472 Dr. Gr. J. Stoney on Texture in Media, and on the 



the real number of blows delivered upon the square millimetre 



is about — x 10 12 . 



Next consider the distance within which the centres of two 

 molecules, one a molecule of the air and the other a molecule 

 of the wall, must approach in order that they may sensibly 

 act on one another. A circular disk, with this distance as 

 radius, may be considered as a target towards which the centre 

 of a molecule of the air must be directed in order that this 

 particular molecule of the wall may be reached. Now the 

 distance within which the centres of the molecules must ap- 

 proach lies more probably in the neighbourhood of a tenth- 

 metret than in the neighbourhood of either a ninth-metret or 

 an eleventh-metret*. Let us for the purpose of an estimate 

 assume that it is a tenth-met ret. The size of the target, sup- 

 posed flat, will then be about three square tenth-metrets. 



This is 3 fourteenthets of a square millimetre (3 x -— ^ of a 



square millimetre). Accordingly the number of encounters 



this molecule will receive in the time t will be approximately 



5 11. 



-x . 10 12 x 3 . yqu — on- This is on the supposition that the 



target to be struck is a disk, whereas it is in reality a sphere. 

 This will double f the number of blows it will be subjected to 

 in the time t; whence, finally, we may take this number to 



be about j^ : in other words, this molecule of the wall is struck 



on the average at intervals of about 40 times t. If the colour 

 of the wall be green, the molecular motions which occasion 

 this colour are repeated in the molecule 2000 each t, and 

 therefore something like 80,000 in the intervals between the 

 shots to which the molecule is subjected by the aerial artillery. 

 This serves to explain why the incessant bombardment bv the 



* See footnote t (p. 

 t N', tlie number of blows that reach a circular disk of radius a, is, 

 according to equation (5) of footnote * (p. 471), 



N' = 7T 2 « 2 A; (6) 



Again, proceeding as in equation (2) of footnote * (p. 471), we find that 

 the number N" which would reach a sphere with radius a, 



= 2 n «. . (7) 



Comparing (6) and (7), we find that 

 N"=2N'. 



