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HISTORY OF PHYSICAL ASTRONOMY.

watch, exactly marking above 40,000 divisions in a foot, by the help oftwo indexes; the one showing hundreds of divisions, the other divisionsof the hundred.” Mr. Townley concludes his letter by stating that, if itshould be found desirable, the Society might obtain a more completedescription of the instrument.

Mr. Townley’s micrometer was actually produced before the Society atthe meeting held on the 25th of July, 1667, and a detailed descriptionof it was subsequently given in No. 29, of the Philosophical Transactions .In principle it exactly resembles the micrometer of Auzout. Two straightedges of metal are made to approach each other at the focus of the telescopeby means of a screw, the mechanism being so contrived that the opticalaxis of the telescope is always situate midway between the two edges. Thenumber of revolutions and parts of a revolution of the screw which arerequired in order to bring the edges into any determinate position willmanifestly indicate the extent of the interval which separates them *.Hooke suggested an improvement of this micrometer by substitutinghuman hairs for the solid edges.

It appears from the observations of Gascoigne, which are still in exist-ence, that he actually used the micrometer in several delicate measure-ments. Flamsteed, in the first volume of the Historia Celestis, has in-serted a series of celestial observations extending from 1638 to 1643,which were made by Gascoigne and his friend Crabtree, and among theseare contained a considerable number of micrometrical measurements byGascoigne. They consist of a determination of the mutual distances ofthe Pleiades, a few values of the apparent diameter of the sun, severaldistances of the moon from the neighbouring stars, and a great number ofmeasurements of the lunar diameter. The results in most instances aregiven in revolutions and parts of a revolution of the screw. In the forty-eighth volume of the Philosophical Transactions , Dr. Bevis has also givenseveral measurements of the apparent diameters of the planets and themoon, made by Gascoigne with his micrometer. They were extracted byhim from a letter written by Gascoigne to Oughtred , the original of which,he stated, was still preserved in the library of the Earl of Macclesfield f.The following comparison instituted by Delambre, between certain mea-surements of the semidiameter of the sun by Gascoigne, and the corre-sponding values in the Connaissance des Temps of the present day, willsuffice to convince the reader that the original inventor of the micrometerdid not fail to bring his instrument to considerable perfection :—

Connaissance des Temps.16' 10"

16 11.416 16.8

Gascoigne.

October 25, O.S. 16' 11" or 10"

October 31, 16 11

December 2, 16 24

Gascoigne makes the greatest variation of the apparent diameter of

* Sherburne, in his translation of the Sphere of Manilius (1675), states that Crabtree,

taking a journey to Yorkshire , in 1639, to see Gascoigne, thus wrote to Horrocks:_

“ The first thing Mr. Gascoigne shewed me was a large telescope, amplified and adornedwith new inventions of his own, whereby he can take the diameters of the sun or moon,or any small angle in the heavens or upon the earth, most exactly through the glass to asecond (see the work cited, p. 92). This is the letter alluded to in the note at thefoot of p. 428.

f Phil. Trans., 1753, p. 190.