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ANEMOMETER. lix 
axle of the wheel and spiral; 7 a loose index under the index m, which the latter 
carries forward by means of a projecting pin near the extremity ; 0 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 
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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 vests 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 mereury cistern, ¢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 n, 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 14 lb. on a 
square foot. Different weights are suspended 
on the wheel f, 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 & acts, is the involute of a circle whose 
3 R : : 5 
radius r= pa where R is the radius of the 
wheel f, and 2 7 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 Forsgs. 
It is easily shewn that if the vessel e be homogeneous, w being the weight of a ring whose depth is one 
