i88 



POPULAR SCIENCE MONTHLY. 



the crater and set loose floods of lava. 

 As the energy expends itself, the vio- 

 lence declines and disappears. The 

 volcano then yields only hot springs and 

 gaseous emissions called fumaroles, 

 until it is stone-cold. 



SOME FACTS AND FIGURES CON- 

 CERNING THE EARTH. 



The earth is easily the most inter- 

 esting and the best known to us of 

 the bodies of the universe ■which have 

 been subjected to scientific investiga- 

 tion. Not only do we know more of 

 the earth than of any other member 

 of the solar system, but we know more 

 of the earth than of any of the smaller 

 bodies which have been studied mi- 

 nutely in laboratories. It is true, of 

 course, that a few bodies, like stand- 

 ards of length and mass, have been 

 determined with great precision with 

 respect to one or two of their proper- 

 ties; but our knowledge of the earth 

 includes many of its properties, and 

 some of them are known with a pre- 

 cision only surpassed by that of the 

 standards referred to. 



The surface of the earth is closely 

 that of an oblate spheroid whose axes 

 are known within about the hundred 

 thousandth part, a precision near the 

 limit possible in laboratory measure- 

 ments of such bodies as inch ball- 

 bearing spheres now made with won- 

 derful exactness for commercial pur- 

 poses. The surface and volume of this 

 spheroid are found to be in round 

 numbers two hundred million square 

 miles and two hundred and sixty 

 thousand million cubic miles respec- 

 tively; and these numbers are known 

 with an accuracy far surpassing that 

 of the measured areas, for example, 

 of the most valuable city properties, 

 which are relieved of the necessity for 

 precise measurement by the legal 

 phrase, 'be the contents of the same 

 more or less.' The magnitude of the 

 work which has led to these results 

 may be appreciated to some extent if 

 one considers seriously how one would 



measure an area of two hundred mil- 

 lion square miles with an error not 

 greater, say than one fifty-thousandth 

 part. 



Less definite but of a higher order 

 of magnitude are the figures express- 

 ing the quantity of mass of the earth, 

 or what is sometimes designated by 

 the scientifically meaningless phrase 

 'the weight of the earth.' We all have 

 a tolerably clear idea of the mass in a 

 ton of coal, but few of us are fitted to 

 realize the nearly equally definite 

 quantity of the earth's mass, which is 

 in round numbers six thousand six 

 hundred million million million tons. 

 Of this total mass, the atmosphere, 

 whose lenticular- shaped envelope in- 

 cludes a volume about one hundred 

 and fifty times that of the solid part 

 of the earth, contributes somewhat 

 more than one millionth part, a small 

 fraction of the whole, but yet millions 

 of millions of tons in amount. 



The regularity of rotation of the 

 earth, or its constancy as a time- 

 keeper, is no less surprising when ex- 

 pressed in numbers. The variation 

 from day to day in the time of rota- 

 tion is probably less than the hun- 

 dredth of a second, or no greater, say, 

 than the ten-millionth part of a day. 

 Our best clocks and chronometers, and 

 they are marvels of mechanical perfec- 

 tion, fall far short of this degree of 

 constancy. Equally remarkable for 

 stately regularity are the precession 

 and nutation of the earth, by reason of 

 which its axis of rotation describes a 

 slightly fluted cone in the heavens, 

 making one complete revolution in the 

 leisurely interval of about twenty-six 

 thousand years and thus rendering it 

 essential for us to change pole stars 

 from time to time. And still more 

 noteworthy are the lately discovered 

 minute wabblings of the earth with 

 respect to its axis of rotation, whereby 

 the latitude of a place varies from 

 month to month, running through a 

 lesser cycle in about fourteen months 

 and through a greater cycle in some- 



