1414 
Eme=0 Smy=0 5 do) 
Em (ey — y' al) =0 HOLWOHOL dav gn (4D) 
and after integration: 
Eme'=1, Smy=T, . Pi BIT Da) 
Emile er dd Rise verde mets al GB 
the 3 constants 7’, 7, and AR still being quite arbitrary. In analogie 
way we get in space 6 constants 7, 7, 7, R,, R‚‚ R,. Itis always 
possible to choose an inertial system in this way that 3 of these 6 
quantities vanish, more of them cannot vanish without a special 
hypothesis. 
The hypothesis of Förer is: 
There are inertial systems, for which all six constants T,, T,, T,, 
RR, R, vanish together. 
Consequently for the plane: 
Dea) Ae a END ae ee ERN 
or also, after (Sa) and (56): 
Sme'=—0 Emy =0. Ste rn enden Me Ale 
ES mey — ya!) =0 zE Krin vpieh et ee alb) 
To these special inertial systems belong also those for which 
moreover = mx’ =O and: 2 my’ =O, the origin thus coinciding 
permanently with the centre of gravity of the system of points, 
such a system is called by Förer a principal system of reference 
(Hauptbezugssystem) ’). 
Such a principal system Xs VY, can be constructed as follows: 
Take a system of axes NX, Y, of which the origin coincides perma- 
nently with the centre of gravity of the system of points, the axis 
of X passing permanently through one of the material points. 
Calculate > mr°8 in X, Vz. Take a second system of co-ordinates 
NXg Yp with its origin also permanently in the centre of gravity 
, i ; = mr" 6 : 
and give it in X, Y, a velocity of rotation w = —— =. In this 
= mr 
way a principal system, consequently an inertial system, has been 
fixed without the aid of an ‘absolute space”. 
1) The hypothesis of FörPL in its original form is: for an inertial system 
with origin in the centre of gravity the total moment of momentum vanishes 
permanently. F 
