The fans keep their axis at right angles to the wind; and with any change of direction they move, carrying with them an outer brass tube, which rests upon friction balls on the top of the pedestal, and is attached to a tubular shaft passing through the interior of the pedestal, and terminating with a mitre wheel. The mitre wheel, working with other cogged wheels, communicates the motion of the direction shaft to a cylinder carrying a pencil, to record the direction.
The shaft carrying the cups is supported upon friction balls, placed in a groove formed on the top of the direction shaft, and passing through the interior of that shaft, comes out below the mitre wheel, where it is terminated in an endless screw, or worm.
Upon the wind moving the cups, motion is given to the innermost shaft, thence to the worm-wheel, whence motion is given to a pencil which registers the velocity.
De la Rue’s metallic paper is used in registration, it having the property of receiving a trace from a brass pencil. The pencils can, therefore, be made in the most convenient form. Mr. Beckley forms each pencil of a strip of brass wrapped round a cylinder, making a very thin threaded screw, so that the contact of the pencil cylinder and the clock cylinder is a mere point of the metallic thread. The pencil cylinders are placed side by side upon the cylinder turned by the clock, and require no spring or other appliance to keep them to their work, but always make contact with the registration paper by their own gravity. They therefore require no attention, and being as long as the trace which they make, they will last a long time.
The velocity pencil has only one turn on the cylinder, and its pitch is equal to a scale of fifty miles upon the paper. The direction pencil has likewise one turn on its cylinder, its pitch being equal to a scale of the cardinal points of the compass upon the paper.
The clock gives a uniform motion of half an inch per hour to the cylinder upon which the paper is fastened.
The registering mechanism of the instrument is very compact, requiring only a space of about 18 inches by 8 inches.
In the Report of the British Association for 1858, Mr. Beckley has given a detailed description of his anemometer, with drawings of all the parts.
129. Self-Registering Lind’s Anemometer.â€”A Lind’s wind-gauge, designed to register the maximum pressure, was exhibited at the International Exhibition 1862, by Mr. E. G. Wood. The bend of the syphon is contracted to obtain steadiness. On the leeward limb a hole is drilled corresponding in size with the contracted portion of the tube. The edge of the hole corresponds with the zero of the scale. On the pressure of the wind increasing, as much of the water as would have risen above the aperture flows away, and therefore the quantity left indicates the greatest pressure of the wind since the last setting of the instrument, which is done by filling it with water up to the zero point.
130. Anemometric Observations.â€”To illustrate the value of anemometric observations, we quote from a paper by Mr. Hartnup, on the results obtained from Osler’s Anemometer, at the Liverpool Observatory. The six years’ observations, ending 1857, gave for the yearly average of the winds: North-easterly, on 60 days, at 7Â·8 miles per hour; North-westerly, on 112 days, at 15Â·4 miles per hour; South-easterly, on 115 days, at 11Â·0 miles per hour; South-westerly, on 77 days, at 13Â·8 miles per hour; and one day calm. From the same observations, the average variation in the strength of the wind during the 24 hours is:â€”11 miles per hour, the minimum force, occurring at 1Â½ a.m.; until 6 a.m. it remains much the same, being then 11Â·3 miles per hour; at 10 a.m. it is 13Â·4 miles per hour; at 1Â½ p.m. the wind is at its maximum strength, being 14Â·8 miles per hour; at 5 p.m. it is again 13Â·4 miles per hour, and at 9 p.m. 11Â·3 miles per hour. Hence it appears that the wind falls to its minimum force much more gradually than it rises to its maximum; that the decrease and increase are equal and contrary, so that the curve is symmetrical; and that generally the force of wind is less at night than during the day.
“There is evidence,” says Admiral FitzRoy, “in Mr. Hartnup’s very valuable anemometrical results, which seems to prove that to his observatory, in a valley, with buildings and hills to the north-eastward, the real polar current does not blow from N.E., but nearer S.E. By his reliable digest of winds experienced there, it appears that those most prevalent were from W.N.W. and S.S.E. But in England, generally, the prevailing winds are believed to be westerly, inclining to south-westerly, and north-easterly; while of all winds, the south-easterly is about the rarest.
“At Lord Wrottesley’s observatory, in Staffordshire, about 530 feet above the sea, there appears to be considerably less strength of wind at any given time, when a gale is blowing generally, than occurs simultaneously at places along the sea-coast: whence the inference is, that undulations of the land’s surface and hills, diminish the strength of wind materially by frictional resistance.
“All the synoptic charts hitherto advanced at the Board of Trade exhibit a marked diminution of force inland compared with that on the sea-coast. Indeed, the coast itself offers similar evidence, in its stunted, sloping trees, and comparative barrenness.”