lxii Introduction to the Makerstoun Observations, 1844. 



the vane enters the mercury, and a double portion s acts as an outer cover to the mer- 

 cury cistern ; t is an aperture, 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 <?, 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 

 graduated 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 surface equal to the weight suspended multiplied by the 

 above ratio. The spiral, on which the weight h acts, is the involute of a circle whose 



T) 



radius r = - — where R is the radius of the wheel /*, and 2 n is the circumference 



to radius of one, if the vessel e were homogeneous throughout its depth, the equal in- 

 crements of motion in the index would correspond to equal increments of pressure.* 



102. The instrument is observed in the following manner : — About 2 m before 

 the observation hour the pressure shewn by the index n is registered as the maxi- 

 mum pressure ; this index is then put back to zero, and from 7 m to 10 m afterwards, 

 the position to which it has again been carried by the index m is noted as the pre- 

 sent pressure ; the index n is then set to zero, and a similar double observation 

 made at the next observation hour. It is conceived that this instrument can be de- 

 pended on for the purpose of determining the laws of variation of the pressure of 

 wind ; for absolute results, an integrating instrument is essential. 



State of the Sky. 



103. The extent of sky clouded is estimated ; the whole sky covered with clouds 

 being noted as 10, and the complete absence of clouds as zero. The motions of the 

 clouds are determined as follows : — A well-marked portion of cloud which passes, 

 or has passed, through the zenith, is watched till the direction is found in which it 

 seems to run down, or parallel to, one corner of the Observatory ; the walls of the 



* 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 

 inch, P the pressure which the wind exerts on the top of e diminishing its weight, /3 the corresponding 

 arc through which the circumference of the wheel/moves (or the length of cord wrapped on the wheel), 

 W the weight of the counterpoise k, and e the specific gravity of the material (zinc) of which e is 

 formed, then 



P W to 

 ~~(3 ~ 2v + T 



a constant ratio. 



