ae ee 
= 
‘. 
a 
CORRECTION TO BAROMETER FOR FORCE OF WIND. 383 
Velocity in Miles Pressure in Pounds Depression 
per Hour. per Square Foot. in Inches. 
14:2 1 ‘010 
20-0 2 015 
24:5 3 -020 
28-3 4 -025 
316 5 ‘030 7 Observed. 
34:6 6 035 
37-4 Uf ‘040 
40:0 8 "045 
42°4 9 “050 
44-7 10 055 
61°6 19 -100 
In this table, I have distinguished the observed depressions corresponding to 
the observed pressures, and have extended the table, shewing that the depression 
would be one-tenth of an inch for a pressure of 19 lb. per square foot, supposing 
the depressions follow the same law; but I do not assert that they do, and rather 
think it probable that they do not, with the higher velocities ; but this is a mat- 
ter for further investigation. 
It is not, therefore, correct to speak of these depressions as oscillations in the 
level of the mercury, above and below a sort of mean tidal line; they are simply 
caused. by diminished pressure, and the barometer resumes its original position 
without passing it, when the gale or gust of wind passes away. With a pressure 
of 2 lb. per square foot during the lulls in a gale, and, with gusts, giving a pressure 
of from 6 to 7 lb., we find the barometer is ‘015 always below what it should be, 
and the effect of the gust is indicated by a further depression of ‘035 and ‘040; so 
that by the indications of barometer alone, we are able approximately to estimate 
the additional force of the gust without reference to the anemometer, and by com- 
paring the readings of the barometer in an exposed and in a sheltered position, 
we may estimate the force of the wind at the time of observation. In applying 
the correction for the force of the wind, the depression due to the force during the 
lulls, should be added to the reading of the barometer when at its highest point. 
Now the corrections hitherto considered absolutely necessary to the readings 
of the barometer, with the view of having the results in a form strictly com- 
parable from different places of observation, are, for barometers with cisterns, 
without zero-points, 
1st, For the capillary action of the tube. 
2d, For temperature to reduce the readings to what they would be at 32’. 
3d, For capacity, depending on the relation of the area of the surface of the 
mercury in the cistern to the area of that in the tube. 
And, 4th, For altitude above the level of the sea. 
Now, if we assume the observed height of the barometer to be 29:500 inches 
at the temperature of 40°, the size of the tube to be °35 inches, and its capacity 
gsth of the cistern, and the neutral point of the instrument at 29-750, the height 
