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This is an instrument for measuring the speed of the wind, and a simple type is shown in Fig. 36. The crosspieces, A and B, are formed from 1/8 in. x 1 in. strip-wood. Each piece is 3 ft. in length. They are joined together at right angles at the centre. Use glue and small fine nails. Check with a set square to ensure that the angles are true. Drill a small hole exactly at the centre. Now take two large glass or wooden beads about 18 mm. in diameter and place them on the crossbars, one at the top and one underneath, exactly over the centre hole in the bars. Glue the beads in position and when the glue has set, pack round the beads with plastic wood, as at C. Allow sufficient time for the plastic wood to harden. Note that the hole in the crossbars is the same diameter as the holes in the beads. Following this, fix four small cups, D, to the ends of the crossbars. Use table-tennis balls, cut in halves with a sharp knife. Screw them in position, using 1/4 in. screws. The cups are fixed underneath the crossbars, and small 1/8 in. thick blocks of stripwood are glued above them.

The next stage is to make the spindle, E. The base is two cotton reels, glued together, into the holes of which is inserted a piece of dowelling, which is glued in place and must be a tight fit. The dowelling is cut off level with the top and bottom of the reels. A strong knitting needle is required, of such a diameter that the crossbars revolve freely but not too loosely on it. Drill a hole in the dowelling in the reel, and drive the needle securely into it. Slip a few 18 mm. beads on to the needle with a small washer on top of them. The third stage is to make the stand, F. This is a piece of 1/4 in. plywood, 3 ft. square, and strengthened underneath with battens, which is screwed to an upright bar of convenient height. The bar may be pointed at the end, so that it can be driven into the ground; or it may be attached to a base-piece, G. Drill a hole at one corner of the stand, F, and insert a 1 1/4 in. length of dowelling, H, and paint it red. This is the marker. One of the cups is also painted red. Place the crossbars on the spindle, and the anemometer is ready for use.



If it is to be left permanently out of doors, then it may be given a coat of size, and when this is dry, enamelled in a suitable colour which offers a strong contrast to the red cup and marker. In order to calculate the speed of the wind, one must first count the number of times which the red cup passes the red marker in the space of one minute. This having been done, the speed in miles per hour can be estimated by an arithmetical calculation. It will be realized that the red cup, as it is blown by the wind, describes the circumference of a circle, the diameter of which is 3 ft. which the reader will realize is the length of the arms. The measurement of this circumference is found by multiplying the diameter by (22/7). When this result is multiplied by the number of revolutions in one minute, the reader will then have found the speed of the wind in feet per minute. From this he can then calculate the speed in miles per hour.

For example:
Number of revolutions in 1 minute = 80
Therefore speed in feet per minute = (3 x 22 x 80) / (1 x 7 x 1)
Therefore speed in miles per hour = (3 x 22 x 80 x 60) / (1 x 7 x 1 x 5,280) = 60/7 = 8 4/7

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