In his book, ‘Opticks’, 1700, Newton made a respectable estimate for both ether’s pressure and density — the best up to about 1905, after which interest in ether sadly waned. Ether pressure is, in fact, the energy per volume in space, and thus is very fundamentally important. But this paper explains, sadly, how the scientists of the 1800’s largely failed to address ether’s pressure. I.e., not even appreciating ether’s causing the high-pressures near the center of the earth and inside more massive bodies. So, Einstein’s great (mc^2) was welcomed even more than otherwise. ((I also give my reasons for my own rough estimate of ether pressure of (10^+34) N/sq.m., (coincidentally, same as scientists use for inside neutron stars.) [1] And for my ether density estimate of (10^-20) Kg/cu.m., (consistent with the highest-to-lowest densities in Nature — roughly equaling the highest-to-lowest forces found in Nature).)) Some 1800’s scientists made good estimate of ether’s density. ((It is said of Einstein’s gravitational theory that "two adjacent bodies bend space more than otherwise". But my view is that the available space for ether flow is naturally more constricted between two close bodies, so ether’s flows must speed-up there to maintain flow continuity. And that causes the well-known ‘Venturi flow effect’ -- reduced pressure, a suction effect, ‘attraction’, i.e., known as ‘gravity’.))