398 Dr. J. R. Mayer on Celestial Dynamics. 



masses, therefore, which falls every minute into the sun amounts 

 to from 94,000 to 188,000 billions of kilogrammes. 



To obtain this remarkable result, we made use of a method 

 which is common in physical inquiries. Observation of the 

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

 phvsicist determines with the torsion-balance the weight of a 

 planet, just as a 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 under constant pressure and 

 under constant volume, and by the help of this number we de- 

 termine the quantity of heat generated 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 de- 

 termination 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 incredible. 

 This quantity, however, may be brought nearer to our compre- 

 hension by comparison with other cosmical magnitudes. 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 distances 

 occurring in the planetary system, Herschel draws the following 

 picture. Let the sun be represented by a globe 1 metre in dia- 

 meter. The nearest planet (Mercury) will be about as large as 

 a pepper-corn, 3 J 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 a quarter of a metre 

 from the Earth will be the Moon, the size of a mustard seed,, %l 

 millimetres in diameter. Mars, at a distance of 160 metres, will 

 have about half the diameter of the Earth \ and the smaller planets 

 (Yesta, Hebe, Astrea^ Juno, Pallas, Ceres, &c), at a distance 

 of from 250 to 300 metres from the sun, will resemble par- 

 ticles of sand. Jupiter and Saturn, 560 and 1000 metres distant 

 from the centre, will be represented by oranges, 10 and 9 centi- 

 metres in diameter. Uranus, of the size of a nut 4 centimetres 

 across, will be 2000 metres ; and Neptune, as large as an apple 



