Stoney — On Texture in Media, etc. 397 



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 disc, 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 approach lies more probably in the 

 neighbourhood of a tenth-metret than in the neighbourhood of 

 either a ninth-metret or an eleventh-metret.(/c) Let us for the pur- 

 pose of an estimate assume that it is a tenth-metret. The size of 

 the target, supposed 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 r will be approximately 



5 11 



— . 10 12 x 3 . tt-t; = tttt. This is on the supposition that the target 

 12 10 14 80 rr ° 



to be struck is a disc, whereas it is in reality a sphere. This will 

 double (/) the number of blows it will be subjected to in the time r : 



Therefore the whole momentum communicated in this way from all inclinations 



f 1 2 



= 2irksa. I cosW cos 6 — - irksa ; 

 Jo 3 



and the pressure thus caused 



= ?^i. (3) 



3 T W 



This is to he equal to the pressure produced hy N downright blows ; whence, equat- 

 ing (1) and (3), 



N= - -ask. (4) 



Again N', the number of blows that reach s, when molecules fly in all directions — 



= I dn' — 2irsk . cos 6d cos Q : 



whence N' = irsk. (5) 



Comparing (4) and (5), we find that 



N^_ 3 

 as in the text. N 2 



(k) See footnote (c) , above. 



(/) N', the number of blows that reach a circular disc of radius a, is, according to 

 equation (5) of footnote (j), 



N' = Tra-k. (6) 



