The Paper which I have the honour of presenting to the Society, is drawn up from letters written by Captain Hall to some of his friends in this country, after a visit made to the Cape of Good Hope, and an excursion to the Table Mountain, in July last. I have given the description, as much as possible, in his own words, and have only connected parts, which, from the nature of the communications, were necessarily disjoined from one another. One of the letters being written to myself, and containing a general view of the whole, has been my guide for arranging the rest.

By comparing the longitudes and latitudes of the fixed stars, as computed from their right ascensions, and north polar distances, determined by observations made at distant periods, Astronomers have been enabled to ascertain, with the utmost accuracy, the precession of the equinoxes. The great accuracy, however, which has lately been introduced into the construction of astronomical instruments, and the more general cultivation of Practical Astronomy, will hereafter be the means of ascertaining the minute changes in the places of the fixed stars, which have been denominated their proper motion, and which, probably, arise from a motion of the solar system in absolute space.

In examining the changes which light undergoes during its passage through transparent bodies, we not only receive information respecting the properties of that mysterious agent; but we are in some measure made acquainted with the composition of the substances themselves, and with the manner in which their ingredients are combined. The optical phenomena, therefore, which bodies exhibit in their action upon light, are so many tests, to which the philosopher may have recourse, either in supplying the place of chemical analysis, or in correcting and modifying its results. A difference in the optical properties of two bodies, is generally an infallible indication of a difference in their elementary principles; and whatever confidence we may place in the skill of the chemist, or in the accuracy of his methods, the mind can never rest satisfied with the results of an analysis which is directly opposed by optical phenomena.

The river Tilt is a principal branch of the Tay, which rises on the borders of Aberdeenshire, and runs towards the south-west, through the north-eastern part of the county of Perth. A portion of the valley along its course, for about ten miles above Blair of Atholl, is called Glen Tilt.

The adjacent country presents the common character of the Highlands. It is mountainous and rugged, and the surface, except in the lower parts of the valleys, is chiefly covered with heath. Peat-moss frequently occurs.

Some months ago, in investigating the anatomy of the eyes of fishes, I washed off the back silvery part of the choroid coat of the haddock, with a hair pencil, and about half a teaspoonful of water. The latter became of a milky colour, and on examining a drop of it, with an Ellis's aquatic microscope, I found the milkiness to be owing to innumerable slender, flat, silvery spicula, which had composed the substance of the choroid. They seemed to be in constant motion, apparently rolling upon their axes, but having no degree of progressive movement. The light reflected from their surface was very brilliant, like that from polished silver, and often disappeared, and again returned, with alternations so rapid, as to produce a twinkling, very like that of a fixed star.

Those philological studies which have been cultivated among polished nations, for their subserviency to literary taste, and their influence on the formation of correct habits of speaking and writing, have gradually suggested the more profound science of Universal Grammar. The business of this science is, not merely to dictate the best manner of connecting words with one another, but to investigate the origin of the various distinctions existing among them, and to discover in what instances these distinctions are dictated by occasional convenience or partial caprice, and in what instances they are essential to the purposes of language. By the aid of these investigations, philosophers have even hoped to trace some of the most important laws of human thought, and to obtain a solution of the most difficult problems of metaphysics. Some of the attempts made to realise these expectations have displayed great ingenuity; but, even with this qualification to recommend them, they have often failed to establish principles on a foundation sufficiently sure to preclude controversy.

An argument which I had stated against the Huttonian Theory of the Earth, in so far as it relates to the operation of a central heat, was honoured some time ago with a reply by a distinguished Member of the Society. Respect for the opinion of Mr Playfair led me to consider attentively the reasoning he employed; and still feeling some confidence in the grounds on which the original argument rests, I propose to offer a few observations with regard to it. The question farther involves the consideration of the mode in which heat is distributed at the surface of the globe. This, when minutely investigated, presents a very perfect arrangement, by which the escape of caloric is prevented, while its equal distribution is more effectually attained; and the subject, under this point of view, may have some interest, independent of its relation to any controversial discussion.

During a series of experiments in which I was lately engaged, for the purpose of determining the law of the polarisation of light, by successive reflexions from plates of parallel glass, I observed that all the images of the luminous body which were formed by more than one reflexion, were crossed by parallel fringes of coloured light, when the two plates had a small inclination to each other; and that these fringes suffered considerable changes, by varying the position of the plate with regard to the incident ray.

I Propose to submit to the Society the analysis of a Mineral Water of the Saline Class, which has lately been discovered in the neighbourhood of Dunblane. The subject may have rather more interest than usually belongs to researches of this nature, from the composition of this water being such as promises to afford a spring of considerable medicinal efficacy, and from its resemblance to another mineral water, of some celebrity,—that of Pitcaithly, the analysis of which I have, from this circumstance, been also led to undertake. The investigation, too, may afford some illustrations of the different methods that may be employed in the analysis of waters of this class, and of the facility and precision which are given to these researches, by the results that have been established with regard to the definite proportions in which many bodies combine, and the uniformity of the relations which thus exist between the compounds they form. And it has led to some views with regard to the constitution of mineral waters of the saline class, which I have applied to the composition of some of the most celebrated mineral waters. In performing the principal experiments on the Dunblane water, I had the advantage of Mr Ellis's co-operation.

The distinguished person who is the subject of this memoir, was born at Boghall, in the parish of Baldernock, near Glasgow, in the year 1739. His father, John Robison, had been early engaged in commerce in Glasgow, where, with a character of great probity and worth, he had acquired considerable wealth, and, before the birth of his son, had retired to the country, and lived at his estate of Boghall.