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HISTORY OF PHYSICAL ASTRONOMY

Oti the 12th of December, 1040, he expressed to his friend n strongdesire to obtain some of Gascoigne’s measurements of the lunar dia-meter*. He also announced to him that his observations of the tideshad already revealed to him many interesting particulars. They werowithal very regular, although subject to many strange inequalities hithertoremarked by no person. He had only prosecuted his researches three months,but he hoped that, by continuing his observations for a year, he should obtainsome valuable results f. In the same letter he entreats his friend to per-severe in Ms astronomical pursuits, adding, that as soon as his own affairswould permit, he purposed resuming his favourite studies. Alas ! ere an-other month had elapsed, his noble spirit had fled from its mortal tenement.

It cannot fail to excite the admiration of the reader, tliatayoutli of twenty-two years of age should have exhibited in his researches such sagacity ofthought and fertility of invention, such enlightened and judicious viewson the various subjects which engaged his attention, and such unwaveringconfidence in the resources of his own mind. Who can doubt that, if hisdays had been more numerous, the history of physical science in theseventeenth century would read very differently from what it now does.Justly may it be affirmed, in the language of the illustrious editor of hisfragments,—“ Qui enivi tain paucis annis, auxiliis parvis, tantisque obsitusdifficultatibus tantos progressus fecerat; quid nonfecisset, si Deus hucusquevitam protelavisset! si necessariis omnibus instructus , doctorum etiamconsortio, adjutus fuisset!"

The utility of the transits of the inferior planets in furnishing an accu-rate method of determining the value of the solar parallax, was firstpointed out by James Gregory, the celebrated mathematician, in histreatise entitled “ Optica Promota,” which was published at London in1003. As the credit due to the original suggestion of this method hasbeen generally ascribed to Halley, it may not be out of place to citethe passage in which Gregory alludes to it. In a scholium to the eighty-seventh problem, the object of which is to determine the parallaxes of twoplanets by observations of their conjunctions, he makes the followingstatement“ This problem has a very beautiful application, althoughperhaps laborious, in observations of Venus or Mercury when they obscurea small portion of the sun; for by means of such observations the parallaxof the sun maybe investigatedJ. It would be impossible to establishthe claims of Gregory to priority of discovery upon a more unequivocalbasis than is afforded by this passage. Halley’s attention was firstdirected to the subject on the occasion of his observation of the transitof Mercury in 1G77, that is to say, fourteen years after the publication

* Gascoigne was the original inventor of the micrometer, as will be shown in the next,chapter, and it is to the results obtained by the use of this instrument that Horroeks herealludes. Crabtree had just returned from a visit to Gascoigne in Yorkshire , and one ofthe principal objects of Horroeks’ contemplated visit to his friend was to obtain an accountof the very remarkable improvements in practical astronomy which Gascoigne had re-cently effected. See, in the next chapter, an extract of a letter from Crabtree to Hor-rocks, in which there is contained an interesting allusion to the micrometer, whichGascoigne showed Crabtree on the occasion of this visit.

+ Horroeks appears to have been the first person who undertook the prosecution of acontinuous course of observations of the tides, for the express purpose of obtaining aseries of facts which might form the groundwork of a philosophical investigation of thesubject.

4 Optica Promota, p. 130. The words in the original are :—“ Hoc problema pulcher-rimum liabet usum, sed forsan laboriosum, in observationibus Veneris vel Mercurii parti-culam solis obscurantis; ex talibus enim solis parallaxis iuvestigari poterit.”