170 LECTURES. 



measuring unit with wliicli to conapare the distance of the earth from 

 the sun, and thus obtain a new unit with which afterwards to compare 

 the distances of the other planets. To give a just idea of the size of 

 the earth we will avail ourselves of the largest tangible measure at- 

 tainable, that is, the highest mountain on the earth's surface. The 

 highest mountains are the Himalayas, their altitude being five and 

 a half miles. Now, we do not exaggerate when we say that, if we 

 could uncover the base of one of those mountains, it would cover four 

 times the original area of the District of Columtjia, or the surface of 

 one of the ordinary counties of our States, rising above that surface 

 to the height of five and a half miles, about equal to the height of 

 Chimborazo added to that of the highest of the Alps. This shall be 

 our standard of comparison with regard to the magnitude of the earth. 

 Such a mountain is rather more than Y^io" <^^ ^^^ earth's diameter or 

 about y\-o of its radius. In making the comparison, after the ordi- 

 nary mode, two difficulties present themselves. It is said that, if you 

 represent the earth by a globe, the highest mountain on its sur- 

 face may be represented by a small grain of sand. You thus proceed 

 from the greater to the less^ whereas, in nature, we must proceed from 

 the less to the greater. Besides, a grain of sand is too small to give 

 an adequate idea of the matter to be illustrated. To avoid this we 

 shall make use of a scale sufiiciently large to present the mountain 

 distinctly, and shall proceed in the natural order from the less to the 

 greater. This diagram before me is thirty-nine feet six inches in 

 length, and is intended to represent two radii of the earth opening to 

 the extent of one degree. At the further end of it is a blue band, 

 representing the atmosphere, and immediately beneath which is a 

 small row of mountains. Their heights, on this scale, is a trifle less 

 than two-thirds of an inch, and their actual height, as compared with 

 the real half diameter of the earth, is as two-thirds of an inch com- 

 pared with thirty-nine and a half feet, and doubling the half diameter 

 we shall have the ratio of two-thirds of an inch to seventy-nine feet. 

 Below the row of mountains you have a dark blue band, representing 

 the ocean. Below that again a darker portion still, representing that 

 portion of the earth's crust through which you must go to find a red 

 heat, and beyond that you have the red color continued until it passes 

 into whiteness ; it indicates the depth at which we would probably 

 arrive at a white heat. 



[It would be impossible, in a wood-cut, to do justice to the illustra- 

 tion here explained by the lecturer. The explanation itself will 

 doubtless be sufficient.] 



The diameter of the earth is, then, a very large unit in comparison 

 with the height of the highest mountain. The circumference, of 

 course, is more than three times the diameter. If you should attempt 

 to walk around the earth at the rate of twenty miles a day, three 

 years and five months would be spent in completing the circuit; and if 

 you should fly around it at the rate at which the steam car travels, say 

 thirty miles an hour, you would accomplish its circuit in thirty-four and 

 a half days ; but, if its circumference be great in comparison with ordi- 

 nary standards, its surface in comparison with that of a sphere of 

 ordinary size must be still more enormous. The illustrations, I would 



