COMMON THINGS THE EARTH. 



Meridians. 20. Which of the two rotations is the more probable ? 

 21 . Rotation of the universe impossible. 22. Simplicity of the supposed 

 rotation of the globe. 23. Direct proofs of this motion. 24. Foucault's 

 experiment. 25. Its analogy to the planets. 26. Conclusion as to the 

 globular form of the earth requires modification. 27. All human 

 knowledge tentative and approximative. 28. Rotation not compatible 

 with the exact globular form. 29. Centrifugal force of the Earth's 

 rotation. 30. The globe rotating would assume the form of an oblate 

 spheroid. 31. The degree of ellipticity would vary with the velocity 

 of rotation. 32. Experimental illustration. 33. Ellipticity corre- 

 sponding to the diurnal rotation. 34. How these circumstances affect 

 the actual state of the Earth. 35. Form of a terrestrial meridian. 

 36. Dimensions of the terrestrial spheroid. 37. Its departure from 

 an exact globe very small. 38. Its density and mass. 39. Deter- 

 mined by Cavendish and Maskelyne. 40. Its total weight. 



1. LOCKE somewhere observes, with his usual felicity of illustra- 

 tion, that the "mind, like the eye, while it makes us see and 

 perceive all other things, can never turn its view with advantage 

 upon itself." We encounter something similar to this in our 

 researches through the universe ; for of all the objects which 

 compose it, one of the most difficult of which to obtain a com- 

 plete and accurate knowledge is the planet which we inhabit. 

 The cause of this is our proximity to it, and intimate connexion 

 with it. We are confined upon its surface, from which we cannot 

 separate ourselves. We cannot obtain a bird's-eye view of it, nor 

 at any one time behold more than an insignificant portion of its 

 surface. We have the same difficulty in obtaining an acquaintance 

 with it that a microscopic animalcule would have in acquiring 

 a perfect knowledge of the form and dimensions of a terrestrial 

 globe twelve inches in diameter, on the surface of which it 

 creeps. 



Still, by a variety of indirect methods supplied by the ingenuity 

 of scientific research, we have been enabled to ascertain its form, 

 dimensions, and physical constitution, with a considerable degree 

 of accuracy. 



2. The first impression produced upon the eye of an observer, 

 who has not carried his inquiries further, is, that the surface of 

 the earth is a flat plane, interrupted only by the inequalities of 

 the land. A little careful observation, however, upon the many 

 phenomena which are easily accessible to every observer, will 

 correct this erroneous impression. 



3. It is well known that if a voyage were made upon the earth, 

 continually preserving one and the same direction, or doing so as 

 nearly as circumstances will permit, we should at length arrive 

 at the place from which we departed. If the earth were an 

 indefinite plane, this could not happen. It is evident, then, that 

 whatever be the exact form of the earth, it is a body which is on 



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