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Proceedings of the Royal Society of Edinburgh. [Sess. 
It is hardly necessary to enter in detail into the methods proposed for 
the driving of the pens. Either of two methods is feasible. In the first, 
the pen is carried round with the final axis to a maximum angular distance 
of 30° from its zero position ere it is released. In this case the ordinates 
of wind-flux on the recording cylinder will be approximately arcs of a circle. 
In the other method, the pen is dragged longitudinally along the cylinder 
by a thread which is kept taut by weights, and which at one part of its 
length passes round a pulley driven by the clockwork of the instrument. 
Experience will be the best guide as to the scale on which the angular 
motion of the wind-wheels should be reduced to give the angular motion of 
the pen. At present, I purpose giving release to the pen every time it has 
recorded a total of 120 kilometres of wind. With a pitch of 2 metres 
for the wind-wheel and a maximum angular motion for the pen of 30°, this 
would give a reduction ratio of 720,000 : 1. 
B. A Portable Barometer on a New Principle. 
The object aimed at in designing the instrument described below is to 
be able to effect in the field a determination of the barometric pressure free 
from the grave uncertainty which attaches to the readings of most aneroids, 
and having an accuracy not far short of that of an ordinary mercurial 
barometer. As the author has only recently arrived in Britain after a year 
of travel, he has been unable to have the instrument constructed so as to 
show it to the Society. 
The instrument is essentially a constant-pressure air thermometer, 
alongside of which is placed a Six’s thermometer for the purpose of enab- 
ling the user to bring the air contained in the air thermometer to constant 
pressure at any temperature. It will be most easily described by reference 
to the diagram (fig. 5). 
The air thermometer consists of an inverted bulb A, of special construc- 
tion ; a descending tube B, the lower portion of which contains mercury ; a 
cistern C, containing mercury, and of capacity adjustable by a screw D ; and 
an ascending tube E, up which the mercury rises to a height depending on 
the pressure of the external atmosphere, E being open to the air at its upper 
end. A scale F, graduated downwards, slides alongside of the tubes D and E. 
If the air in the thermometer is always to be brought to the standard 
pressure of (say) 80 cms., then the lower end F of the scale would be marked 
80, and the readings upwards would be 79, 78, etc. If now the air in the 
bulb be brought to 80 cm. pressure by the adjusting screw of the cistern, 
and if the lower end of the scale be brought to the top of the mercury in 
