154  Lovering on Magnetic Observations at Cambridge. 
One word is necessary in regard to the barometric observations 
made at Cambridge. They have been corrected for temperature, 
and the data are furnished for the correction due to elevation, capil- 
larity, and the constant instrumental error. But the result indi- 
cates not the pressure of the air merely, but the sum of the 
pressures of the gas and the vapor in it. Now it appears, from a 
paper which Mr. Sabine read before the British Association in 1844, 
that in many places, if not everywhere, and always, the changes 
in the amount of vapor in the air correspond in such a way to 
the changes in the true gaseous pressure that they are mutually 
masked so far as the barometer indicates them. In this way we 
account for that want of distinctness in the barometric curve 
(either the daily or the yearly one) which we find in the curves 
which represent the corresponding variations of temperature and 
the magnetic meridian. It seems from the Toronto observations, 
that when the vapor pressure is separated from the gaseous press- 
ure, both are subject to regular mean daily and yearly fluctuations, 
which almost entirely neutralize one another when taken together, 
leaving a smaller residuum of change which exhibits, also, a less 
striking relation to the hour of the day or the season of the year. 
I may illustrate these remarks by the daily curve. The maximum 
of gaseous pressure occurs at eight in the morning, and the mini- 
mum at two or three in the afternoon: but the maximum of vapor 
pressure takes place at the last period, and the minimum at the 
former. Mr. Sabine further shows, that the periods of maxima 
and minima for vapor and gaseous pressure not only correspond 
in this way, but that they are the same as the times of maxima 
and minima for temperature and the force of the wind. In a 
word, the maxima and minima for temperature, force of the wind, 
and vapor pressure, are all found at the same general periods, 
