But as this apparent Motion of the Stars at the Earth, must, from its Nature, be very small, so as scarce to be discovered in some of them in less than an Age, with any Instrument by the nicest Observer, I judge it will be extremely proper in this Place to propose some Method, by which, in process of Time, the Truth of the Theory may be ascertained. The Way I think most likely to succeed is this.
PLATE XIX.
Is a Plan of the principal Stars that form the Pleiades, correctly taken by a Combination of Triangles, as in the Figure, from whence it will naturally follow, all the whole Form being comprehended in much less than one Degree. That the most minute local Motion in any one of those Stars in a very few Years, will be made sensible to an Eye at the Earth. For Instance, if any of the Stars that form the Letter A, or T, within the Term of ten or twenty Years, be found in the least to deviate from the Lines of their present Position and Direction, it will be evident beyond a Contradiction, that they have a Motion amongst themselves, and since at such a Distance they cannot possibly be affected by the Earth, it must be a Motion of their own; and thus if any one can be proved, to change its Situation, with regard to the rest, we can have no new Difficulty in concluding that they all may do the same.
Thus if any of the regular Triangles M B Z, Z P H, A Z M, Y A Γ, or Π Ο I, &c. in due Time be carefully noted, we may venture to say with great Safety, that the thousandth Part of a Degree will be plainly discovered.
PLATE XX.
Is a true Plan and Combination of the principal Stars that form the Persedes, in which other Observations may be made in a different Part of the Heavens, and perhaps with an Opportunity of being still more exact, the Areas of these Triangles, particularly that of Θ I K, and those of ρ and δ, being much less than the former, where the least Alteration possible must render them sensibly distorted. But here it must be considered, that the real Motion of the Stars, as well as their apparent, may be, and in all Likelihood, is extreamly slow, for the most minute, visible, local Motion, will answer all the Purposes we know in Nature, and the greatest seems to be that of the projectile, or centrifugal Force, which not only preserves them in their Orbits, but prevents them from rushing all together, by the common universal Law of Gravity, which otherwise, as a finite Distribution of either regular or irregular Bodies, they must at length do by Necessity.
Plate XIX.
Plate XX.
