102 Prof. Encke on the Calculation 



Now the weight deflected is known, the surface of the plane 

 is known, and also the angle passed through by the links ; 

 hence, by an easy calculation, the force acting on the surface 

 of the plane is readily deduced ; and from this the velocity of 

 the wind, by means of the tables published of the relation be- 

 tween that and the force. 



I hope I have made my ideas sufficiently clear without en- 

 cumbering your valuable pages with a sketch, or with the 

 details of construction. And for the same reason I shall not 

 make any comparisons between this and other anemometers, 

 except that I conceive the suspended sphere, as it always 

 presents an equal and similar surface to the wind, makes a 

 very simple anemometer, and by counterpoising as I have 

 done, it might be rendered sensible to almost any degree; 

 but I think it is liable to objections, from which I believe that 

 which I propose to be free. 



I shall feel obliged if you will give insertion to this paper 

 in your extensively read and valuable journal ; my object 

 being to assist in attaining that desideratum which would be 

 of such advantage in many cases of practical mechanics, as 

 well as in meteorological observations. 



I am. Gentlemen, your obedient Servant, 



Dublin, Oct. 28, 1831. Hugh Hamell, 



XV. On the Calculation of the Orbits of Double Stars. By 

 Professor Encke. 



[Continued from p. 46.] 



NOTHING remains now but the determination of the 

 epoch, or the perihelion passage. Here we may follow the 

 same course, which will afterwards be found the most convenient 

 for calculating the places from given elements. If r and v 

 stand for the radius vector and the true anomaly in the true 

 ellipsis, the coordinates of the points to which they belong in 

 the plane of projection, referred to the line of nodes, as axis 

 of one of the coordinates, are : 



r cos (v + co') and r sin (v + w') cos ?', consequently 

 ^ = r cos (v + o)') cos 9, — r sin (v + w') sin S3 cos i\ 

 rj = r COS (v + w') sin S8 + ?• sin (v + w') cos £3 cos i', 

 or expanding the functions of the compound angles: 



g = ?• cos v{cos w' cos Sh — sin w' sin ^ cos i'} 



—r sin v{sin w' cos ft + cos w' sin ft cos i'} 



ij = r cos v{cos cJ sin ft + sin w' cos ft cos i } 



—r sin v{sin cu' sin ft — cos «' cos ft cos i'} 



and 



