The most prejudicial resistance in any part of the anemometer, when in good condition, is that occurring in the top bearing of the spindle. This bearing should first be made very smooth and polished and afterward kept clean and freely lubricated, if good results are to be expected. When well made and properly cared for the anemometer will show only a very little wear after years of exposure, even with comparatively high winds. The small bright spot upon the lower end of the steel spindle, where it wears against the steel plate, should never be larger than a pin head. If this portion becomes dry of oil by neglect, as some times happens, it will quickly wear flat and in troduce a very great amount of friction. The instrument can then be restored to good con dition only by recutting and polishing the pivot end, also the small plate, so as to conform to the above specifications.
Certain advantages result from the use of ball-bearings in anemometers, chiefly in in creasing the service to be obtained and in re ducing the attention necessary to maintain the bearings in proper lubrication. The reduction in friction is not such an important gain as might be supposed, unless the instrument is used in very slight wind. The slight friction in any good anemometer is quite inconsequential in light and fresh winds, such as usually pre vail at practically every station.
The ideal anemometer exposure is secured when the instrument is placed where the move ment of the wind is unobstructed from any di rection, su..h for example, as would be ob tained in the centre of a large open expanse or plain. Only an approximation to the ideal ex
posure is possible in most places, but every effort should be made to better anemometer ex posures in order that the records from the several stations may be comparable. Usually, with an anemometer exposed in a city or town, the great interference offered by buildings and other natural obstructions to the free move ment of the wind causes the velocity to be much less in the vicinity of these obstructions than beyond such influence; therefore, in se lecting the location for an anemometer prefer ence should be given to the more elevated points in the vicinity of the station, and some rigid support should be used to raise the in strument as far as practicable above the im mediate influence of the office building itself.
The Robinson anemometer has the fault of °slurring" its records of gusts and lulls of the wind, due to a certain amount of inertia which cannot be overcome. Moreover, in very light winds it is not dependable, and it is customary to check its record with that of a U-tube in strument placed beside it. The anemometer must be regarded as an instrument of con venience rather than of precision, the difficulty lying in the fact that there is no fixed constant for moving gases, except within narrow limits. Elaborate scales have been devised for ease and rapidity of calculation, but the results, from a scientific standpoint, are at best ap proximate. Consult report of the director of the meteorological office of Great Britain (London 1906). See WIND VANE.