The fall of the barometer is thus connected, not with the exist➡ ence of clouds or the fall of rain, but with their formation. And we are enabled by this view of the subject to account at once for the undoubted connexion between the fall of the barometer and the fall of rain in general, and for a fact equally well established and frequently observed in the summer of 1823, that the barometer does frequently fall during fine weather and rise during rain, the first phenomenon depending on the precipitation of water into the form of clouds, and the second on the circumstance of that precipitation having ceased; in this manner, when fair weather and rain succeed each other rapidly, the barometer may fall whilst clouds are forming, and rise when this process has ceased, though the rain fall.

The connexion is not less obvious between the formation of clouds and rain and the fall of the barometer, and the occurrence of gales and of changes in the wind.

It is probable, indeed, that these phenomena act and react in the atmosphere, and assume in turns the character of cause and of effect. The clash of winds is the cause of the first precipitation of moisture from the atmosphere, and the precipitation of moisture becomes, in its turn, a fresh cause of wind.

That state of the barometer which is most generally observed, and which usually continues longest without change, may be considered to be its most natural condition. This state approaches to its highest elevation, the barometer being far more frequently and for longer periods high than low, this state prevailing when no changes are going on in the atmosphere, and when the weather is of course calm and serene.

The causes of the movements of the barometer are to be sought, therefore, not only in such phenomena as may be supposed to add to the weight of the atmosphere, but in such as may temporarily diminish its elasticity, and concur with the disturbance of its transparency and tranquillity, or with the formation of clouds. or rain, and of winds.

To shew how much the low state of the barometer is connected with the formation of clouds or rain, I quote a remark from the VOL. XX.

C

Meteorological Observations of Mr. Dalton*. "Very dark and dense clouds," he observes, "pass over without rain when the barometer is high; whereas when the barometer is low, it sometimes rains without any appearance of clouds."

The views just given are greatly supported by the observation that the variations in the barometer are confined to the lower region of the atmosphere, and diminish rapidly as we approach the upper limit of the region of the clouds.

I now proceed to make a few observations on the influence of the condensation of atmospheric moisture as a cause of wind. The causes of the north-east wind have been already stated as given by various authors t. But I am not aware that any satisfactory cause has been assigned for the prevalence of winds from the south or from the west. It is plain, however, from the preceding remarks, that such winds must be produced by great and rapid formations of clouds and rain, the air being drawn from all the adjacent regions to that in which this condensation of atmospheric vapour takes place, and thus under particular circumstances a south-west wind or gale will be produced.

Mr. Playfair has made some observations which appear to confirm this view of the subject. He observes, "the sudden sinking of the barometer almost always indicates a gale of wind, though a gale that is sometimes at a considerable distance." And further, "there is in our climate hardly any instance of rain without a change of wind, and very rarely a change of wind without rain in a greater or less quantity §." Mr. Dalton ob serves, "the barometer generally rises with a wind between the north and east, and is very low in winter when a strong and warm S. or S.W. wind blows]." He adds, the lowest extreme of the barometer for five years, was accompanied with a strong S. or S.W. wind and and heavy rain, and occurred just after the

* Page 196.

+ See LA PLACE, Systeme du Monde, t. 2. p. 174; PLAYFAIR'S Outlines of Natural Philosophy, v. I. p. 293.

highest extreme which took place during a long and uninterrupted frost.

It is impossible to show more clearly than by these facts the connexion between rain, wind from the S. or S.W., and a low state of the barometer; I merely venture to suggest one mode of accounting for this association of phenomena:-during a period of atmospheric tranquillity, evaporation goes on from the surface of the earth, and expands and forms a part of the atmosphere, to the absolute weight of which it must necessarily add, and the barometer accordingly takes its highest station; at length, from the collision of different winds induced by irregularities on the earth's surface, this vapour is precipitated, on the principle of the beautiful theory of Dr. Hutton, viz. that, as the power of the air to imbibe moisture increases in a higher ratio than the temperature, two portions of air of different temperatures, and saturated or nearly so with moisture, cannot be mingled together without its precipitation; this precipitation is, in its turn, a fresh cause of wind, of the fresh collision of different airs, and of the renewed formation of clouds and rain,-and the contraction which thus takes place in the atmosphere diminishes its elasticity, lowers the barometer, and again becomes a cause of wind.

