People have been working on extending Newtonian gravity ever since Newton discovered it. The major problem of the theory, as admitted by Newton, was its 'action at a distance' character and for which he was unable to come up with a solution. As various people worked on thos the generic term for the medium of transmission was called the aether. Notably, given the prominance of mechanics, the aether was hypothesised mechanically.

It's only after electromagnetism was discovered with Faraday introduced his lines of force that another way of conceptayalising the aether came to the fore, that is by the field. A number of authors were working on aetherial theory of gravity, the most important of which, according to Einstein, was Nordstroms scalar-tensor theories, his second theory even satisfied Einsteins weak equivalence principle! However, it failed to predict light deflection and also the anomalous precession of Mercury, which Einsteins 1915 theory did.

People have been working on extending Newtonian gravity ever since Newton discovered it. The major problem of the theory, as admitted by Newton, was its 'action at a distance' character and for which he was unable to come up with a solution. As various people worked on those extensions, the generic term for the medium of transmission was called the aether. Notably, given the prominence of mechanics, the aether was hypothesised mechanically.

It's only after electromagnetism was discovered with Faraday introduced his lines of force that another way of conceptualising the aether came to the fore, that is by the field. A number of authors were working on aetherial theory of gravity, the most important of which, according to Einstein, was Nordstroms scalar-tensor theories. His second theory even satisfied Einsteins weak equivalence principle! 

However, it failed to predict light deflection and also the anomalous precession of Mercury, which Einsteins 1915 theory did.