284 CELESTIAL DYNAMICS. 



which is common in physical inquiries. Observation of the 

 moon's motion reveals to us the external form of the earth. 

 The physicist determines with the torsion-balance the weight 

 of a planet, just as the merchant finds the weight of a parcel 

 of goods, whilst the pendulum has become a magic power in 

 the hands of the geologist, enabling him to discover cavities 

 in the bowels of the earth. Our case is similar to these. By 

 observation and calculation of the velocity of sound in our 

 atmosphere, we obtain the ratio of the specific heat of air un- 

 der constant pressure and under constant volume, and by the 

 help of this number we determine the quantity of heat gene- 

 rated by mechanical work. The heat which arrives from the 

 sun in a given time on a small surface of our globe serves as 

 a basis for the calculation of the whole radiating effect of the 

 sun ; and the result of a series of observations and well- 

 founded conclusions is the quantitative determination of those 

 cosmical masses which the sun receives from the space 

 through which he sends forth his rays. 



Measured by terrestrial standards, the ascertained number 

 of so many billions of kilogrammes per minute appears in- 

 credible. This quantity, however, may be brought nearer to 

 our comprehension by comparison with other cosmical mag- 

 nitudes. The nearest celestial body to us (the moon) has a 

 mass of about 90,000 trillions of kilogrammes, and it would 

 therefore cover the expenditure of the sun for from one to 

 two years. The mass of the earth would afford nourishment 

 to the sun for a period of from 60 to 120 years. 



To facilitate the appreciation of the masses and the dis- 

 tances occurring in the planetary system, Herschel draws the 

 following picture. Let the sun be represented by a globe 1 

 metre in diameter. The nearest planet (Mercury) will be 

 about as large as a pepper-corn, 3 millimetres in thickness, 

 at a distance of 40 metres. 78 and 107 metres distant from 

 the sun will move Venus and the Earth, each 9 millimetres in 

 diameter, or a little larger than a pea. Not much more than 



