ASTRONOMY. 



[BOTATIO* or THE KIKTII. 



v I 

 Liiitud*. 



Peru 1 31' 1* 



India 12 s 3T 21' 



France and Spain . 46' 8' fl* 

 Englan.l .... 62" 2* 20* 

 Upland . . . . 66 ^ 10- 

 In European latitudes, actual observation 



RUltOM. MMBUt. 



Formentera ... 40 0" 60* 



Mountjouy ... 42' 17' 29* 



Carcassonne ... 44 41' 49* 



Evaux 47 SO' 46* 



Paris Pantheon . 49 66' 29* 



Dunkirk .... 61 16' 25* 

 From Bemel's Elements the next table is 



Lrn1tiof 



K'.irt'-h ft. 



989,808 



363,013 

 884.640 



364,914 



305,782 



gives: 



Knli.h ft. 



304,206 

 304,239 

 304,347 

 304,935 

 306,052 

 305,110 

 formed : 



The elements of the earth's figure, deduced by Profes- 

 sor Airy,* are 



Equatorial diameter in miles . 7925-648 

 Polar diameter in miles . . . 7899-170. 



The equatorial circumference being a Little less than 

 26.000 accurately, 24,899. 



To illustrate the very trifling proportion which subsists 

 between the inequalities of the earth's surface and its 

 entire volume, we may suppose an artificial ball eighteen 

 inches in diameter, to represent our globe, when the 

 proper proportionate elevation to be assigned to its 

 highest mountains would be jWh of an inch. 



ROTATION or THE EARTH. Two principal motions be- 

 long to our planet ; one of rotation upon its axis, called 

 its aiunial motion, producing the succession of day and 

 night ; and another of progression in space, or revolution 

 round the sun, called its annual motion, causing the vicis- 

 situde of the seasons. Both of these motions are to be 

 understood of the whole earth, its interior substance, its 

 superficial masse* of land and water, the surrounding 

 atmosphere, and the clouds in suspension over it ; ana 

 both motions are in the same direction from west to 

 east. 



The exact time occupied by the diurnal rotation is 23 

 hours, 66 minutes, and 4-09 seconds. This forms a 

 sidereal day ; so called, because, in that time, the stars 

 appear to complete one revolution round the earth. A 

 star wfiich is on the meridian of a place at a given 

 period, will be on the meridian again after that interval. 

 Hut as, while the earth rotates upon its axis, it is also 

 moving in iU orbit round the sun, it will require twenty- 

 four hours, upon an average, throughout the year, for the 

 snn to pass from the meridian of a place to the same 

 meridian again. This forms a solar day, longer than a 

 sidereal ; and consequently, in the course of the earth's 

 annual revolution round the sun, while wo have 365 of 

 . Miftltfmilt Murof., Art FtftK-i of llu Eartk. 



the former, we have 360 of the lat tor, or that number of 

 cnmplrtu rotations of the earth upon its axis. Hence the 

 well-known fact, that, in travelling round the globe, a 

 person finds, on arriving at the point whence he set nut, 

 that he has gained or lost a day in his reckoning of time, 

 according as he has travelled east or west, aa compare 1 

 with the reckoning of those who have remained at rest. 



The earth's motion upon its axis is perfectly uniform 

 and equable. Sidereal days, therefore, are always of the 

 same length, every rotation being accomplished in the 

 same time ; but the motion of our planut in its orbit 

 being unequal, sometimes faster, sometimes slower, solar 

 days vary in length at different times of the year. Henoe 

 the hour shown oy a well-regulated clock and a true sun- 

 dial is scarcely ever the same ; the difference between 

 them, sometimes, amounting to 16V minutes, being called 

 the equation of time. About the 21st of December, the 

 solar day is half a minute longer, and about the 21st of 

 September nearly as much shorter than 24 hours ; which 

 is an average of all the solar days throughout the year. 



The rotation of the earth upon its axis is not suscep- 

 tible of ocular evidence like that which the observation 

 of spots upon the sun, and some of the planets, affords 

 of the same fact in relation to those bodies. Neverthe- 

 less, the truth of the doctrine is established by various 

 considerations. 



Either the globe revolves upon an axis every twenty- 

 four hours, or the whole universe, including the sun, 

 moon, comets, and fixed stars, accomplishes a revolution 

 round the earth in the same time. No third opinion 

 upon the subject can possibly be held. In the latd-r 

 case it is evident, from the distance of the celestial 

 bodies, that their diurnal revolution around our planet 

 must involve a rate of motion that is utterly inconceiv- 

 able. The sun must travel at the rate of 400,000 miles 

 a minute, the nearer stars with the velocity of upwards 

 of 1,000,000,000 of miles a second, and the more distant 

 with a rapidity which no numbers can express. It is 

 absurd to suppose this, when the end to be gained re- 

 quires only our little globe to revolve upon itself. 



The following considerations present themselves. If 

 the diurnal movement which we see, were attributable 

 to a movement of the stars, E, E', E* (Fig. 9) would 

 describe uniformly a circle situated in a plane, T P, per- 

 pendicular to the plane of the poles and the centres of 



rig. 9. 



the circle, C*, C, CX, for the reduced perpendiculars of 

 the stars upon this line ; that is to say, the most distant 

 points from the earth T. But we know that when a 

 body describes a circle with a uniform motion, it is at- 

 tracted towards the centre of the circle by a constantly 

 acting force, of which the magnitude depends at once 

 upon its rate of motion and the radius of the circle which 

 it describes. The stars E, E' E" cannot move in the 

 circle of which wo speak without being attracted towards 

 the points C, C', C , situated upon the line of the poles. 

 But something like demonstrative proof of real cir- 

 cumstances may be adduced. If the earth rotate, the 

 summit of a high tower, having a larger circle of rotation 

 to describe in the same time than the base, must ob- 

 viously move with greater rapidity ; a stone, therefore, 

 dropped from the summit, leaving it with a greater 



