90 Remarks on Prof. Moseley's Principle of least Pressure. 



have taken a wrong view of the matter, (which I confess is not 

 improbable, since what I have written is so very obvious that it 

 must have occurred to Mr. Moseley and been considered by 

 him invalid,) I cannot comprehend why we are to consider the 

 forces as functions of the coordinates of the points at which they 

 act. If there be a reason, it most certainly does not appear 

 on the face of the demonstration of the principle. 



In Mr. Moseley's communication to your Journal of this 

 month, he mentions, as a valuable instance for showing the in- 

 correctness of the notions which have hitherto been held on the 

 subject of pressures, the case of a mass supported on three 

 props, at the angular points of a triangle, whose centre of gravity 

 is in the same vertical line as that of the given mass. The 

 pressure upon each prop in this case, as is well known, is 

 equal to one third of the weight of the body supported. 

 " Now," says Mr. Moseley, " this condition continuing to be 

 satisfied, let us suppose the third point of support to move 



into the same with the other two. The fraction ^ expressing 

 the evanescent ratio of each elementary triangle to the whole 

 triangle, will then manifestly have the value %. And three 

 points of support in the same straight line will each of them 

 sustain the same pressure." I am, however, of opinion that 

 the conclusion from this reasoning is not so manifest as Mr. 

 Moseley seems to think it is ; for let A, B, C be the angular 

 paints of the triangle in its finite state, then if we inquire into 

 the cause of the pressure at any one of them (suppose C), we 

 shall find that it arises from the circumstance that the side 

 AB is in a certain respect a fixed axis, about which the body 

 has a tendency to move, and about which it is only prevented 

 from moving by the pressure of the prop at C: as soon, there- 

 fore, as the points of support are brought into one line, AB, in 

 which of course, from the nature of the hypothesis, the centre 

 of gravity is situated, the body ceases to have a tendency to 

 move round AB, and the office of C is abolished, and the 

 case of the triangle is not applicable to this. 



To put this idea in another view, let the body be supported 

 on three props, A, B, C, of which A, B are so situated that 

 the line joining them passes through the centre of gravity, 

 and C is situated without. Then the body will balance upon 

 AB, and there will be no pressure at all upon C ; for if C ex- 

 erted any pressure, it would overthrow the body by turning it 

 round AB. Let now C come into the line AB, and have the 

 precise situation assigned 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 on C ; which is directly at variance with Mr. 

 Moseley's results . 



