lx INTRODUCTION TO THE MAKERSTOUN OBSERVATIONS, 1845 AND 1846. 
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 
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 1b. on this surface is equivalent 
to 144 Tb. on a square foot. Different weights are suspended on the wheel f, acting 
oppositely to the vessel ¢, 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 & acts, is the involute of a circle whose 
: R : : : : 
radius 7 = = where R is the radius of the wheel f, and 2 7 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™ 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™ to 10™ afterwards, 
the position to which it has again been carried by the index m is noted as the pre- 
sent pressure ; the index m 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 
Observatory are in the meridian and prime vertical, and the points of the compass, 
reckoning from each corner as a centre, are marked upon the paling surrounding 
the Observatory ; the observer, therefore, sees at once the direction of motion of the 
cloud on the paling ; when a portion of cloud cannot be seen which has passed, or is 
* The application of the involute of the circle as the spiral is due, I believe, to Professor Forsss. 
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 ¢ diminishing its weight, 8 the corresponding 
are through which the circumference of the wheel f moves (or the length of cord wrapped on the wheel), 
W the weight of the counterpoise k, and o the specific gravity of the material (zinc) of which e is 
formed, then 
W ™ 
a 
a constant ratio, 
