English

Noun

æquilibrium ‎(plural æquilibria)

1. Archaic spelling of equilibrium.^[1]
   • 1734, John Keill, Maupertuis, An Examination of Dr. Burnet’s Theory of the Earth, page 93:

     Now in a ſpherical fluid, all whoſe parts gravitate towards the Centre, I think it is evident from the principles of Hydroſtaticks and fluidity, that all thoſe Bodies which are equally diſtant from the Centre, muſt be equally preſt with the weight of the incumbent fluid, and if one part come to be more preſſed than another, that which is moſt preſſed will thruſt that out of its place which is leaſt, till all the parts come to an æquilibrium one with another, and this is known by a common and eaſy experiment, if you take a recurved tube as in the figure, [Fig. 4. Plate II.] and fill it with water or any other fluid, it will riſe equally in both Legs of the Tube, ſo that the ſurfaces C E and F I are equally preſſed by the weight of the incumbent columns B C E D, G F I H, but if one of the Legs of this Tube ſhould be filled with oil, or ſome other lighter fluid, and the other with water, the lighter fluid will riſe higher than the other, for otherways, theſe ſurfaces which are equally diſtant from the Centre could not be equally preſſed.

References

1. ↑ “equilibrium” in the Oxford English Dictionary (second edition), giving æquilibrium as a 17th–19th-century spelling.