To the assiduous observations and the indefatigable activity of Halley, the natural history of the atmosphere, of the ocean, and of magnetism, are all under the greatest obligations. For the purpose of inquiring into these objects, this ardent and philoso phical observer relinquished the quiet of academical retirement, and, having gone to St. Helena, by a residence of a year in that island, not only made an addition to the catalogue of the stars, of 860 from the southern hemisphere, but returned with great acquisitions both of nautical and meteorological knowledge. His observations on evaporation were the foundation of two valuable papers on the origin of fountains ; in which, for the first time, the sufficiency of the vapour taken up into the atmosphere, to maintain the perennial flow of springs and rivers, was established by undeniable evi dence. The difficulty which men found in conceiving how a precarious and acci dental supply like that of the rains, can sufficiently provide for a great and regular expenditure like that of the rivers, had given rise to those various opinions concerning the origin of fountains, which had hitherto divided the scientific world. A long re sidence on the summit of an insulated rock, in the midst of a vast ocean, visited twice every year by the vertical sun, would have afforded to an observer, less quick-sighted than Halley, an opportunity of seeing the work of evaporation carried on with such rapidity and copiousness as to be a subject of exact measurement. From this ex treme case, he could infer the medium quantity, at least by approximation ; and he proved that, in the Mediterranean, the humidity daily raised up by evaporation is three times as great as that which is discharged by all the rivers that flow into it. The origin of fountains was no longer questioned, and of the multitude of opinions on that subject, which had hitherto perplexed philosophers, all but one entirely disappeared.' Beside the voyage to St. Helena, Halley made two others ; the British government having been enlightened, and liberal enough to despise professional etiquette, where the interests of science were at stake, and to entrust to a Doctor of Laws the com mand of a ship of war, in which he traversed the Atlantic and Pacific Oceans in various directions, as far as the 58d degree of south latitude, and returned with a col lection of facts and observations for the improvement of geography, meteorology, and navigation, far beyond that which any individual traveller or voyager had hi therto brought together.
The variation of the compass was long before this time known to exist, but its laws had never yet been ascertained. These Halley now determined from his own obser vations, combined with those of former navigators, in so far as to trace, un a nautical chart, the lines of the same variation over a great part both of the Atlantic and Pacific Oceans, affording to the navigator the ready means of correcting the errors which the deviation of the needle from the true meridian was calculated to produce. In his dif ferent traverses he had four times intersected the line of no variation, which seemed to divide the earth into two parts, the variations on the east side being towards the west, and on the west side towards the east. These lines being found to change their position in the course of time, the place assigned to the magnetical poles could not be permanent. Any theory, therefore, which could afford an explanation of their changes must necessarily be complex and difficult to be established. The attempt of Halley to give such an explanation, though extremely ingenious, was liable to great objec tions, and while it has shared the fate of most of the theories which have been laid down before the phenomena had been sufficiently explored, the general facts which be established have led to most of the improvements and discoveries which have since been made respecting the polarity of the needle.
Besides the conclusion just mentioned, Dr Halley derived, from his observat ions, a very complete history of the winds which blow in the tropical regions, viz. the trade-wind, and the monsoons, together with many interesting facts, concerning the phenomena of the tides. The chart which contained an epitome of all these facts was published in 1701.
The above are only a part of the obligations which the sciences are under to the observations and reasonings of this ingenious and indefatigable inquirer. Halley was indeed one of the ableit and most accomplished men of his age. A scholar well versed in the learned languages, and a geometer profoundly skilled in the ancient ana lysis, he restored to their original elegance some of the precious fragments of that analysis, which time happily had not entirely defaced. He was well acquainted also
with the algebraical and fluxionary calculus, and was both in theory and practice a profound and laborious astronomer. Finally, he was the friend of Newton, and of ten stimulated, with good effect, the tardy purposes of that great philosopher. Few men, therefore, of any period, have more claims than Halley on the gratitude of suc ceeding ages.
The invention of the thermometer has been already noticed, and the improvements made on that instrument about this period, laid the foundation of many future dis coveries. The discovery of two fixed temperatures, each marked by the same expan sion of the mercury in the thermometer, and the same condition of the fluid in which it is immersed, was made about this time. The differences of temperature were thus subjected to exact measurement ; the phenomena of heat became, of course, known with more certainty and precision ; and that substance or virtue, to which nothing is impenetrable, and which finds its way through the rarest and the densest bodies, apparently with the same facility,—which determines so many of our sensations, and of which the distribution so materially influences all the phenomena of animal and vegetable life, came now to be known, not indeed -in its essence, but as to all the characters in which we are practically or experimentally concerned. The treatise on Fire, in Boerhaave's Chemistry, is a great advance beyond any thing on that subject hitherto known, and touches, notwithstanding many errors and imper fections, on most of the great truths, which time, experience, and ingenuity, have since brought into view.
• It was in this period also, that electricity may be said first to have taken a scientific form. The power of amber to attract small bodies, after it has been rubbed, is said to have been known to Thales, and is certainly made mention of by Theophrastus. The observations of Gilbert, a physician of Colchester, in the end of the sixteenth century, though at the distance of two thousand years, made the first addition to the transient and superficial remarks of the Greek naturalist, and afford a pretty full enumera tion of the bodies which can be rendered electrical by friction. The Academia del Cimento, Boyle, and Otto Guericke, followed in the same course ; and the latter is the first who mentions the crackling noise and faint light which elec tricity sometimes produced. These, however, were hardly perceived, and it was by Dr Wall, as described in the Philosophical Transactions, that they were first distinctly observed.' By a singularly fortunate anticipation, he remarks of the light and crackling, that they seemed in some degree to represent thunder and lightning.
After the experiments of Hauksbee in 1709, by which the knowledge of this my sterious substance was considerably advanced, Wheeler and Gray, who had discovered that one body could communicate electricity to another without rubbing, being will ing to try to what distance the electrical virtue might be thus conveyed, employed, for the purpose of forming the communication, a hempen rope, which they extended to a considerable length, supporting it from the sides, by threads which, in order to pre vent the dissipation of the electricity, they thought it proper to make as slender as pos sible. They employed silk threads with that view, and found the experiment to suc ceed. Thinking that it would succeed still better, if the supports were made still more slender, they tried very fine metallic wire, and were surprised to find, that the hempen rope, thus supported, conveyed no electricity at all. It was, therefore, as being silk, and not as being small, that the threads had served to retain the electri city. This accident led to the great distinction of substances conducting, and not conducting electricity. An extensive field of inquiry was thus opened, a fortunate ac cident having supplied an instantia crucis, and enabled these experimenters to distin guish between what was essential and what was casual in the operation they had per formed. The history of electricity, especially in its early stages, abounds with facts of this kind ; and no man, who would study the nature of inductive science, and the rules for the interpretation of nature, can employ himself better than in tracing the progress of these discoveries. He will find abundant reason to admire the ingenuity as well as the industry of the inquirers, but he will often find accident come in very opportunely to the assistance of both. The experiments of Wheeler and Gray are described in the Transactions for 1729.