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tuation in the density of the air to be effected, and in what manner the preceding general facts relative to the variation of the barometer may be accounted for, is what we shall now attempt to explain.” This is referred to the varying amount

of vapour.

In section 5, “observations on the height of clouds,” there is given the summary of 5381 observations, made by Mr. Crosthwaite, an evidence of the intellectual diligence to be found at the lakes even before they became the haunt of poets.

At p. 127 he gives a table of the temperature of water made to boil at different atmospheric pressures, bearing on the fact, which he is there explaining, that “ aqueous vapour always exists as a fluid sui generis diffused amongst the rest of the aerial fluids;" and at p. 129, “that it may be determined a priori what weight of vapour a given bulk of dry air will admit of, for any temperature, provided the spec. grav. of the vapour be given.” These conclusions appear more fully in a note* to a paper read in 1797, after having made confirmatory experiments. This must be taken as an elucidation of a subject which afforded much discussion at that period. We know that Saussure and others knew well that moisture existed in the air at very low temperatures, and there was a variety of opinions as to the state in which it existed. Many writers of the period believed, that because warm air was sensibly drier, it contained less moisture than cold air; all these points Dalton has elucidated and spoken on with decision.

In the third and sixth essays, the precise point where the burden of his argument is contained is difficult to find, the reasoning being a constant process to prove what it is supposed we knew beforehand to be the result. In the appendix, he says more pithily, p. 188, “ I am confirmed in the opinion that the vapour of water, and probably of most other liquids, exists at all times in the atmosphere, and is capable of bearing any known degree of cold without a total condensation, and that the vapour so existing is one and the same thing with steam, a vapour of the temperature of 212° or upwards. The idea, therefore, that vapour cannot exist in the open atmosphere under the temperature of 212°, unless chemically combined therewith, I consider as erroneous; it has taken its rise from a supposition, that air pressing upon vapour condenses the vapour equally with vapour pressing upon vapour, a supposition we have no right to assume, and which I apprehend will plainly appear to be contradictory to reason and unwarranted by facts; for, when a particle of vapour exists between two particles of air, let their equal and opposite pressures upon

* Memoirs of the Manchester Philosophical Society. Vol. V., p. 351.

it be what they may, they cannot bring it nearer to another particle of vapour, without which no condensation of vapour can take place, all other circumstances being the same; and it has never been proved that the vapour in a receiver from which the air has been exhausted, is precipitated upon the admission of perfectly dry air. Hence, then, we ought to conclude, till the contrary can be proved, that the condensation of vapour exposed to the common air does not in any manner depend upon the pressure of the air.

At p. 135, after contending for the theory that the vapour of water is mixed with the air and not combined, he explains how it is precipitated by cold and taken up by heat, and how it is that clouds consisting of light drops do not fall so readily as clouds with heavy drops, as the resistance of the drops is as the square of the diameter, in which his mathematical knowledge helps his meteorology. This was suggested by Mr. Gough.

There is in these Essays, and everywhere in Dalton's writings, a great rapidity of reasoning, a direct

passage

from premise to conclusion without fear, as if more than usually persuaded that true reason could not misguide him, so that he is utterly regardless of consequences. .

At p. 168 we find a fair example of his mode of reasoning, and one also of his daring theories.

“ The light of the aurora has been accounted for on three or more different suppositions:-1. It has been supposed to be a flame arising from a chymical effervescence of combustible exhalations from the earth. 2. It has been supposed to be inflammable air, fired by electricity. 3. It has been supposed electric light itself.”