According to this view of the subject, the barometer may be regarded as an instrument measuring the actual weight of a column of the atmosphere in serene weather, but in serene weather only. On other occasions it measures the changes which take place in the elasticity of the lower region of the atmosphere, the pressure of the superincumbent part of which is cut off for a time until the equilibrium be restored. From this view, too, we observe that it is only in serene weather, and with a transparent and tranquil atmosphere, that the barometer should be employed to measure. the heights of mountains. That the variations of the barometer do, in fact, depend upon changes which occur in the lower regions of the atmosphere, is proved by the observations of M. Laval, recorded in the Memoirs of the Royal Academy at Paris for 1709. During ten days the barometer was observed to vary two lines and *Trans. of the R.S. of Edin. v. I. p. 41.

three-fourths at Marseilles, whilst it varied only one line and three-fourths at the top of St. Pilon, which is 960 yards above the level of the sea.

I must observe, in this place, that I am fully aware that it may be objected to the preceding observations, that the quantity of water in the atmosphere is not sufficient to account for the movements of the barometer, and for winds, from the changes in its state from transparent vapour to that of clouds or rain. To this objection I would reply, first, that it is the object of this paper to propose this, only as one cause of the movements of the barometer, and of wind, whilst it is admitted that there may be, and undoubtedly are, many others; and secondly, that the circumstances attending these changes in the state of the atmospheric moisture, and all the effects of these changes, are probably not yet fully understood and appreciated. Two points, at least, have been established:-the first, that there is an unequivocal connexion between the fall of the barometer, the production of wind, and the formation of clouds or rain; the second, that the transition from the elastic to the fluid state of the atmospheric moisture does, in part, at least, account for these phenomena-must be attended by a certain diminution of bulk and elasticity-and consequently by a movement in the adjacent regions of the atmosphere, and by a fall in the barometer. The principles here pointed out may obtain in the minor but more usual changes in the atmosphere; whilst the more extraordinary phenomena of gales and tempests may, and doubtless do, involve many other. Our atmosphere is so complicated a structure, that no simple theory will ever explain the whole of its phenomena. It has just been suggested, that as all the causes of the movements of the barometer are not hitherto ascertained, so all the circumstances attending the precipitation of the atmospheric vapour, assumed as one of these causes, are probably not yet fully appreciated:-one effect of the condensation of the atmospheric moisture, is an actual diminution of the weight of the atmosphere, and this would operate on the barometer, were the elasticity of the air unaffected; but as the vapour really constitutes a part of the atmosphere, and adds to its volume,

its subtraction or precipitation must also diminish the elasticity of the remaining portion of the air, and, in this manner especially, lower the barometer; and this effect will be the greater in consequence of the atmospheric vapour being accumulated, and its precipitation chiefly effected, in the lower region of the atmosphere, in which the movements of the barometer are consequently observed to be proportionately greater than in more elevated situations.

In conclusion, I would observe, that as I should probably never have taken the pains with this subject which I have done, but with the view of laying it before our little Society, so my wishes will be fully attained if this paper either excite interest or afford instruction even to the youngest of our members, and thus tend to further the object of our meeting together. My great aim, indeed, has been to allure and aid the beginner in philosophical studies; and such, I imagine, to be the specific design of our institution. With this view conjectures and observations which might otherwise never have deserved attention become extremely valuable. If any further apology were required for the preceding pages, I would say that the current of my studies has of late years flowed in a totally different channel from that of philosophy; but that it was only by recurring to the studies of former days that I could select any subject which would be appropriate to the present occasion.

ART. IV. Description of an Instrument for destroying Stone in the Bladder. By J. R. Griffiths, Esq.

[Communicated by the Author.]

I HAD long entertained the idea that an instrument might be constructed, by means of which a calculus in the bladder could be broken down into fragments small enough to pass by the urethra, and about three years since, with the assistance of an instrumentmaker, I attempted to put my plan in execution; but not at that time being able to construct a drill that would work in a curve, I gave up the attempt. Some time after this I heard that an in