Anemometee. 



lix 



axle of the wheel and spiral ; n a loose index under the index m, which the latter 

 carries forward by means of a projecting pin near the extremity ; o a tube passing 

 under the cistern a, which, entering the bottom, proceeds upwards within the vessel e 

 till its open extremity is above the level of the water in a neck of the vessel e ; the 

 other end of the tube o is six feet above the outer wall of the observatory, where it 



is capped by a vanep; at the top of the tube o 

 three brass rods are joined, which carry a 

 small tube in which a pin within the top piece 

 q rests or turns ; the tube o is double at the 

 top, containing between the tubes a quantity 

 of mercury to the level r, the continuation 

 of the cylindrical body of the vane enters the 

 mercury, and a double portion s acts as an 

 outer cover to the mercury cistern, t is an aper- 

 ture, 2 inches square. When the wind blows, 

 this aperture is presented to it, the wind then 

 presses on the column of air within the tube o 

 (being prevented from escaping under the 

 vane by the mercury), and ultimately on the 

 top surface of the vessel e, forcing the latter 

 up, turning the axle carrying the index mi, 

 which carries before it the index w, leaving 

 it at its farthest excursion. The dial is gra- 

 duated as follows : — The surface of the top 

 of the vessel e on which the wind presses is 

 78 square inches, therefore a pressure of 1 lb. 

 on this surface is equivalent to ^^ lb. on a 

 square foot. Different weights are suspended 

 on the wheel/, acting oppositely to the vessel e, 

 and the position of the index for each weight 

 shews the pressure on a square foot of sur- 

 face equal to the weight suspended multiplied 

 by the above ratio. The spiral, on which the 

 weight k acts, is the involute of a circle whose 



T) 



radius r= - — where R is the radius of the 



27r 



wheel/, and 2 'tt is the circumference to radius of one, if the vessel e were homo- 

 geneous throughout its depth, the equal increments of motion in the index would 

 correspond to equal increments of pressure.* 



* The application of the involute of the circle as the spiral is due, I believe, to Professor Forbes. 

 It is easily shewn that if the vessel e be homogeneous, w being the weight of a ring whose depth is one 



