for a long time with a known velocity; and thus the actual value of the indications of the instrument might be determined. And a small Comparative Anemometer, more easily transferable from place to place than the working instrument, might be employed to obtain the value of the scale of the instrument in this manner. This process might be performed at any time, and might therefore serve to compare the Anemometer with itself at different times. The relation between the velocity of rotation produced, in a wheel with oblique blades, and the velocity of a fluid which flows past it, is so steady, that the rotation of such a machine has already been used in measuring the velocity of the motion, in Masson's Patent Log, and Saxton's Current-meter. The same process which would compare an instrument with itself, would also compare it with another instrument of the same kind. But, as we have not yet any such means of judging what is the comparative going of different Anemometers, we may say a word or two of the comparison of them by means of their results. The station at the Society’s house and the Observatory are so near each other, that there can hardly be any great difference in the quantity of wind which blows at the two places. Assuming these quantities to be equal, it appears that the index at the Observatory moves nearly twice as fast as that at the Society's house. The equality of the wind at Cambridge and Edinburgh cannot so safely be assumed; but if we proceed upon the equality for March, as our only accessible basis, we shall find that the index of the Society's Anemometer moves more than twice as fast as that of the Edinburgh one. But I shall return to this comparison in another form. In order to exhibit the general course of the winds at each place I have adopted the following graphical method. Assuming, on a sheet of paper, the proper relative directions of the points of the compass, I begin from a point and draw a line in the direction of the first recorded wind, and of such a length as to represent this wind in magnitude on a scale of equal parts. From the extremity of this line, I draw another line representing in direction and magnitude in like manner the second recorded wind; and from the extremity of this line, a third; and so on. In this manner I obtain a continuous line, which represents the course of the wind as long as it is continued. Such lines were drawn for February and March 1837; those for March are exhibited in Plate VII, which represents the curves for March, drawn for the stations, at the Society, at the Observatory and at Edinburgh. In all the cases the observations experienced interruptions, which make it difficult to draw any general conclusions from them. But we may remark that in February the wind blew almost constantly from a more westerly point at the Observatory than at the Society. It is not difficult to conceive this result to be occasioned by the peculiar circumstances of the Anemometer at the Observatory: but it is also possible that it may be a general fact that such differences obtain at neighbouring places, in consequence of the direction of valleys, &c. Further observation alone can clear up this and similar points. It has been deemed an important point by Meteorologists to obtain the mean direction of the wind at a given place for a given time, for instance, a year. Kämtz in his Meteorologie, Vol. II. p. 218, has collected several results of this kind. But in these researches the force of the wind has entirely been left out of the account, and each wind was reckoned according to the number of days which it blew. It is clear that such a procedure is entirely fallacious; for the high wind of one day may be greater, with regard to every possible effect, than the gentle breezes of a week. The mean annual direction is probably constant at each place within certain limits: and the mean directions at different places are perhaps connected by certain general relations, depending upon the quantity of fluid transferred, and upon other atmospherical conditions, which may hereafter be found to be important elements of meteorological speculation. But it is not at all likely that this will hold if the mean direction be taken without reference to the strength of the wind; and no mode of measurement can be good for this purpose which does not give the whole quantity of the aerial current, depending both upon velocity and upon time. The Anemometer here referred to is, as I have said, the only one, so far as I know, which has been constructed with the view of thus registering both the quantity and direction of the wind; and however imperfect its construction may yet be, it must give some approximation to the quantity which it is our object to measure, and must thus afford the means of a better estimate of the mean direction for a year (or for any other time) than has hitherto been possible. It is obvious that the mode of obtaining the mean direction of the wind for any time would be to resolve each partial wind into its component parts E. and W. and N. and S. The sum of all the west components, subtracting the east elements, give the effective west wind; and the sum of all the south elements, subtracting the north elements, give the effective south wind. The magnitude and proportion of these two effective winds compounded will give the magnitude and direction of the effective wind, between west and south, which belongs to the whole time. And the same may be said of any other cardinal points. The reduction of any wind to these cardinal directions is of course to be performed by considering it as the hypotenuse of a right-angled triangle, and here the multipliers by which the reduction is to be performed are easily found. We may take fractions which are sufficiently accurate, and yet simple enough to be easily used. Thus the intercardinal winds, NE, SE, SW, NW, are reduced to the cardinal direc tions N, S, E, W, by multiplying by i. The subordinate winds NNE, ENE, ESE, SSE, SSW, WSW, WNW, NNW, are reduced to the cardinal directions by the multipliers 12 and #: thus a wind 13 10 NNE 65, is equivalent to N 60 and E 26. The oblique winds N by E, &c. might be reduced in the same manner by the multipliers i. and T 100' But these last I have not used. I annex calculations made in this manner for the months of January, February and March, 1837; in which I have resolved the days into periods during which a certain group of neighbouring winds were prevalent. Thus from January 1 to 14, the prevailing winds were SSW, SW and WSW ; from January 26 to 30 they were NE, ENE and E. It appears in this manner that on the scale of the Society's Anemometer the total wind for these three months was W. 321, S. 558. It may be observed that the graphical method offers at once the mean direction of the wind, and the resolution of the winds into their cardinal parts. A straight line drawn from the beginning of the curve to its end is the direction and magnitude of the resulting wind; and if lines E and W and N and S be drawn from its extremities, they will give its component parts. If we were to draw the graphical curve of the wind as registered by the Anemometer for a year, and were to do this for several years at the same place, beginning from the same point, we should probably have a set of curves in which a considerable resemblance might be traced; for there is a kind of annual cycle of the winds at each place. The mean of such curves for a sufficient time would be the mean annual type of the winds for that place. The mean annual type of the winds at different places would vary very much, as is clear from the materials which Kämtz has collected. Thus he finds (Vol. II. p. 223) that the mean direction at Paris is S 68° W, at Montmorenci N 48° W, at Utrecht N 85° W, and Amsterdam S 61° W. And though, as we have seen, his method of obtaining these results is very insufficient, it still serves to show that they would probably be various by any method. If Anemometers of the kind now described were fixed in various parts of the world, and the annual type, and other circumstances of the wind thus obtained, it cannot be doubted but that this portion of meteorology, and probably other portions which are connected with this, would soon make great progress. |