332 Prof. W. A. Norton on the Diurnal and Annual 
minima and maxima of the horizontal force. If we make the 
comparison for other years, and also for the quarters of years, we 
find a rat ae eee ne correspondence. 
If end to a minute Copa RATED, we find that the inter- 
vals beets ‘the: recise hours of maxinga and minima in some 
instances amount to as much‘as two hours ; in fact that the sec- 
ondary or morning minimum of the barom eter sometimes pre- 
variations of the horizontal force, it is to be observed that the 
Fs Variations both of barometer and horizontal force are compara- 
tively small during the night, and also in general about the times 
“of maxima and: minima ; see therefore that such comparatively i 
smail differences may be expected to subsist, unless the two pie- 
nomena be supposed to be identical in their origin. If the times 
force are in some way directly qepuiide nt upon the-diurnal varia~ ad 
tions of the pressure of the air; as it is, the more probable con- 
clusion is, that these two different phenomena are two different 
effects or consequences of the same meteorological p enomenon. 
hen I had arrived at this point in the progress of my inves- 
tigations, it at once occurred to me that, as the diurnal variations 
of the horizontal: force had been explained by referring them to 
the daily changes in the temperature and humidity of the earth’s 
surface, the diurnal variations of the barometer ‘were probably at- 
tributable to daily changes in the temperature and humidity of - 
the air. It was seen that the same evaporation by day which _ 
tended to diminish the horizontal force, would tend, by adding 
to the quantity of vapor in the air, to augment the height of the 
barometer, and that che same condensation of vapor at the earth’s 
surface at night, which tended to increase the horizontal force, 
would, by diminishing the quantity of vapor in the air, tend to 
make the barometer fall: also that if these as ce in conjunc- 
tion with those due to variations of temperature, are the ‘actual 
producing causes of the diurnal variations of the barometer an 
horizontal force, there would doubtless be an approximate corres- 
pondence between the maxima of the one element and the minima 
It is somewhat curious that I should have been 
condueted in this indirect manner, to the explanation of the daily 
oo of the barometer, which, as I have since found, has 
n conclusively established, to be the true explanation of this 
jheisadibnibe, by direct observation. This is known to meteor- 
