Remarks on the " Principle of Least Pressure" 199 



the line A B, and have the precise position assigned to it by 

 Mr. Moseley in the instance mentioned by him, and (if we 

 may be allowed to stretch the reasoning of a finite state to an 

 evanescent state,) there will still be no pressure upon C ; which 

 is directly at variance with Mr. Moseley's results." 



The answer to this argument is analogous to the preceding. 

 Let G be the centre of gravity of the triangle, then 



pressure upon C A A G B 

 weight of mass A A C B 



Now when G is in the line A B, A A G B is evanescent ; 



pressure upon C 



weight of mass A A C B' 



In this case, therefore, as Mr. Earnshaw observes, there 

 will be no pressure upon C, provided only the A A C B have 

 a finite value. But if we give to this triangle also an evanescent 

 value, the conditions of the case will be altered, and we shall have 



pressure upon C 



weight of mass 



from which no conclusion can be drawn, unless we know the 



evanescent value of the fraction A a p tv Now supposing 



the point G to take up the position which I have assigned it, 



the evanescent value of this fraction is — - . 



The evanescence of the pressure upon C arises out of a par- 

 ticular application of the general principle of the equality of 

 moments ; it is not therefore absolutely, but only relatively 

 evanescent; a fact which Mr. Earnshaw does not appear to 

 have considered. 



I have stated in one of my former papers that the result 

 which I have obtained in the case of parallel forces is verified 

 by the known condition of the question when the points of 

 support are two only. It has occurred to me to seek for a 

 similar verification in the case which I have just been discuss- 

 ing of a pressure equally divided between three points of 

 support in the same right line. 



As above, let C and G come into the line A B, 



-B X 



b 1} C~ 



Let D be the bisection of A B. Then D G = * D C. 



