MEASURE OF THE SUN's HEAT. 275 



straight line towards the centre of the sun arrives at the solar 

 surface is expressed by the formula 



wherein c expresses the final velocity in metres, and h the 

 original distance from the centre of the sun in terms of solar 

 radius. If this formula be compared with the foregoing, it 

 will be seen that a mass which, after moving in central mo- 

 tion, arrives at the sun's surface has the same velocity as it 

 would possess had it fallen perpendicularly into the sun from 

 a distance* equal to the major axis of its orbit ; whence it is 

 apparent that a planet, on arriving at the sun, moves at least 

 as quickly as a weight which falls freely towards the sun 

 from a distance as great as the solar radius, or 96,000 geo- 

 graphical miles. 



What thermal effect corresponds to such velocities? Is 

 the effect sufficiently great to play an important part in the 

 immense development of heat on the sun ? 



This crucial question may be easily answered by help of 

 the preceding considerations. According to the formula given 

 at the end of Chapter II., the degree of heat generated by 

 percussion is 



= 0-000139 Xc 2 , 



where c denotes the velocity of the striking body expressed in 

 metres. The velocity of an asteroid when it strikes the sun 

 measures from 445,750 to 630,400 metres ; the caloric effect 

 of the percussion is consequently equal to from 27| to 55 mil- 

 lions of degrees of heatf. 



An asteroid, therefore, by its fall into the sun developes 



* This distance is to be counted from the centre of the sun. 



f Throughout this memoir the degrees of heat are expressed in the 

 Centigrade scale. Unless stated to the contrary, the measures of length 

 are given in geographical miles. A geographical mile 7420 metres, and 

 an English mile = 1608 metres. TR. 



