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LXXVIII. A Proposal for discovering the Annual Parallax of Sirius.
Rev. Nevil Maskelyne,* A. M., F. R. S. p. 889.
The Royal Society had resolved to send persons of ability to proper places, to observe the approaching passage of Venus over the sun, the 6th of June in the year following; (as first proposed by Dr. Halley 44 years ago, as a proper means of determining the sun's parallax to a great degree of exactness, Dr. M. recommends a very important object in astronomy, which he apprehends may be cleared up at the same time, by the astronomers sent to one of those places which will probably be judged convenient for the observation of Venus's transit, viz. the island of St. Helena.†
This object is the determination of the annual parallax of the Orbis Magnus; the finding out of which, from observation, would be the fullest and most direct proof of the Copernican system, as the want of this proof hitherto has been the strongest argument made use of by those who have withheld their assent to an hypothesis, which so fully satisfies all the other phenomena.
No one indeed will now venture to assert that, even if no annual parallax could be found, after the greatest exertion of human art and industry, the Copernican system was not therefore true; since the quantity of this parallax may be so small as to escape the reach of our sight, though assisted to the utmost. But though the defect of it would be no just argument against the Copernican system, yet the actual demonstration of it, from observation, would be a direct and con vincing proof of the truth of that system. It remains then to be considered, what hope there is now left, after astronomy has been brought to such a great degree of perfection, of being able to find out an annual parallax in any of the fixed stars.
Mr. M. is sensible he may here seem to be presumptuous, in venturing to treat on this subject, after the many accurate observations made by Dr. Bradley, with an instrument constructed for this very purpose. He would just beg leave to take notice, that the stars which this astronomer observed, were such only as lay within a few degrees of his zenith; and though his observations do not seem to show a sensible parallax in any of them, yet we cannot thence absolutely conclude, that among the great number of visible stars, there are none in which it may be perceptible, till they have all of them, especially those of the greatest lustre, been observed in proper places near the zenith, with the like care and accuracy which he has used: for, as Dr. Bradley has himself remarked, where any stars are remote from the zenith, the uncertainties of refraction, and the irre
*The present astronomer royal, 1807.
+ Mr. M. was himself the person sent to St. Helena, to make the observations, as will appear hereafter in due place.
gular motions of the air, become so great, as to take away from us all hopes of observing them to an equal degree of exactness.
The particular star which Mr. M. proposes should be carefully observed, with a view of discovering its annual parallax, if sensible, is Sirius, the brightest of all the stars in the firmament, and which is therefore probably the nearest to us of them all. With us, this star passes the meridian, at the altitude of 22°, where the refractions are too irregular to admit of our discovering a very. minute quantity, by observation: but, at the island of St. Helena, Sirius passes only half a degree south of the zenith; and, on this account, he has for some time considered that as the most proper place to make observations at, for this purpose.
Mr. M. mentions a particular argument, which had for 2 years induced him to think it probable, that the annual parallax of Sirius is not so small, as to elude the nice discernment of our modern astronomers; and he thinks it affords a sufficient presumption to undertake a careful and assiduous series of observations of the distances of Sirius, from the zenith of the island of St. Helena. This argument is drawn from an examination which he had made of the observations of the zenith distances of Sirius, taken at the Cape of Good Hope, in the years 1751 and 1752, by the Abbé de la Caille. Every particular observation of the same star was reduced to one epoch, that of the beginning of the year 1750, by applying the equations of aberration, precession; and deviation to the observed places; so that the places corrected ought all to agree together, if the observations were perfectly exact, and the star was affected with no sensible motion that was unaccounted for. Being satisfied with the excellence of the observations, Mr. M. was tempted to examine those of some of the principal fixed stars, in hopes of discovering some sensible differences in the observations made at different times of the year, when a parallax, if there had been any, would have had the greatest effect. But he found very few stars, the observations of which were sufficient in number, or taken at proper seasons of the year, to give room for any inference at all. Fortunately however the observations of the zenith distances of Sirius were more in number, and, what is still of more consequence in this case, made in various, and some in opposite seasons of the year: and, on looking them over, he was agreeably surprized to find a very sensible difference in the observations made at different times, agreeing in direction with what a parallax ought to produce, the zenith distance of Sirius in July coming out no less than 8" greater than in the opposite season of the year, viz. December and January; the zenith distance also in March and April being of an intermediate quantity, as it ought to be: for Sirius being in conjunction with the sun in June and July, it is evident he must be then farther from the earth than in December and January, when he is in opposition
to the sun, and consequently his latitude and declination, which are both south, must be less in the former case than in the latter; and therefore, as he passes north of the zenith at the Cape of Good Hope, his zenith distance must be greater in the former season than in the latter, as the observations indicated: but, in March and April, when Sirius is in quadrature with the sun, and equally distant from the sun and earth, his apparent latitude and zenith disance must be the same as the true, or that which would obtain, if the earth was translated to the sun, and consequently, a mean between the zenith distances in July and December, agreeably to the observations.
