X. Observations on atmospherical refraction as it affects astro­nomical observations; in a letter from S. Groombridge, Esq. to the Rev. Nevil Maskelyne, D.D. F. R. S. Astronomer Royal. Communicated by the Astronomer Royal

Abstract

Dear Sir, The great advantages we derive from the labours and writings of those learned men, who have preceded us ; the profound researches in physical astronomy by the mathema­ticians of the last century; and the improvements in the con­struction of instruments, effected by the ingenuity of our modern artists: these circumstances should stimulate all those who have inclination and opportunity, to the endeavour of contributing to the common stock of knowledge. I beg leave to add my tribute of admiration to the general voice, for the large share you, Sir, have taken in the promotion of science, not only by your learned publications, but also by the liberal encouragement and assistance you have given to those who have been honoured with your acquaintance. I am, therefore, induced to commit to your protection the inclosed paper, ac­knowledging myself indebted to your friendly communications for much useful information. I could have wished the task had been performed at an earlier period ; but as you are well acquainted with the labour required in the reduction of so many observations by one person, it will appear to have been impracticable. I remain, Dear Sir, Your Obliged and very obedient servant, Stephen Groombridge. To the Rev. Dr. Maskelyne, Astronomer Royal, &c. &c. When I had fixed my four feet transit circle, made by Troughton, it was my intention to make observations of a sufficient number of circumpolar stars, at different altitudes, to ascertain the latitude of my observatory with the greater precision; and also to endeavour to settle the true quantity of refraction: especially as my instrument was better adapted for that purpose, both from the construction and convenient size thereof, than those which had been heretofore made. Being fixed on stone piers, it is not so liable to partial expan­sion, as those instruments which are supported on brass frames: and having both sides divided, with two microscopes to each face; the same observation has the advantage of four made with a quadrant. I have also contrived, by means of sliding shutters in the roof, to prevent the rays of the sun from falling on any part, excepting on the object glass. By these means I flatter myself that, the observations having been made with great care and attention, the result will prove both satisfactory and useful.