HISTORY OF PHYSICAL ASTRONOMY.
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Auzout first announced liis improvement of the micrometer in a letter tothe Royal Society of London , dated December 38, 1666, which is in-serted in the 21st Number of the Philosophical Transactions . Birchstates that, upon this letter being read before the Society, two of themembers, Wren and Hooke, took occasion to mention several ways ofmeasuring small angles which were already known in England. Thesephilosophers have nowhere given a detailed description of the micrometersdevised by them. Sprat, however, in his “ History of the Royal Society,”alludes, in a general summary of the labours of the Society , to theirconstruction of telescopes of different lengths, with several contrivances inthem for measuring the diameters and parts of the planets * * * § . In anotherpart of the same work, he states that Wren had added many sorts ofretes, screws, and other devices to telescopes, for taking small distancesand apparent diameters to seconds f. In a work which he published in1665, Hooke proposes to determine the parallax of the moon or any ofthe planets by observing its distance from the neighbouring stars in twodifferent latitudes, by the aid of a telescope fitted with a rete or dividedscale £. It is evident that the micrometrical contrivances of Huyghens , theMarquis Malvasia, and Hooke, were all essentially similar to each other.Indeed, as soon as telescopes composed of two convex lenses began to becommonly used, the invention of such a micrometer as that devised byeither of these philosophers was a very obvious step.
In the 25th Number of the Transactions of the Royal Society there is aletter from Richard Townley, of Lancashire , relative to the micrometer ofGascoigne. The date of this letter is March 25,1667 §. “ Finding,” sayshe, “ in one of the last Philosophical Transactions , how much M. Auzoutesteems his invention of dividing a foot into near 30,000 parts, and takingthereby angles to a very great exactness, I am told I shall be lookedupon as a great wronger of our nation, should I not let the world knowthat I have, out of some scattered papers and letters that formerly cameto my hands of a gentleman of these parts, one Mr. Gascoigne, found outthat, before our late civil wars, he had not only devised an instrument ofas great power as Mr. Auzout’s, but had also for some years made use ofit, not only for taking the diameters of the planets and distances uponland; but had further endeavoured out of its preciseness to gather many
certainties in the heavens |j. I shall only say of it that it is
small, not exceeding in weight, nor much in bigness, an ordinary pocket
* “ History of the Royal Society,” p. 250, 4to, London , 1667. The account of thescientific labours of the Royal Society , inserted in this work, was compiled from the re-gisters of the Society down to 1665.
f Ibid., p. 314.
| Micographia, p. 237, fob, Lond., 1665. Hooke, on a subsequent occasion, describeda species of micrometer of a more perfect construction, but he was then acquainted withthe contrivances for a similar purpose devised by Gascoigne and Auzout.
§ The date of the letter is not mentioned in the number of the Philosophical Trans-actions cited in the text, but the omission is supplied by Birch, who alludes to the letterin his History of the Royal Society (vol. ii. p. 164). This letter, as well as the state-ment respecting it by Birch, has entirely escaped the notice of Delambre, who censures thetardiness of the English astronomers in not bringing forward any account of Gascoigne’smicrometer before the 25th of July, 1667.
|| Townley here states that Gascoigne determined the value of the lunar parallax bymeasuring with his micrometer the apparent diameter of the moon when she was in thehorizon, and again when she was on the meridian. He mentions also that he had in hispossession the very micrometer that Gascoigne originally used in his observations.
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