Similarly it is necessary, in view of the hydrostatical relations of water and mineral oils, and the volatile character of the latter, that the porous stratum should be protected from water and air by an overlying shale or other impervious deposit.
In casting a thin hollow object like a bell, it will be seen that the resultant upward thrust on the mould may be many times greater than the weight of metal; many a curious experiment has been devised to illustrate this property and classed as a hydrostatic paradox (Boyle, Hydrostatical Paradoxes, 1666).
In 1738 Dr Robert Smith published Cotes' Hydrostatical and Pneumatical Lectures, a work which was held in great estimation.
'The following are the more important of his works in addition to the two already mentioned: - Considerations touching the Usefulness of Experimental Natural Philosophy (1663), followed by a second part in 1671; Experiments and Considerations upon Colours, with Observations on a Diamond that Shines in the Dark (1663); New Experiments and Observations upon Cold (1665); Hydrostatical Paradoxes (1666); Origin of Forms and Qualities according to the Corpuscular Philosophy (1666); a continuation of his work on the spring of air (1669); tracts about the Cosmical Qualities of Things, the Temperature of the Subterraneal and Submarine Regions, the Bottom of the Sea, &c. with an Introduction to the History of Particular Qualities (1670); Origin and Virtues of Gems (1672); Essays of the strange Subtilty, great Efficacy, determinate Nature of Effluviums (1673); two volumes of tracts on the Saltness of the Sea, the Hidden Qualities of the Air, Cold, Celestial Magnets, Animadversions on Hobbes's Problemata de Vacuo (1674); Experiments and Notes about the Mechanical Origin or Production of Particular Qualities, including some notes on electricity and magnetism (1676); Observations upon an artificial Substance that Shines without any Preceding Illustration (1678); the Aerial Noctiluca 0680); New Experiments and Observations upon the Icy Noctiluca (1682) a further continuation of his work on the air; Memoirs for the Natural History of the Human Blood (1684); Short Memoirs for the Natural Experimental History of Mineral Waters (1685); Medicina Hydrostatica (1690); and Experimenta et Observationes Physicae (1691).
The following is Nicholson's statement on this point: "One of the greatest difficulties which attends hydrostatical experiments arises from the attraction or repulsion that obtains at the surface of the water.