Scientific Lectures. 271 



rate of about tlireeand one-tenth miles (4,983 meters) in ever j second. 

 The relative size of the sun is represented on the diagram, which 

 represents in true proportion the disks of the larger planets and the 

 orbit of the moon, traced on a circle representing the face of the sun. 



The Sun's WEiGirr. 



The sun's distance and size thus being known, our next question 

 has regard to its weight, or mass, as it is called, when very large 

 bodies are spoken of. Here, too, we have abundant and trusty 

 information, and it has proved easier to weigh the sun than to mea- 

 sure his distance. The weight of any terrestial object is simply the 

 attraction subsisting between it and the earth, being measured by 

 the force necessary to lift it. JSTow, by means of large masses of 

 metal or other very heavy material, it has been found possible to 

 exert a measurable gravitative attraction upon small objects, and 

 comparison between the amount of attraction which a body of 

 known weight thus exerts and that exerted by the earth, as shown 

 by the weight of the object itself, gives us the actual mass of our 

 planet. In this way the mass of the earth has been found to be a 

 little less than 6,000,000,000,000,000,000,000 of metric tuns (of about 

 2,205 lbs. averdupois). One more step of the same kind enables us 

 to compare the attraction exerted upon other heavenly bodies by the 

 sun and the earth respectively ; and it has thus been established that 

 the mass of the sun is very nearly 327,000 times that of our planet. 



We have seen that the diameter of the sun is about 108.8 times 

 that of the earth. His volume or total size, therefore, is about 

 1,288,000 times, while his weight is only 326,800 times that of our 

 planet. Consequently, he must consist of material fourfold lighter 

 on the average than that of which tiie earth is composed, his specific 

 gravity being on the whole about 1.38 times that of water. 



Since the attraction exerted by a sphere is proportionate to its 

 mass divided by the square of the distance from its center, it is easily 

 seen that the weight of any object at the surface of the sun must be 

 27.0 times as great as the weight of the same object upon the earth. 

 Anything let drop near the surface of the earth will fall about six- 

 teen feet during the first second, but near the sun's surface, it will 

 fall more than 444 feet during the first second; at the end of the 

 nexf, it would have acquired a velocity equal to that of a cannon 

 ball as it leaves the most powerful gun; and even it' let fall at the 

 height of two miles, it would traverse this distance in less than five 

 seconds. 



