thermometers, making experiments on hygrometers of whip cord, sending specimens of butterflies to Mr. Crosthwaite's museum in Keswick, and afterwards engaging for a very small sum to send him dried specimens of plants.

Mr.

T. P. Heywood, of the Isle of Man, has in his possession eleven volumes of his Hortus Siccus. The first is a thick volume, containing the general title page, Hortus Siccus seu Plantaram diversarum in Agris Kendal vicinis sponte nascentium Specimina, Opere et Studio Johannis Dalton collecta, et Secundum classes et ordines disposita. 1790." The other volumes are thin. They are not preserved with the greatest care which collectors are capable of, but they are a proof of great industry and of considerable attainment, even in a branch of science which he did not profess. He supplied Mr. Crosthwaite with a barometer and thermometer, although not knowing how to make them. He nevertheless begins and learns their faults by experience. A letter given by Dr. Henry shews us how his knowledge stood at this time, and how also he was in the habit of acquiring it. Speaking of the mercury in the barometer, he says, "I intend to renew mine as soon as convenient; if thou do the same, be careful in undoing it; and attend to the cautions I give. Be sure to rub the inside of the tube well with warm dry cotton or wool, and have the mercury when poured in at least milk warm, for moisture is above all things else to be avoided, as it depresses the mercury far more than a particle of air does; mine is, as I have said, at least th of an inch too low, and yet it is clear of air, and to all appearance dry; but I doubt not but attending to these precautions, which I knew nothing of when it was filled, will raise it up to its proper height."

At this time he felt uneasy, the sphere of his simple school was too small, his impatience took the form of variety in his pursuits, and he wandered over nearly every branch of science. He seems now, although at a later time than generally happens to young men, to have been "yearning

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for that large" existence which he knew to be somehow attainable. He thought of a profession. His uncle Thomas Greenup, the barrister, thought that it was entirely out of the reach of a person in Dalton's circumstances to be a barrister or a physician, and recommended rather as less difficult, but still much above him, that of an attorney or apothecary. It was at this time that he began to make medical experiments, wishing to ascertain the loss from the human body by insensible perspiration, a sufficient proof that however much he might have advanced the knowledge of that profession, he was too much an experimenter and solitary thinker to have been pleased with the active life of some of our medical men. The discouragement received from his friends seems to have vented all exertion in the new direction he had contemplated, and he remained three years longer or until 1793 in Kendal, when Dr. Barnes asked Mr. Gough for a suitable person to teach mathematics in the New College of Manchester. This college had arisen out of the Warrington academy, where Priestley had taught, as well as Dr. Aikin, Dr. Enfield, Reinhold Forster, and Gilbert Wakefield. At that time the college was in the present "College-buildings," in Mosleystreet. He lived in the establishment, and remained tutor of mathematics and natural philosophy for six years. Dr. Barnes was the principal. This college is now transferred to Gordon-square, London. Whilst here we find from papers lent me by Mr. Woolley, that in 1794 he had twentyfour pupils for mathematics, mechanics, geometry, algebra, book-keeping, natural philosophy, and chemistry. He used Lavoisier's Elements of Chemistry and Chaptal's amongst others. In 1799 there were twenty-two students. Although Manchester is now multiplied by four, it cannot shew the same number, and I fear that the love of external things has overpowered the love of science.

As soon as he gave up the intention of studying for a profession, he seems to have decided at once on a regular course

of investigation and in the interval from that time to 1793, when he went to Manchester, produced his first work properly so called, his "Meteorological Observations and Essays;” they were not, however, published until he had taken up his position at the college.

