Apergy is a fictional antigravity substance described by the author Percy Greg in his 1880 science fiction tale Across the Zodiac. Here are some extracts from the book that outline the concept:

 

"I had satisfied myself that only one thing needful was as yet wholly beyond the reach and even the proximate hopes of science. Human invention could furnish as yet no motive power that could fulfil the main requirement of the problem – uniform or constantly increasing motion in vacuo – motion through a region affording no resisting medium. This must be a repulsive energy capable of acting through an utter void. Man, animals, birds, fishes move by repulsion applied at every moment. In air or water, paddles, oars, sails, fins, wings act by repulsion exerted on the fluid element in which they work. But in space there is no such resisting element on which repulsion can operate. I needed a repulsion which would act like gravitation through an indefinite distance and in a void – act upon a remote fulcrum, such as might be the Earth in a voyage to the Moon, or the Sun in a more distant journey. As soon, then, as the character of the apergic force was made known to me, its application to this purpose seized on my mind. Experiment had proved it possible, by the method described at the commencement of this record, to generate and collect it in amounts practically unlimited. The other hindrances to a voyage through space were trivial in comparison with that thus overcome; there were difficulties to be surmounted, not absent or deficient powers in nature to be discovered.

 

"...For obvious reasons, those who possessed the secret of the Apergy had never dreamed of applying it in the manner I proposed. It had seemed to them little more than a curious secret of nature, perhaps hardly so much, since the existence of a repulsive force in the atomic sphere had been long suspected and of late certainly ascertained, and its preponderance is held to be the characteristic of the gaseous as distinguished from the liquid or solid state of matter. Till lately, no means of generating or collecting this force in large quantity had been found. The progress of electrical science had solved this difficulty; and when the secret was communicated to me, it possessed a value which had never before belonged to it...

 

"At 12 pm, or 24h by the chronometer, I directed a current of the requisite strength into the eastward conductor, which I had previously pointed to the Earth's surface, but a little short of the extreme terrestrial horizon, as I calculated it. At 1 am I found myself, judging by the stars, exactly where I wished to be, and nearly stationary as regarded the Earth. I instantly arrested the eastward current, detaching that conductor from the apergion; and, directing the whole force of the current into the downward conductor, I had the pleasure of seeing that, after a very little adjustment of the helm, the stars remained stationary in the mirror of the metacompass, showing that I had escaped from the influence of the Earth's rotation."