Anemometee. 
lix 
axle of tlie 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 vane p; 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 m, 
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 <?, 
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 
R 
radius r= — — where R is the radius of the 
2 IT 
wheel /, and 2 ir 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 
