temporarily produced in Isotropic Bodies. 



251 



variation ; if, for example, it be greater between and - than it 



\ X . . 



is between -: and rr, this variation will manifest itself in the 



successive values of P& which are found by means of our formula. 

 If, on the contrary, the proportionality exists between these two 

 limits, we shall obtain, by calculation, the same value of P^ that 

 has been found by direct compression. 



The following table contains some of these determinations : — 



By direct measure- 

 ment. 



Pa. 



P*. 



Equivalent weights. 



Calculated. 



P,. 



Pft. 



Two compressions, flint and 

 crown.. , 



Two compressions, plate 

 glass and crown 



Two compressions, crown 

 and alum 



Compression and traction, 

 crown and plate glass ... 



Compression and traction, 

 crown and flint 



140 



r292 



r 75 



r 68 



68 



185 

 27 



210 

 155 



58 

 90 



120 

 50 

 80 



120 

 14 

 34 

 71 



120 

 12 

 19 

 16 

 24 

 33 

 44 

 52 



15-0 

 28-0 

 450 

 600 

 310 

 500 

 87-0 

 50 

 12-5 

 250 

 45-0 

 35-0 

 55-0 

 35-0 

 550 

 75-0 

 950 

 1150 



650 



67-0 



700 



700 



1810 



1820 



187-0 



26-8 



27-6 



26-4 



281 



1.98-0 



197-0 



1490 



1560 



1560 



1470 



1500 



These examples are sufficient to enable us to form a judgement 

 as to the exactitude of the procedure ; they show us at the same 

 time the little variations of compressibility and extensibility 

 which occur within the limits of the first half ring, and which 

 we are to take into account in the construction of the curve 

 which represents the route of the compressions and elongations. 



After the details into which I have just entered, I can in the 

 subsequent portion of this memoir dispense with describing for 

 each piece the whole series of operations to which it has been 

 submitted, and confine myself to stating the charges which, by 

 traction or pressure, have successively produced difiierences equal 



to ^, or to the length of a semi-undulation of the mean yellow 



ray. For example, I will designate by Tg - and Pg - the weights 



which it is necessary to add, so that the difference of path which 



was ^ may be equal to 2 jr, and so of the rest ; consequently the 



total charge will always be equal to the stum of the successive 

 values of T and P. 



