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to the pole, so then the land being separated from the water, their ratio could immediately be found for each of the thirty-six parts, without any reduction for the different magnitudes of the several zones.
*...* The different portions of ink on the different parts of the paper may be thought to affect their weights. These are generally in larger quantities on the land than on the sea, but not always: there is uniformly a kind of shading, which extends to some distance from the several coasts, and when the interior of a country is little known, it is comparatively blank. There is reason also to believe, that when the ink is thoroughly dried, it adds very little to the weight. The difficulty of reducing the paper, at different times, with any certainty, to the same degree of dryness, prevents a direct trial of the alteration, which might be produced in printing, but workmen consider it to be very small. Two pieces as nearly as possible of the same size, having been cut out of the same gore, the one which was perfectly white weighed 8.2 grains, while the other which was covered with names weighed only 8.1. The difference must have been occasioned by some accidental circumstances; but the experiment, as far as it goes, will tend to shew
that no sensible error was likely to be occasioned by the attempt not having been made to introduce an allowance for this particular.
XII. On the Results of Observations made with a new Anemometer. By the Rev. W. WHEwell, M.A. Fellow and Tutor of Trinity College.
[Read May 1, 1837.]
IN the present paper I shall give an account of the mode which has been employed in using an Anemometer which I have invented and caused to be erected. By this account I hope to shew that such instruments may be made to give consistent and comparable results, and may lead to a more complete knowledge of the course of the winds than we yet possess.
It is not necessary to describe in detail the construction of the instrument here spoken of ; its general principles may be easily explained. A fly (resembling the fly of a revolving ventilator or the sails of a windmill) is fixed to the small end of the vane of a weathercock, so as always to be turned with its circular disk to the wind; and it consequently revolves by the action of the wind with a rapidity increasing as the strength of the wind increases. The revolutions of the axis of this fly are converted, by a train of toothed wheels and screws, into a vertical motion, by which a pencil is carried downwards touching the surface of a vertical cylinder, the cylinder having the axis of the weathercock for its axis. As the vertical rod on which the pencil slides is attached to the vane of the weathercock, the point of the compass from which the wind blows is recorded by the side of the cylinder on which the mark is made, while the quantity of the wind is represented by the extent of the descent of the pencil.
In the instruments which I have had constructed, the pencil
descended one-twentieth of an inch for ten thousand revolutions of
the fly, and the cylinder on which the marks were made was sixteen or eighteen inches high. By this means the surface of the cylinder would contain the trace of the wind of one or two days when there was much wind, and of several days when the winds were lighter.
These instruments were made at first by Mr Newman of Regent Street, and since, by Mr Simms of Fleet Street. They have been erected and observed by Professor Forbes and Mr Ranken at Edinburgh, by Mr Southwood at Plymouth, and also at other places; but the observations of which I am able to give the most complete account are those made under the direction of Professor Challis at the Cambridge Observatory, and under my own direction at the house of this Society. The Anemometer at the Observatory was placed over the portico, in which situation it was free on the other sides, but considerably sheltered by the dome of the equatoreal, on the north side. The Anemometer placed on the top of the Society's house is favourably circumstanced, being higher than any neighbouring building which is near enough to intercept the wind. The observations were made with care and regularity by Mr Crouch the Society's housekeeper.
Various improvements in the instrument were suggested by using it; and as it had not been foreseen what strength of workmanship would be requisite to resist the weather, all the instruments were, at one time or other, disabled, so as to interrupt the observations.
One of the difficulties which most interfered with the precision of the observations, was that which arose from the wavering of the wind. The weathercock is in almost constant motion, swinging to and fro through an arc often not less than a quadrant, and the consequence is, that the pencil describes upon the cylinder, not a single line, but a broad path of irregular form, made up of the transverse lines which the oscillation of the vane occasions. It might at first be supposed that this oscillation arose from the momentum of the vane, and might be remedied by some contrivance which should cause the change of direction of the wind to come into effect more slowly; such for example as the tail of a windmill. But the cause of this oscillation is in reality almost entirely the constant shifting of the wind, as may be seen by examining the motions of the vane, for it often swings into a new position or stands still awhile before it swings back again.
In consequence of this circumstance the direction of the wind cannot be ascertained with very great precision. By carefully taking the middle of the broad path, the direction may be read off to a single point of the compass; but in the observations at the Society, we contented ourselves, for the most part, with reading off to the double points (one sixteenth of the circumference).
The vertical scale is divided into tenths of inches, and read off by means of two indexes, which slide on the same vertical rod which guides the pencil. The detail of the process of observation will be best understood by attending to the following directions. It may be observed that the cylinder is of brass japanned white, on which common pencil marks can be rubbed out in the way described below.
Directions for observing with Whewell's Anemometer.
1. PLACE the instrument in a situation well exposed on all sides, and fix it so that when the wind is South, the pencil is on the line S on the barrel.
This may be done by clamping the weathercock part of the instrument with the pencil on the line S, then turning the box till the vane points due north, and then fixing it in that position.
2. Read off the instrument every day at a constant hour.
The pencil in descending will make a broad path, in consequence of the wavering of the wind. The darkest part of this path must be taken; and from this, the direction of the wind determined, by reference to the points of the compass marked at the bottom of the cylinder: and, as the wind changes, the directions of the successive strips of wind must be noted.
3. To read off the amount of the wind in each of these successive strips;–slide the lower index so that the point is upon the top of the first strip; then slide the upper index to touch the lower; then slide the lower index to the bottom of the first strip, or the top of the second strip of wind; then read off on the graduated rod, the interval (in tenths of inches,) through which the lower index has moved; then again slide down the upper index to touch the lower, slide down the lower index to the bottom of the second strip, and read off the interval; —and so on. Write down these intervals under the corresponding directions of the strips of wind, observed as above.
4. When the pencil has reached the bottom of the barrel, the instrument must be wound up, by unscrewing the clamping screw of the nut, removing it to the top of the barrel, and clamping it.
At the same time the barrel must be cleaned, by rubbing it with a soaped cloth enclosing a smooth wooden rubber.
5. The following is suggested as a simple way of marking the points of the compass; for example, from the North to the East the points may be
N. Ne. NNE, NEm. NE. NEe. ENE. Em. E.
and so on for the other quadrants.
The only ambiguities which can arise by this method, are Ne, Nw, Se, Sw; which must be distinguished from NE, NW, SE, SW.
Ne is N by E.; and so of the rest.
I shall now give the Register of the wind as observed for the months of January, February, March and April of the present year at the Society's house. I shall add also the observations made at the Observatory for a portion of the month of February. The readings are in tenths of inches on the scale.
* The asterisk indicates the times when the instrument was wound up.