena are equally comprehended 

 in it, and a stronger confirmation still, when such 

 phenomena can actually be predicted. Tho two 

 fundamental hypotheses of Astronomy the rotation 

 of the earth, and the law of gravitation have this 

 character in the highest degree : every new discovery in 

 the science has only the more firmly established tho 

 truth of both. 



The learner will perceive that we could not, with any 

 propriety, speak in this way of the truths of geometry : 

 Hi- (i are quite independent of the confirmations of expe- 

 rience, and hence the marked difference between physical 

 and geometrical science. When Dr. Halley had predicted 

 the return of the comet of 168-' in 17.VJ, and Clairault 

 had computed, from the hypothesis of gravitation, tho 

 time when it ought to appear, its return was watched for 

 by astronomers with the greatest interest not from any 

 inxiety to see the comet, but to learn how tho hypo; 

 of gravitation would stand so severe a test ; and the 

 reappearance of the same comet, in 1835, was anti- 

 cipated with like interest, solely in reference to tho 

 planetary attractions that is, to the general theory of 

 gravitation. 



"The rude supposition of the uniform revolution of 

 tho moon in a circle about the earth as a centre, led 

 Newton at once to the true law of gravity, as extending 

 from the earth to its companion. The uniform circular 

 motions of the planets about the sun, in times following 

 the progression assigned by observation in Kepler's rule, 

 co n tinned the law, and extended its influence to tho 

 boundaries of our system. Everything more refined than 

 this the elliptic motions of tho planets and satellites 

 their mutual perturbations the slow changes of their 

 orbits and motions, denominated secular variations tho 

 deviation of their figures from the spherical form the 

 oscillatory motions of their axes, which produce nut:. 

 and procession of tho equinoxes the theory of the tides, 

 both of the ocean and the atmosphere have all in 

 -sion been so many trials for life and death, in which 

 this law has boon, as it were, pitted against nature : 

 trials, whoso event no human foresight could predict, 

 and where it was impossible oven to conjecture what 

 modifications it might \w found to need. ihis 



moment, if, among the innumerable inequalities of the 

 lunar or planetary motions, any one, however RI< 

 .-lioiild bo discovered decidedly not explicable on the 

 hypothesis of a force varying as tho inverse square of 



