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



obtained by a given weight in passing from a state of rest into 

 that of motion. The work done is then expressed by the 

 product obtained by the multiplication of the weight by the 

 square of its velocity. The first method, however, because it is 

 the more convenient, is the one usually adopted ; and the num- 

 bers obtained therefrom may easily be expressed in other units. 



The product resulting from the multiplication of the number 

 of units of weight and measures of height, or, as it is called, the 

 product of mass and height, as well as the product of the mass 

 and the square of its velocity, are called "vis viva of motion," 

 " mechanical effect," " dynamical effect," " work done," " quan- 

 tite de travail," &c. &c. 



The amount of mechanical work necessary for the heating of 

 1 kilogramme of water 1° C. has been determined by experiment 

 to be =367 Km; therefore Km = O00273 units' of heat*. 



A mass which has fallen through a height of 367 metres 

 possesses a velocity of 84*8 metres in one second; a mass, 

 therefore, moving with this velocity originates 1° C. of heat 

 when its motion is lost by percussion, friction, &c. If the 

 velocity be two or three times as great, 4° or 9° of heat will be 

 developed. Generally speaking, when the velocity is c metres, 

 the corresponding development of heat will be expressed by the 

 formula 0-000139° x c\ 



III. On the Measure of the Sun's Heat. 



The actinometer is an instrument invented by Sir John 

 Herschel for the purpose of measuring the heating effect pro- 

 duced by the sun's rays. It is essentially a thermometer with 

 a large cylindrical bulb filled with a blue liquid, which is acted 

 upon by the sun's rays, and the expansion of which is measured 

 by a graduated scale. 



From observations made with this instrument, Sir John 

 Herschel calculates the amount of heat received from the sun to 

 be sufficient to melt annually at the surface of the globe a crust 

 of ice 29*2 metres in thickness. 



Pouillet has recently shown by some careful experiments with 

 the lens pyrheliometer, an instrument invented by himself, that 

 every square centimetre of the surface of our globe receives, on 

 an average, in one minute an amount of solar heat which would 



* How this important result is obtained has been explained in my paper 

 " Die organische Bewegung in ihrem Zusammenhauge mit dem Stoff- 

 wechsel." 



[This essay was published in 1845. At that time de la Roche and Berard's 

 determination of the specific heat of air was generally accepted. If the 

 physical constants used by Mayer be corrected according to the results of 

 more recent investigation, the mechanical equivalent of heat is found to be 

 77 1 '4 foot-pounds. Mr. Joule finds it =772 foot-pounds. — Tr.] 



