A primary astronomy .. . ical bodies are to each other as the cubes of their diame-ters. Now 8,000 X 8,000 X 8,000 = 512,000, and 3,200 X 3,200 X 3,200 = 32, 512,000 -f- 32,668 = _J, nearly, the comparative bulk of Mercury.] 188. What is the comparative size of Venus ? PKIMARY ASTEO]STOMY. 43 About t9q- that of the Earth. 1§9. How do Jupiter, Saturn, Herschel, and Neptune comparewith our globe? Jupiter is more than 1,400 times as large; Saturn1,000 times ; and Herschel and Neptune each 90 times. [This subject may be illustrated by the following cut, exhibiting the COMPARATIVE MAGNITUDE
A primary astronomy .. . ical bodies are to each other as the cubes of their diame-ters. Now 8,000 X 8,000 X 8,000 = 512,000, and 3,200 X 3,200 X 3,200 = 32, 512,000 -f- 32,668 = _J, nearly, the comparative bulk of Mercury.] 188. What is the comparative size of Venus ? PKIMARY ASTEO]STOMY. 43 About t9q- that of the Earth. 1§9. How do Jupiter, Saturn, Herschel, and Neptune comparewith our globe? Jupiter is more than 1,400 times as large; Saturn1,000 times ; and Herschel and Neptune each 90 times. [This subject may be illustrated by the following cut, exhibiting the COMPARATIVE MAGNITUDE OF THE SUN AND PLANETS. J S /O H HI m © The student may think it almost incredible that what appears only as a star in theheavens should be larger than the mighty globe upon which he dwells; but when heconsiders their immense distance, and remembers the effect it must have upon theirapparent magnitude, as illustrated under Question 178, he will see that they could notbe seen at all if they were not very large bodies.]. DENSITY. 190. What is meant by Density ?Compactness or closeness of parts. [1. For example, cork is less dense than iron, and stone is more dense than commonearth. 2. Density and solidity are not the same; for while ice is more solid than lead, it isfar less dense, and is consequently lighter.] 191. What can you say of the Density of the planets ? They differ in the compactness of the substances ofwhich they are composed. 192. What is the Density of Mercury? About the same as lead, or three times the averagedensity of our globe. 193. State the Density of Venus and Mars. It is about the same as that of the Earth. 44 PEIMAEY ASTBONOMY. 194. How is it with the other large planets 1 Jupiter and Herschel have but \ and Saturn T^ thedensity of our globe ; the first two answering to water,and the latter to cork. GRAVITATION. 195. What is Gravitation ? The tendency of all bodies toward each other. [Gravitation is a species of attraction, and is often called attraction
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