Among the more advanced students who, in the years 1748 and 1749, attended the lectures on di vinity in the university, was Mr. Thomas Melvin, so well known by his mathematical talents, and by those fine specimens of genius which are to be found in his posthumous papers, published in the second volume of the Edinburgh Essays Physical and Literary. With this young person Mr. Wil son then lived in the closest intimacy. Of several philosophical schemes which occurred to them in their social hours, Mr. 'Wilson proposed one, which was to explore the temperature of the at mosphere in the higher regions, by raising a num ber of paper kites, one above another, upon the same line, with thermometers appended to those that were to be most elevated. Though they ex pected in general, that kites thus connected might be raised to an unusual height, still they were somewhat uncertain how far the thing might suc ceed upon trial. But the thought being quite new to them, and the purpose to be gained of some im portance, they began to prepare for the experiment in the spring of 1749.
In the year 1752, Mr. Wilson, who had married the daughter of William Sharp, a reputable mer chant of St. Andrews, brought his family to Glas gow. About five years afterwards he invented the hydrostatical glass-bubbles, for determining the strength of spirituous liquors of all kinds. From the minutes of a Philosophical and Literary So ciety, composed of the professors and some of their friends, whose meetings were held weekly within the college, it appears that these hydrostatical bubbles made the subject of a discourse delivered by Mr. 'Wilson in the winter of 1757. At this time he also showed how a single glass-bubble may serve for estimating very small differences of specific gravity of fluids of the same kind, such as water taken from different springs, or the like. This he did by varying the temperature of such fluids, till the same bubble, when immersed, be came stationary at every trial, and then expressing the differences of their specific gravity, by degrees of the thermometer, the value of which can be computed and stated in the usual manner.
In the year 1758 he read another discourse to the same society upon the motion of pendulums. On this occasion he exhibited a spring-clock of a small compass, which beat seconds by means of a new pendulum he had contrived, upon the principle of the balance, whose centres of oscillation and mo tion were very near to one another. At one of the trials it performed so well as not to vary more than a second in about forty hours, when compared with a very exact astronomical clock near to which it was placed.
Not long after this, he also put in execution a remarkable improvement of the thermometer, which consists in having the capillary bore drawn very much of an elliptical form, instead of being round. By this means the thread of quicksilver upon the scale presents itself broad, and much more visible than it does in a cylindrical bore of the same capacity. The difficulty of constructing thermo meters of this kind had nearly hindered him from completing his invention, as the thread of quick silver was found extremely liable to disunite when descending suddenly in so strait a channel. But, by his long experience, he at last discovered a method of blowing and filling thermometers with flattened bores, which freed them entirely from this defect.
About the same time, also, he conceived the de sign of converting a thermometer graduated for the heat of boiling-water, into a marine barometer, in consequence of the well-known difference of tempe rature which water, when boiling, acquires under the variable pressure of the atmosphere. This he
effected by making a boiling-water thermometer about a foot in length, with a pretty large ball, and having a thread of quicksilver as broad and as visible as was consistent with a very perceptible run upon small alterations of temperature. The stem of this thermometer he fortified, by inclosing it in a cylindrical case of white iron, having sol dered to it, at its lower end, a socket of brass for receiving half of the ball, which afterwards became entirely defended, by screwing to the socket a hemispherical cap. At the other end of the case which environed the stem, there was soldered a tube of brass wide enough to admit a scale of pro per dimensions, before which there was an opening in the tube, defended by glass.
The utmost range of the scale he determined by the points, where the thermometer was found to be stationary when the ball, and a certain part of the stem were immersed in water, boiling under the greatest variations of pressure which the climate afforded. The interval so found, he subdivided by other observations into degrees, which corresponded to inches of the barometer, and which were so de nominated upon the scale.
In the year 1756, the college of Glasgow, upon the death of Dr. Alexander Macfarlane of Jamaica, a great lover of, and proficient in the sciences, re ceived a legacy of a valuable collection of astrono mical instruments, which that gentleman had got constructed at London, by the hest artists, and had carried out with him to Jamaica, with a view of cultivating astronomy in that island. The college, upon this, soon built an observatory for their re ception, which, by medals placed under the found ation, was called by the name of their generous benefactor; and Mr. Wilson was immediately thought of as a proper person for making the astronomical observations. At this juncture his Grace Archibald, Duke of Argyle, procured his Majesty's presentation, nominating him professor of practical astronomy, with an annual salary of fifty pounds; and accordingly, in 1760, he was ad mitted to this new office.
In 1769, Dr. Wilson made that discovery concern ing the solar spots, of which he has treated in the Philosophical Transactions of London for 1774. Not long after he entered upon this new field, the nature of the solar spots was announced by the Royal Society of Copenhagen as the subject of a prize essay. This induced him to transmit thither a paper written in the Latin language, containing an account of his observations, and of the conclu sions drawn from them. In return, he obtained the honourable distinction of a gold medal of near six teen guineas.
He published in the Philosophical Transactions for 1783, the second paper upon that subject, wherein, upon the authority of many new observa tions, he obviates objections, and maintains the reality of his discovery with an entire conviction. The amount of it is, " That the spots are cavities or depressions in that immensely resplendent sub stance which invests the body of the sun to a certain depth; that the dark nucleus of the spot is at the bottom of this excavation, which commonly extends downwards to a space equal to the semidiameter of our globe; that the shady or dusky zone which sur rounds the nucleus, is nothing but the sloping sides of the excavation reaching from the sun's general surface downwards to the nucleus or bottom." In March and April 1786, when he had nearly completed his seventy-second year, it became appa rent to his family and friends, that his constitution and strength were fast declining. After a gradual decay, which he bore with the utmost resignation, he expired on the 16th day of October.