329 



ANEMOMETER. 



ANEMO'METER. 



330 



first magnitude in the head of Andromeda, marked a, and called 

 Alpherat. A line drawn through e Cassiopeia;, at the other corner, and 

 the pole star, passes through Almach in the foot of Andromeda, marked 

 y, while in the line between the two stars thus found, lies Mirach, 

 marked 0, in the girdle of Andromeda. 



The following is an enumeration of the principal stare in this con- 

 stellation, classified according to their magnitudes : 



Magnitude. 

 1st . 

 2nd 

 3rd 



Number of Stars. 



. 1 



. . 1 



. 2 



Magnitude. 

 4th . 

 5th 

 6th . 



Number of Stars. 



. 6 



. . 8 



. 45 



Hence the total number of the stars in this constellation which are 

 visible to the naked eye amounts to 63. 



The following are the designations of the various stars in Andromeda, 

 down to the 4th magnitude inclusive : 



Character, 

 a 

 n 



t 

 S 



C 



No. in Catalogue No. in Catalogue of 

 of Flamsteed. British Association. Magnitude. 



21 

 29 

 80 

 31 

 34 

 35 

 37 

 43 

 51 

 57 



4 



155 

 164 

 166 

 215 

 227 

 252 

 334 

 487 

 628 



ANEMO'METER (from ivefios, the wind, and nn-pea, to measure), 

 is an instrument for measuring the force of the wind, by finding what 

 mechanical effect the wind to be measured will produce upon the 

 apparatus. The first anemometer seems to have been invented by 

 Dr. Croune, in 1667, but this did not answer its purpose well, and a 

 better instrument was devised in the last century by Wolfius, which 

 is described by him in his ' Elemeuta Matheseos,' vol. ii. p. 319 (Geneva 

 edition, 1746). It consists of four sails, similar to those of a windmill, 

 but smaller, turning on an axis. On the axis is a perpetual screw, 

 which turns a vertical cog-wheel round a second axis, placed trans- 

 versely to the former. To the second axis is attached a bar, on which 

 a weight is fixed, so that the sails cannot turn without moving round 

 the bar in a vertical circle. When the wind acta upon the sails the 

 bar rises, and this continues until the increased leverage of the weight 

 furnishes a counterpoise to the moving force of the wind. The number 

 of degrees the bar moves through to produce this effect is measured 

 on a dial, the hand of which turns on the axis of the cog-wheel. 



Another form of anemometer was invented by Leslie, depending 

 for ite action upon the principle, that the cooling power of a current of 

 air varies as its velocity. Another instrument depended on the evapo- 

 ration of water, which, for any time, is proportional to the velocity of 

 the wind. In all these forms, however, the force is measured either by 

 the compression of a spring, or by the raising of some weight to a 

 height varying with the force to be measured. The former method, 

 though more convenient, is, owing to the diminution in elasticity by 

 frequent compression, liable to give varying results. 



The principle of I>r. Lind's anemometer is as follows : A, a curved 



Fig. 1. 



tube of glass, as represented in fyure 1, is 

 jiartially filled with water. The bore of the tube 

 it diminished at the bottom, as a check on the 

 oscillations to which, the water is subject from 

 sudden variations in the force of the wind. The 

 wind acts upon the open end A, and depresses 

 the water to B, until the column of water b C, 

 the difference between the levels B and C, is a 

 counterpoise to the force of the wind on B. 

 This difference can be ascertained by the gradu- 

 ated scale. Hence, when the area of the bore at 

 B is known, and the height of b C observed, the 

 column of water is found the weight of which is equivalent to the 

 force of the wind. The velocity may thence be found by observing 

 (AERODYNAMICS) that the velocities are nearly 

 aa the square roots of the resistances, and that 

 the moving force of a wind of 20 feet per 

 second on a square foot is 12 ounces. 



Lind's anemometer has been improved by 

 .Sir W. Snow Harris, who has reduced one of 

 the limbs to the diameter of one-fourth of 

 the tube which is open to the wind, and by 

 making the first part of the scale horizontal 

 the delicacy of the instrument has been 

 much increased. He has also provided it with 

 a plumb line, and with a light vane to 

 facilitate the operation of observing. (See 

 fg. 2.) Sir John Herachel, in the ' Manual of 

 ific Inquiry, 1 recommends that in using this instrument in cold 



Fig. 2. 



climates, a saturated brine which does not freeze should be substi- 

 tuted for the water ; its specific gravity is 1-244, so that the force given 

 by the table must be multiplied by this factor. 



The following table, calculated by Dr. Hutton, and given in his 

 ' Mathematical Dictionary,' is based upon some experiments made with 

 Dr. Lind's anemometer, at Woolwich ; it may be used with that instru- 

 ment, and indicates what velocity of wind corresponds to various 

 differences between the levels of the liquid, and the consequent force 

 of the wind. Thus, when the column of liquid is 9 inches, the velocity 

 is 108 miles per hour, and the pressure on the square foot is 43'9 Ibs., 

 producing a most violent hurricane ; so that in the greatest storms the 

 difference between the atmospheric pressures on the windward and 

 leeward sides of any object does not amount to -^th of the pressure on 

 the leeward side, which, we know, is capable of supporting a column of 

 water 32 feet, or of mercury 30 inches. 



Difference of Levels 

 in inches. 



10 

 11 

 12 



Force of Wind 

 in pounds. 



1-3 



2-6 



5-2 

 10-4 

 li-6 

 20-8 

 26-0 

 31-3 

 3G-5 

 41-7 

 46-9 

 52-1 

 67-3 

 62-5 



Velocity ner hour 

 in miles. 



18.0 



25.0 



36-0 



50-8 



62-0 



76-0 



80-4 



88-0 



95-2 

 101-6 

 108-0 

 113-0 

 119-2 

 124-0 



In Regnier's anemometer, a bar, carrying a flat wooden surface at 

 right angles to it, protrudes from a box, through a hole in the front 

 of which it slides. This bar is met by a spring, which resists its 

 further entry, until force is applied against the wooden surface. In 

 the interior of the box, the under side of the bar carries rackwork, 

 which plays on a cog wheel, the axis of which, passing through a side 

 of the box, carries a hand round a dial-plate. The flat surface of 

 wood is presented to the wind, which presses upon it and forces back 

 the bar, carrying the cog wheel and hand through an angle, greater or 

 less, according to the greater or less impulse of the wind. 



Various other contrivances have been proposed, the most important 

 of which are by Dr. Whewell and Mr. Osier. In Whewell's anemo- 

 meter ( fiy. 3) a windmill fly is, by the action of a vane, constantly 

 presented to the wind, and the velocity of the revolutions of the fly 



Fig. 3. 



depends, of course, on that of the current. By means of an inter- 

 mediate train of wheels moved by the fly, a pencil is made to descend 



