-1859] WRITINGS OF JOSEPH HENRY. 139 



pose we will briefly recall some of the elementary facts of 

 astronomy. 



The earth is a globe very slightly flattened at the poles, 

 isolated in space, supported upon nothing, and only connected 

 with other bodies of the universe by the all-pervading force 

 of attraction. In this free space it turns upon itself with a 

 regular motion around an ideal axis which pierces its surface 

 at two opposite points or poles, which have never sensibly 

 varied their position. It also moves in space describing 

 around the sun in the course of a year a slightly elliptical 

 curve called its orbit. But this movement of translation 

 around the sun does not interfere with the rotation of the 

 earth around its axis ; for in accordance with the second fun- 

 damental law of motion, two motions of this kind may exist 

 in a body at the same time. If the earth's axis were at right 

 angles to the plane of its orbit, but slight variations would 

 be found in the temperature at its surface in different periods 

 of the year. The axis is not however thus placed, but is in- 

 clined at an angle of about twenty -three and a half degrees 

 to the plane above mentioned ; and this fact, which at first 

 sight might appear of little consequence, in reality produces 

 all the alternations of seasons, and is connected with all the 

 changes of climate of the surface of the globe. Gradual 

 changes of climate cannot be produced by a change in the 

 axis of rotation, as some have supposed, since this would alter 

 the whole form of the earth, and produce other changes in- 

 compatible with the facts of observation. 



The position, the form, and the movement of the earth are 

 similar to those of the other planetary masses which we see 

 isolated in space under the form of globes, turning around 

 on an axis within themselves, and around the sun in ellip- 

 tical curves. While we observe that the earth is the centre 

 of the orbit of our moon, we see that four moons turn around 

 Jupiter, seven around Saturn, and six around Uranus. A 

 planet, with the moons which accompany it, form what is 

 called a planetary system, and all the planets taken together, 

 with the sun, constitute what is denominated the solar system. 

 In this system the earth occupies the third place from the 