Mr. M. then lays down the observations themselves, with the calculations which he made of of the values of the parallax for each, the maximum being assumed 9", which he found would best reconcile the observations with one another.
The first column shows the year and day of the observations; the 2d the zenith distances of Sirius at the Cape of Good Hope, as delivered in Abbé de la Caille's recital; the 3d contains the computed values of the parallax at different times, taking that of the maximum 9"; the 4th column gives the observations reduced to the mean, by applying the parallax computed in the 3d column. to the observations in the 2d; which quantities ought all to agree together, if the observations were liable to no error, and the parallax was rightly assumed :: but, taking a mean of them all, the last column shows how much each of them, differs from that mean, which in general is very small, and scarcely exceeds 2",,
except in two observations, in one of which it amounts to 3", and the other to 4" but these differ as much from the mean of the six other observations made at the same season of the year. Thus, assuming a parallax, the observations will be found to agree as well with that supposition, as they do with one another. But if the observations are considered in themselves without any allowance for parallax, they will differ sensibly from one another: nor is this difference to be found only in two observations, in which case it may easily be attributed to the account of the unavoidable errors; but five observations in July opposed to two in December and January, make the zenith distances vary 8", in the direction which a parallax ought to produce.
Mr. M. is aware, that it may be objected, that two observations made in the winter season in December and January, at one of the maxima of the parallax, are too few to determine a point of such consequence, and readily agrees that the argument is weakened in proportion to the paucity of the observations: but then, it should also be considered, that the observations made in March and April concur with the rest in supporting the supposition of a parallax; and, on the whole, the observations will perhaps be judged to afford a sufficient presumption of the existence of a parallax, to encourage the undertaking of a careful trial.
LXXIX. Further Experiments in Electricity. By Mr. Benjamin Wilson, F. R. S. p. 896.
Being provided with a large square of glass, polished on both sides and fixed upright on one edge, Mr. W. placed, for a conductor, a slender piece of ivory, about one foot long, having one end within of an inch from the centre of the glass at the other end were suspended 2 small balls of pith, by threads 4 inches long. The ivory was supported horizontally by a stand made of baked wood. When the glass was made a little warmer than the external air, his finger rubbed that side which was furthest from, and opposite to, the ivory. On which, the two sides of the glass were electrified plus, as were likewise the balls; which continued plus, even after they were removed from the glass into any part of the room. That the fluid here flowed from the finger into the glass he thinks may be inferred from the following experiment.
A piece of silver, being fixed on a slender rod of prepared wood, he rubbed the same glass with it, as he had done before with the finger; on which the silver was electrified minus, and both sides of the glass, with the conductor and balls, plus. There are therefore, he thinks, certain circumstances under which the electric fluid passes through glass; he says in certain circumstances, because in others, for instance, the Leyden bottle, the fluid does not
pass through the glass, but electrifies one side plus, and the other side minus, as Dr. Franklin has shown in his letters on electricity.
Hence he collects, that the three different effects, viz. the electrifying glass plus on both sides; or plus on one side, and minus on the other; or, lastly, minus on both sides, are occasioned by the different degrees of the same power and resistance in the respective experiments with the same glass.
Mr. W. concludes with an experiment made by Mr. Hamilton, professor of philosophy in the university of Dublin, as it seems to illustrate the doctrine of resistances, at least, so far as respects the air.
Let a slender brass, or iron wire, 5 or 6 inches long, and finely pointed at each end, be fitted in the middle, with a brass cap, void of angles; then let half an inch at each extremity be bent in opposite directions, till they are perpendicular to the rest of the wire, and in such a manner, that when the wire is suspended, by means of its cap, on a point of metal, it may lie in a plane parallel to the horizon. The pointed metal, which supports this wire, must be 2 or 3 inches long, and have its other end fixed into a small block of wood. Now, if this block, with a wire suspended, be set upon an electrified body, the wire will turn round with a very great velocity, moving always in a direction contrary to that in which the electric fluid issues from its points, without having any conducting substance near it, save that of the air: and if the wire be made to turn round by any other force, in the opposite direction, so that its points go foremost, it will when electrified, soon be deprived of that motion, and be made to turn round the contrary way.
This experiment, he says, was contrived, to try whether the electric fluid, which issues so freely from pointed bodies, would have any effect to move these bodies by its reaction; and that it has such an effect seems sufficiently manifest from the event. Mr. Hamilton apprehends, that the electric particles, by their elastic force, issue directly forwards from the points, and endeavour to expand themselves; but meeting with some resistance from the air, force the wire to move backward in a contrary direction, much in the same manner that a Catherine-wheel is made to turn round in a direction, contrary to that in which the small rockets affixed to its periphery discharge themselves. And therefore, he is inclined to think, that it might be made use of as an electrometer, by having it to turn round in a plane perpendicular to the horizon, and loading the wire with small weights near one of its extremities, which will be raised to a greater distance from the perpendicular line, as the electric fluid is stronger.