Ix INTRODUCTION TO THE MAKERSTOUN OBSERVATIONS, 1845. 
95. 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. 
96. It is conceived that this instrument is trustworthy. It is occasionally, 
however, liable to slight derangements ; the cup at the top containing mercury fills 
with rain, which, when frozen, prevents the vane from turning its aperture to the wind. 
§ 13. SravTE oF THE Sky. 
97. The quantity of clouds is estimated, the whole sky covered with clouds 
being noted as 10, the complete absence of clouds being zero. The motions of the 
clouds are estimated as follows :—A well marked portion of cloud which passes 
through, or nearly 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 ; this direction is 
then estimated very nearly, as the walls are in the meridian and prime vertical. 
About the end of 1843, the points of the compass, with reference to each corner, 
were marked on the paling surrounding the Observatory. I have no hesitation in 
saying, that the motions of upper currents of air thus observed are much better de- 
termined than the motion of the lower or surface current by the vane. 
98. The nomenclature adopted is that of Mr Howarp, with certain combina~ 
tions, which are, in general, sufficiently descriptive. The term scud refers to that 
loose, generally amorphous, and often rainy cloud, which is the lowest of all except- 
ing the stratus. 
§ 14. Clock, STovE, AND COMPUTING Room. 
99. The mean time clock in the Observatory is by Dent of London ; it is kept 
at Géttingen mean time by means of comparisons with the transit clocks in the 
Astronomical Observatory, the errors of which are determined by Sir THomas 
BRISBANE, by myself, or my assistant. The rate of the mean time clock is kept 
small by placing small weights on, or taking them off, the bob of the pendulum. 
inch, P the pressure which the wind exerts on the top of ¢ diminishing its weight, @ the corresponding 
are through which the circumference of the wheel f moyes (or the length of cord wrapped on the wheel), 
W the weight of the counterpoise &, and o the specific gravity of the material (zinc) of which e is formed, 
then 
P Ww w 
age eh Rg 
a constant ratio. 
