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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 2
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. R. R.