صور الصفحة
PDF
النشر الإلكتروني
[blocks in formation]
[merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][ocr errors][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][ocr errors][merged small][merged small][merged small][merged small][merged small][ocr errors][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small]

Several questions obviously offer themselves respecting the numbers thus registered. What is their real import? How far is each instrument consistent with itself? In what manner are two such instruments comparable?

I cannot at present answer these questions completely, but I will make a few observations on each.

As to the import of the indications of this Anemometer, it is evident, that their magnitude will increase with the force of the wind, and with the time to which each number refers. If we could assume that the velocity of revolution of the fly of the Anemometer is always proportional to the velocity of the wind, the space which the index passes over on the scale would be proportional to the velocity of the wind, and the time during which it has blown, jointly; that is, to the total quantity of the aerial current which has passed the point: and however the velocity of the wind might vary, the instrument would give the sum of all the elements of the current, or in other words, would integrate the velocity multiplied into the differential of the time. Hence I term the amount registered by this instrument the Integral Effect of the wind. That the velocity of the fly is thus proportional to that of the wind, I have not yet ascertained; and till that is done, I can only urge, that it appears highly probable that the instrument will afford at least some approximation to such a result; which no instrument hitherto erected, so far as I am aware, has ever pretended to do.

The question whether the instrument be consistent with itself, is one of considerable difficulty; for it does not readily appear how we are to obtain any permanent standard by which we may test its indications at different times, and thus ascertain whether its scale has varied. It is certainly very conceivable that the friction and other impediments to motion should alter considerably from month to month, so as to affect materially the rate at which the instrument would move with a given wind. We might however imagine means by which the actual velocity of the current of air which turns the instrument should be ascertained, and thus this difficulty overcome. For example, the Anemometer might be placed on some part of a large machine which moves VOL. VI. PART II.

RR

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

« السابقةمتابعة »