This book contains an extensive series of observations on old and new subjects, comprising ideas sometimes new, sometimes old, and at other times modifications of the old. He enters into the discussion of the cause of the rise and fall of the barometer, which he decides to be the existence of the vapour of water in the air. Also he discusses the state of water in the atmosphere, shewing it to be an elastic vapour existing like any other gas not in chemical combination. Then he treats of evaporation from the earth's surface, clouds and rain, and allied phenomena, bringing, as Professor Sedgwick says, "the elements themselves under his own intellectual domination." The extent and variety of these inquiries prove the earnestness with which he studied in his almost solitude in Cumberland. The work seems to have been at first intended as a popular treatise on meteorology. It begins with a description of the barometer, then come the thermometer, hygrometer, and rain gauges; connected with these are tables of observations made at Kendal and Keswick. There seems to be a looseness of description in the first part of the volume, which would seem to imply that the matter was easily understood, and the readers could make out the particulars for themselves. As he proceeds, however, he seems to feel that he has a harder task to perform, and speaks rather to scientific than to popular hearers, whilst we gradually become aware that he is a close and precise reaHis style is very simple; he goes directly to his point; all inessential parts are left out. He seems to move forward with a heavy dogged tread, never turning his head aside, but as any style may become a fault if too far carried out, we find that in his there are left out many things that

soner.

are certainly needful as accessory or confirmatory, leaving what to the eyes of many is a want of finish, so that others have been needed to complete what was in reality sufficiently complete had it been laid out entire as it existed in his own mind.

As an example of his style, at p. 97, we find

"It appears from the observations (see table p. 15) that the mean state of the barometer is rather lower than higher in winter than in summer, though a stratum of air on the earth's surface always weighs more in the former season than in the latter; from which facts we must unavoidably infer that the height of the atmosphere, or at least of the gross parts of it, is less in winter than in summer, conformable to the table p. -80. There are more reasons than one to conclude that the annual variation in the height of the atmosphere, over the temperate and frigid zones, is gradual, and depends in a great measure on the mean temperature at the earth's surface below, for clouds are never observed to be above four or five miles high, on which account the clear air above can receive little or no heat, but from the subjacent regions of the atmosphere, which we know are influenced by the mean temperature at the earth's surface; also, in this respect, the change of temperature in the upper parts of the atmosphere must in some degree be conformable to that of the earth below, which we find by experience increases and decreases gradually each year, at any moderate depth, according to the temperature of the seasons.

"Now with respect to the fluctuations of the barometer, which are sometimes very great in twenty-four hours, and often from one extreme to the other in a week or ten days, it must be concluded, either that the height of the atmosphere over any country varies according to the barometer, or otherwise that the height is little affected therewith, and that the whole or greatest part of the variation is occasioned by a change in the density of the lower regions of the air. It is

very improbable that the height of the atmosphere should be subject to such fluctuations, or that it should be regulated in any other manner than by the weekly or monthly mean temperature of the lower regions; because the mean weight of the air is so nearly the same in all the seasons of the year, which could not be if the atmosphere was as high and dense above the summits of the mountains in winter, as it is in summer. However, the decision of this question need not rest on probability; there are facts which sufficiently prove, that the fluctuation of density in the lower regions has the chief effect on the barometer, and that the higher regions are not subject to proportionable mutations in density. In the Memoirs of the Royal Academy at Paris, for 1709, there is a comparison of observations upon the barometer, at different places, and amongst others, at Zurich, in Switzerland, in lat. 47° N., and at Marseilles, in France, lat. 43° 15′ N.; the former place is more than 400 yards above the level of the sea; it was found that the annual range of the barometer was the same at each place, viz., about 10 lines; whilst at Genoa, in latitude 44° 25′ N., the annual range was 12 lines, or 1 inch; and at Paris, latitude 48° 50′ N., it was about 1 inch 4 lines. In the same memoir it is related that F. Laval made observations, for ten days together, upon the top of St. Pilon, a mountain near Marseilles, which was 960 yards high, and found that when the barometer varied 2 lines at Marseilles, it varied but 1 upon St. Pilon. Now had it been a law, that the whole atmosphere rises and falls with the barometer, the fluctuations in any elevated barometer would be to those of another barometer below it, nearly as the absolute heights of the mercurial columns in each, which in these instances were far from being so. Hence then it may be inferred, that the fluctuations of the barometer are occasioned chiefly by a variation in the density of the lower regions of the air, and not by an alternate elevation and depression of the whole superincumbent atmosphere. How we conceive this fluc