FOUCAF/LT'S EXPERIMENT.] NAVIGATION NAUTICAL ASTRONOMY. 



1073 



experiment. The path of the bob is between the points 

 a a (Fig. 23): 



fig. 23. 



APPLICATION TO AN UNEXPLAINED PHENOMENON IN 

 FALLING BODIES. The idea of the cone of latitude will 

 subserve the purpose of accounting for circumstances in 

 the late M. Oersted's experiments on falling bodies, 

 hitherto, we believe, involved in some obscurity. The 

 following quotation is from the Literary Gazette of March 

 22, 1851 : " One of the most important observations 

 first made by Oersted, and since then confirmed by 

 others, was, that <t body falling from a height, not only 

 fell a little to the east of the true perpendicular which 

 is, no douht, due to the earth's motion but that it fell 

 to the smith of that line : the cause of this is, at present, 

 unexplained. It is no doubt connected with some great 

 phenomenon of gravitation which yet remains to be 

 discovered. " 



The explanation of this phenomenon is very easy. 

 Suppose a heavy body to be let fall from a point at a 

 MnudmtUe height vertically over P (Kig. 22) : when it 

 is let go, the body will have a progressive velocity to- 

 wards the east, greater than the velocity of P at the foot 

 of the vertical ; and this velocity it will preserve through- 

 out its descent, which, from the nature of the gravity, 

 must be in a vertical plane through P C, C being the 

 centre of the earth. Now the point P, at the foot of 

 the vertical line, ncede* from this plane, towards the 

 north, during the descent of the body : it always keeps 

 in the plane through PC, and perpendicular to the axis 

 of the earth, and describes a circle whose radius is cP on 

 the cone of latitude. The body, therefore, must neces- 

 sarily fall towards the south of P, as well as towards the 

 east. If the experiment be made in south latitude, the 

 deviation will, of course, be north instead of south. 



It is plain that, in Oersted's experiment, the falling 

 body, by the rotation of the earth, and, therefore, by its 

 own more rapid easterly motion, had advanced more to- 

 wards the east, when it reached the ground, than the 

 point P at the foot of the vertical, but it had not ad- 

 vanced at all towards the south ; it was the foot of the 

 vertical the point P that had receded towards the 

 north. 



These experiments prove, in the most satisfactory 

 manner, that the earth really rotates on its axis : we are 



TOL. I. 



made sensible of this rotation in other ways also. "It 

 is well known to engineers, that when railway carriages 

 are going north, their tendency is to run off the raile on 

 the east side ; but when the train is going south, their 

 tendency is to run off on the west side of the track ; that 

 is, always on the right hand."* In the former case, the 

 train, at starting, is moving eastward with a velocity 

 greater than that with which any more northerly point 

 of the track moves ; and in the latter case, it is 

 moving more slowly towards the east than auy more 

 southerly point of the track, and hence the uniform 

 tendency to escape the confinement of the rails towards 

 the right, t 



EXPLANATION or TERMS IN NAUTICAL ASTRONOMY. 

 What are called the heavenly bodies, appear, to an ob- 

 server on the earth, to occupy a surrounding spherical 

 concavity, at the centre of which our planet is placed : 

 the phenomena of their rising and setting are appearances 

 which necessarily present themselves in consequence of 

 the rotation of the earth about its axis. This apparent 

 concavity is called the celestial sphere, and the imagi- 

 nation traces upon it a variety of circles, analogous to 

 those conceived to be traced on the terrestrial globe. 



To assume, however, that what we call the starry 

 heavens is really a concave sphere, whose centre coincides 

 with that of the earth ; and, therefore, that all the 

 celestial bodies situated in it are at equal distances from 

 that centre, would be to oppose what is well known to 

 be truth ; but the part of Astronomy with which we are 

 at present concerned is occupied mainly with appearances, 

 not with realities ; or, we should rather say, it is chiefly 

 occupied with the consideration of those astronomical 

 phenomena which are independent of actual distances, 

 and which would equally present themselves were these 

 distances other than what they are, or, as they all appear 

 to be, the same. 



The learner will readily perceive how this assumption 

 of a surrounding celestial sphere is perfectly consistent 

 with correct deductions in certain departments of astro- 

 nomy all those departments, for instance, which regard 

 only the angular distances of the stars from one another, 

 or from the imaginary circles before alluded to. The 

 angular distance of two objects whether on the earth 

 or in the heavens is the angle formed at the eye of the 

 observer by lines drawn to it from the objects observed. 

 If one or both of these objects move nearer to the eve, 

 along the line of vision, or recede further from it, it is 

 ilain that the auyular distance of the two must remain 

 ;he same ; the objects cannot, in this way, increase or 

 diminish their angular separation. The observer there- 

 ore may, if he please, consider the linear distances of 

 ;he two objects from his eye to be the same. 



Nautical Astronomy has a good deal to do with ob- 

 servations of this kind that is, with the measurement 

 of angular distances, and but very little with linear dis- 

 ances. It would have nothing to do with linear dis- 

 ;ances if the earth were really a point, or if observations 

 were all carried on at the centre instead of on the sur- 

 ; but, as it is, the semi-diameter of the earth is a 

 inear measure of which cognizance must be taken, 

 imply because appearances in reference to the sun and 

 moon, but not in reference to the stars are different at 

 he surface, from what they would be at the centre, 

 lence, the angular distances are not exactly the same 

 mm the two points of observation. 



What is here said of distances applies equally to 

 magnitudes. The linear diameters of the sun and moon 

 re not matters of concernment in Nautical Astronomy, 

 nly their apparent diameters, the diameters (taken in 

 ngnlar measure) they would appear to have, to an ob- 

 erver at the centre of the earth. 



All observations made upon these two bodies, for the 



'iirpose of determining the latitude and longitude at sea, 



are reduced to what they would be if tho place of 



Maury's Fhyiiml Orography of the Stti, p. 39. 



+ \Vc may observe, that whilst personally agreeing with most of the 



riruments which Professor Young has advanced, ami having made the 



V'Timeut repeatedly, the whole subject has excited great difference of 



i ; and we are inclined to suggest, that the theory of the pendulum 



experiment is more satisfactorily established than its facts. KD. 



6x 



