In the first Case, the Moon and Sun attract the Atmosphere of the Earth conjointly, or with united Force: in the second Case; the Earth being between them, they act in Opposition to each other, still nearly in the same Line.
At these Times, the spring Tides are at the highest i. e. once every Fortnight; and in the two interval Weeks are the neap or lowest Tides: for a like Reason.
Because, in the latter Case, a Line supposed to be drawn from the Moon to the Earth, and another from the Earth to the Sun, woud form nearly a right Angle: or in other Words; because the Moon and Sun woud attract the Earth at right Angles to each other, or in a lateral Direction:—the Moon woud draw one Way and the Sun another:—their Forces woud be divided.
Now it is a Fact, that the Ocean is raised considerably twice every twenty-five Hours, by the Attraction of the Moon, when she comes to the Meridian. So that the Surface of the Sea, instead of putting on the Form of a Sphere, or Globe, will be changed into an oval Figure, whose longest Diameter being produced, woud pass throu’ the Moon.
In like Manner a similar Elevation must take Place, as often as the Sun is in the Meridian; either above or below the Horizon.
Moreover, this Elevation is greatest on the New and Full Moon, because the Moon and Sun do then conspire in their Attractions: and least in the Quarters: as they will then draw different Ways; the Difference of their Actions only producing an Effect.
Lastly, the Intumescence will be of a middle Degree, at the Times between the Quarters, and New and Full Moon.
289. As in the Ocean, so in the Air above it; a Tide of Air must roll along the Atmosphere, throu’ the whole Extent of it; and rise upwards twice in about 24 Hours.
And since the Height of the Atmosphere is computed by Halley at 45 Miles, and the Depth of the Ocean at an Average, but half a Mile; the Air will more easily and quickly obey the Attraction of the Moon and Sun, than the Tide of the Ocean: and, as it revolves in a Sphere which is about 100 Times larger than that of the Ocean, the Agitation and the Velocity of its Tide, will be something greater, in Proportion to its Elasticity, and inferior Density to the Water of the Ocean.[92]
290. The Weight of the Air must now be considered.