Captain Cockburn devised in 1817 a form of anemometer founded on that of Dr. Robin son, but calculated for use on ship-board. It is seven inches high ; the diameter of the wings is sixteen inches ; there are four cups placed at right angles, attached by arms to a spindle on which is an endless screw, working in the teeth of two multiplying wheels, marked up to 10,000 revolutions. The concave side of each cup is made so as to receive or con tain one-third more wind than the convex side.
Osler's anemometer bears some such rela tion to the older forms, as the aneroid baro meter bears to the common barometer : that is, it acts chiefly by the pressure of the air on a spring, rather than by the rise and fall of a liquid in a tube. The spring is not so delicate in its indications, but it is mare manageable in yielding self-registered observations. In Osler's anemometer, machinery connected with a spring is made to move a pencil over a piece of paper ; so as to keep a continuous register of the variations of wind, both in intensity and direction. A fine instrument of this kind is placed on the roof of the Royal Exchange, so arranged as to register its indications in a room below. It also acts as a pleviameter or rain-guage.
Mr. Goddard's anemometer, somewhat re sembling in principle Mr. Osiers, registers no less than twelve classes of phenomena in respect to the wind; viz.: miles of wind blown during a day ; miles of wind blown in each direction ; miles of wind blown between any two given periods ; hour and minute of the strongest wind ; hours in which most wind Ihas blown ; times and lengths of calms ; velo city of wind at any hour ; time occupied by the wind going any certain distance at any period of the day; direction of wind at any minute ; mean direction ; direction of longest continuance ; and direction of the greatest passage of wind.
Professor Phillips described to the British Association in 1816 a simple form of anemo meter which depends on a well-known prin ciple. When the bulb of a thermometer, covered with cotton wool, is immersed in water and then exposed to the air, a diminution of temperature ensues, consequent on evapora tion from the surface; and when the thermo meter is moved through the air, or air be blown upon it, the rapidity of evaporation is increased. Professor Phillips took advantage of this principle, to measure the rapidity of cooling by the rapidity of motion in or through the air, or, conversely, to measure the motion by the cooling. He ascertained the amount of diminution of temperature by simple expo sure, and then raised the temperature by the heat of the hand to that of the air, and marked by a seconds-watch the rapidity of cooling when the hand was withdrawn. He next repeated the process when the thermometer was in motion, and he was thus enabled to determine the velocity of a railway carriage in swift movement.