“ The first of these suppositions I pass by as utterly inadequate to account for the phenomena. The second is pressed with a great difficulty how to account for the existence of strata of inflammable air in the atmosphere, since it appears that the different elastic fluids intimately mix with each other; and even admitting the existence of these strata, it seems unnecessary to introduce them in the case, because we know that discharges of the electric fluid in the atmosphere do exhibit light, from the phenomenon of lightning. From these and other reasons, some of which shall be mentioned hereafter, I consider it almost beyond doubt that the light of the aurora borealis, as well as that of falling stars and the larger meteors, is electric light solely, and that there is nothing of combustion in any of these phenomena. Air, and all elastic fluids, are reckoned amongst the non-conductors of electricity. There seems, however, a difference amongst them in this respect : dry air is known to conduct worse than moist air, or air saturated with vapour. Thunder usually takes place in summer, and at such times as the air is highly charged with vapour; when it happens in winter, the barometer is low, and consequently, according to our theory of the variation of the barometer, there is then much vapourized air; from all which it seems probable, that air highly vapourized becomes an imperfect conductor, and, of course, a discharge made along a stratum of it will exhibit light, which I suppose to be the general cause of thunder and lightning."

“Now, from the conclusions in the preceding sections, we are under the necessity of considering the beams of the aurora borealis of a ferruginous nature, because nothing else is

"*

known to be magnetic, and consequently, that there exists in the higher regions of the atmosphere an elastic fluid partaking of the properties of iron, or rather of magnetic steel, and that this fluid, doubtless from its magnetic property, assumes the form of cylindric beams. It should seem, too, that the rainbow-like arches are a sort of rings of the same fluid, which encompass the earth’s northern magnetic pole, like as the parallels of latitude do the other poles; and that the beams are arrayed in equidistant rows round the same pole. * Things being thus stated, I moreover suppose that this elastic fluid of magnetic matter is, like vapourized air, an imperfect conductor of electricity; and that when the equilibrium of electricity in the higher regions of the atmosphere is disturbed, I conceive that it takes these beams and rings as conductors, and runs along from one quarter of the heavens to another, exhibiting all the phenomena of the aurora borealis.'

In the edition of 1834 he still adheres to the same theory; some will look on it as absurd; it is certainly the result of great daring, or in other words, it may be viewed as the reasoning of a man who has exhausted all his knowledge in finding a cause, feels certain that there is one, and decides upon that which is most conformable to his knowledge, without waiting for a wider view, or for a time when something perfectly new might entirely change the scene.

This essay on the aurora he considered as of great importance. He begins with these words ; “ As this essay contains an original discovery which seems to open a new field of inquiry in philosophy, or rather, perhaps, to extend the bounds of one that has been as yet but just opened; it may not, perhaps, be unacceptable to many readers to state briefly the train of circumstances which led the author to the important conclusions contained in the following pages.” And yet we take up

treatises on the aurora, and do not even find Dalton's name

* The pages refer to the new edition.

mentioned. Wargentin, Halley, and Celsius had all observed the action of the aurora on the magnetic needle. Dr. Halley had supposed it to be caused by magnetism. Dalton went more fully into the subject than his predecessors, without, however, taking all difficulty from it. In this treatise we see an instance of the pertinacity with which he held ideas which he had formed. But we find him altering his opinion on the height of the aurora ; his observations led him to believe the height to be about 150 miles; afterwards he considered it to be about 100. Numerous as have been the attempts to ascertain the height, the differences ranging from feet to thousands of miles, Dalton still, in 1834, severely criticised all the observations which differed greatly from his early results. To this treatise on meteorology he added little, although a new edition appeared after forty years.

He then says, that it is printed verbatim (adding only a small appendix), “as I apprehend it contains the germ of most of the ideas which I have since expanded more at large in different essays, and which have been considered as discoveries of some importance.” But he says also, that “the subject here treated of appeared to the author to be very imperfectly appreciated, or little understood, by some of the modern writers on meteorology,” and it is probably true that the facts and theories he advanced had, in some or many instances, been worked out by others with little aid from his book, because, although occasionally quoted, it was really very little known. This arose from a peculiarity in his mode of publishing it. It was like all his books printed for himself, and was never allowed to make its due way in an independent manner among the booksellers, nor had the essays the advantage of being read to a society, or given out by any journal.

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