276 Meteorological Observations at Hudson, Ohio. 
followed in nine hours. When weconsider that such instances 
of dryness occur on an average only eight times in a year, we are 
led to infer that the dryness not only does not prevent a speedy 
rain, but that it may be the effect of an approaching rain, or may 
operate as an efficient cause to produce it. Both of these cases I 
conceive to be possible, and may happen in the following manner. 
If, as the effect of an approaching storm, distant one or two hun- 
dred miles, a strong current is forced from a higher latitude to a 
lower, where it is warmer, it will be rendered drier, and moreover 
the heat which is given out in the condensation of vapor may 
produce an effect at a considerable distance in advance of the 
storm. 
The dry air may also be an efficient cause of rain in the fol- 
lowing way. ‘The prevalent current of the atmosphere over the 
United States, is from west to east. So long as the entire mass of 
the atmosphere moves on together with uniform velocity in this di- 
rection, we seldom ifever have rain. But ifalower stratum stands 
still or flows back towards the west, rain is usually the consequence. 
Now this reversal of the lower current may be the effect of in- 
creased pressure, or increased specific gravity of the air; that is, 
an unusually high barometer, an unusually cold or dry air, may 
be a cause of rain. ; 
The following table exhibits all the cases for two years in 
which the thermometer at Greenwich was 25° above the dew 
point, with the hours of observation in Géttingen mean time, 
which is about 40 minutes in advance of Greenwich. 
te. Hygrom. Rain followed; 
1841, April 30, 4h. 25-0° in 2 days. 
June 4,4 26°8 : Lfis his 
*: 46,9 27°5 15 minutes. 
1842, April 28, 4 27:0 5 days. 
June 6, 2 31-1 zs % 
“28, 4 30:1 A2 hours... 
July 15,4. ~~ 26:0 ' . 3-days. 
Aug. 15, 4. 27°6 Ars; $4 
i AB, 4 30°7 22 hours. 
Thus it appears that the atmosphere at Greenwich is less dry 
than that at Hudson, but it is quite remarkable that in 1841 rain 
followed in 15 minutes after the driest time of the whole yeat- 
The following are the only instances in which the dew ‘point 
at Hudson has risen to 80°, all at 3 p. m. 
