=130. Anemometric Observations.=--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 7·8 miles per hour; North-westerly, on 112 days, at 15·4 miles per hour; South-easterly, on 115 days, at 11·0 miles per hour; South-westerly, on 77 days, at 13·8 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:--11 miles per hour, the minimum force, occurring at 1-1/2 a.m.; until 6 a.m. it remains much the same, being then 11·3 miles per hour; at 10 a.m. it is 13·4 miles per hour; at 1-1/2 p.m. the wind is at its maximum strength, being 14·8 miles per hour; at 5 p.m. it is again 13·4 miles per hour, and at 9 p.m. 11·3 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."[14]
