Volume 1

A mathematical and philosophical dictionary: containing an explanation of the terms, and an account of the several subjects, comprized under the heads mathematics, astronomy, and philosophy both natural and experimental: with an historical account of the rise, progress, and present state of these sciences: also memoirs of the lives and writings of the most eminent authors. Both ancient and modern, who by their discoveries or improvements have contributed to the advancement of them ... With many cuts and copper-plates / By Charles Hutton.

  • Charles Hutton
Date:
1795-1796
Catalogue details

Licence: Public Domain Mark

Credit: A mathematical and philosophical dictionary: containing an explanation of the terms, and an account of the several subjects, comprized under the heads mathematics, astronomy, and philosophy both natural and experimental: with an historical account of the rise, progress, and present state of these sciences: also memoirs of the lives and writings of the most eminent authors. Both ancient and modern, who by their discoveries or improvements have contributed to the advancement of them ... With many cuts and copper-plates / By Charles Hutton. Source: Wellcome Collection.

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    motion of aline parallel to itfelf, much after the manner that the compofition and reiolution of forces are ex- plained. If light have a progreffivc motion, let the proportion of its velocity to that of the eaith in her orbit, be as the line 13 C to the line AC; then, by the compofition of thefe two motions, the particle of light will feem to deferibe the line B A or D C, infiead of its real coiirfe B C ; and will appear in the direilion A B or C D, in- ftead of its true direction C B. So that if A B reprefent a tube, carried with a parallel motion by an obferrer along the line A C, in the time that a particle of light would move over the fpace B C, the different places of the tube being AB, cd^ CD; and when the eye, or end of the tube, is at A, let a particle oB light enter the other end at B ; then when the tube is at a the particle of light will be at c, exadlly in the axis of the tube ; and when the tube is at c .7, the particle of light wall arrive at fy llill in the axis of the tube ; and laftly, when the tube arrives at C D, the par- ticle of light will arrive at the eye or point C, and con- fequently will appear to come in the dire6Iion D C of the tube, inftead of the true direclion B C. And fo on, one particle fucceeding another, and forming a con- tinued dream or ray of light in the apparent diredtion 1) C. So that the apparent angle made by the ray of light witli the line A E, is the angle D C E, inftead of the true angle BCE; and the difference, B C D or A B C, is the quantity of the aberration. M. de Maupertiiis, in his Elements of Geography, gives alfo a familiar and ingenious idea of the aber- ration, in this manner: “ It is thus,” fays he, “ con- cerning the direction in which a gun muft be pointed to ftrlke a bird in its flight ; inftead of pointing it ftraight to the bird, the fowler wall point n little before it, in the path of its flight, and that fo much tlic more as the flight of the bird is more rapid, with refped: to the flight of the fhot.” In this w^ay of confidcring the matter, the flight of the bird reprefents the motion of the earth, or the line A C, in our fcheme above, and the flight of the fliot reprefents the motion of the ray oi light, or the line B C. Mr. Clairaiit too, in the Memoires of the Academv of Sciences for the year 174^, illuftrates this elfeci in a familiar w’ay, by fuppofing drops of rain to fall rapidly and quickly after each other from a cloud, under wliicli a perfon moves with a very narrow tube ; in w’hich cafe it is evident that the tube muft have a certain inclina- tion, in order that a drop which enters at the top, may fall freely through the axis of the tube, wn'thout touching the Tides of it ; w’hich inclination muft be more or lefs according to the velocity of the dro])s in refpctl to that of the tube: then the angle made by tlie direAlon of the tube and of the falling drops, is the aberration arii- Mig from the combination of thofe two motions. This difeovery, which is one of the brighteft that have been made in the prefect age, we owe to the accuracy and ingenuity of the late Dr, Bradley, Altronorncr jR-oyal; to which he was occafioHaHy led by the refult of fome accurate obfervations which be had made with another view, namely, to determine the annual parallax of the fixed ftars, or that which arifes from the motion of the earth in its annual orbit about the fun. The annual motion of the earth about the fun had been much doubted, and warmly contefted. The de- fenders of that motion, among other proofs of the re- ality of it, conceived the idea of adducing an inton- feftable one from the annual parallax of the fixed ftars, if the ftars fhould be within fuch a diftance, or if in- ftruments and obfervations could be made with fueft ac- curacy, as to render that parallax fenfible. And with this view' various attempts have been made. Before the obfervations of M. Picard, made in 1672, it was the general opinion, that the ftars did not change their po- fition during the coiirfe of a year. Tycho Brahe and Ricciolus fancied that thev had aftiired themfelves of it from their obfervations; and from thence they concluded that the earth did not move round the fun, and that there was no annual parallax in the fixed ftars. M. Pi- card, In the account of his Voyage Uranihourg^ made in 1672, fays that the pole ftar, at different times of the year, has certain variations which he had obferted for about 10 years, and which amounted to about 40 a year: from whence fome who favoured the annual motion of the earth w'cre led to conclude that thefe va- riations w'ere the effetft of the parallax of the earth’s orbit. But it w'as impoffible to explain it by that pa- rallax ; becaufe this motion was in a manner contrary to w'hat ouglit to follow only from the motion of the earth in her orbit. [n 1674 Dr. Hook publifhed an account of obfer- vations which he faid he had made in 1669, and bj which he had found that the ftar y Draconis was 23 ' more noitlierly In July than in Odlober: obfervations which, for the prefent. Teemed to favour the opinion of the earth’s motion, although it be now known that there could not be any truth or accuracy in them. Flamfteed having obferved the pole ftar with hi* mural quadrant, in 1689 and the fbllow’ing years, found that Its declination was 40“^ lefs In July than in De- cember; which obfervations, although very juft, were yet however improper for proving the annual parallax ; and he recommended the making of an inftrument of 15; or 20 feet radius, to be firmly fixed on a ftrong foundation, for deciding a doubt wdiich w'as otherwile not Toon likely to be brought to aconclufion. In this Hate of uncertainty and doubt, then, Dr. Bradley, in coiijun61ion with Mr, Samuel MoHneux, in the year 1725, formed the proje6l of verir)ing, by a feriea of new obfervations, thofe wftiich Dr, Hook had communicated to the public almoft 50 years before. And as it was his attempt that chiefly gave rife to this, fo it w’as his method in making the obfervations, in fome meafure, that they followed; for they made choice of the fame ftar, and their inftrument was conftrufted upon nearly the fame principles ; but had it not greatly exceeded the former in exadlnefs, they might itill have continued In great uncertainty as to tl\e parallax of the fixed ftars. And this w as chiefly owing to the accuracy of the Ingenious Mr. George Graham, to w'hom the lovers of ailronomy are alfo in- debted for fcvcral other exa(ft: and convenient inftru- meiUh, B 2 The