350 ON THE METEOROLOGY OF IRELAND. 



convertible into the former. The six degrees of wind-force were de- 

 signated as follow : 1. Light breeze; 2. Moderate breeze; 3. Strong 

 breeze ; 4. Moderate gale ; 5. Strong gale; 6. Storm. 



In order to know the amount of confidence which may be 

 placed in such observations, it is necessary to determine how far, 

 in respect of accuracy, six degrees of wind- force can be estimated, 

 the observations being supposed to be made by practised observers. 

 And to be able to apply the observations, we must further know 

 what are the pressures and velocities of the wind corresponding 

 to the several terms of the scale. For these purposes I made a 

 somewhat extended series of observations, estimating the force 

 of the wind according to the prescribed scale, and, at the same 

 time, measuring its velocity by means of Robinson's anemometer. 

 The following Table gives the mean results of these observations. 

 The numbers in the first column are the terms of the scale ; those 

 in the second are the corresponding times of 100 revolutions of 

 the instrument, expressed in seconds ;* the third column contains 

 the corresponding velocities of the wind, in feet per second; 

 and the fourth the calculated velocities, deduced as hereafter 

 described. 



* Dr. Robinson has shown (Trans. S. I. A., vol. xxii., p. 167) that the velocity of 

 the wind is to that of the centres of the hemispherical cups, as 3 to 1. But r being 

 the length of the horizontal arms of the instrument, measured to these centres, the 

 circumference of the circle described by them is 2irr ; and if r be expressed in feet, and 

 n be the number of revolutions performed in a second, their velocity is 2*r x n. The 

 corresponding velocity of the wind therefore is V= 6- x . In the instrument in 



my possession the radius is 5-5 inches. Hence 2r = , and substituting for IT its nu- 

 merical value, V= 8-64 x n. 



Instead of noting the number of revolutions, and parts of a revolution, performed 

 in a given time, I have found it convenient to observe the time of performing 100 revo- 

 lutions. I have had the instrument accordingly provided with a little hammer, which 

 is pressed against the registering wheel by a spring, and which, being raised by a pro- 

 jecting pin at one point of its circumference, falls again with a sharp noise when this 

 has passed. The interval between two such strokes of the hammer, therefore, is the 

 time of one whole revolution of the registering wheel, or of 100 revolutions of the 

 arms. Accordingly, a chronometer being held close to the ear, the whole observation 

 effected by the help of that organ. The velocity of the wind in this case is given 

 by the formula V = ~, T being the observed time of 100 revolutions. 



