218 



NINTH ANNUAL REPORT OF 



The next table, which gives the variation of the barometer, shows 

 that the indications of this instrument are not precisely the same as 

 those obtained by actually weighing a portion of air. The table ex- 

 hibits two maxima and two minima in the course ot" the twenty-four 

 hours. 



TABLE VIIL 



MEAN DIURNAL VARIATION OF ATMOSPHERIC PRESSURE AT GREENWICH. 



At first sight, it might appear that the weight and pressure, or in 

 other words the density and pressure, ought to vary together, but a 

 little reflection will make it plain that an increase of pressure on a por- 

 tion of the surface of the earth may be counteracted by a variation of 

 temperature. The height of the barometer indicates the weight of a 

 column of air, extending to the top of the atmosphere ; now, this 

 column may be unduly heated at one point, as we ascend, and unduly 

 cooled at another, and while the absolute weight of a portion oi" tlie air 

 at any point of the column may vary, the pressure of the whole column 

 may remain the same, or the converse. It is probable, however, that 

 the variation of the weight which is found at the surface does not ex- 

 tend to a great elevation, and that when it is not indicated b}'' the 

 barometer it is because the change is too small to be noted by that in- 

 strument. 



The fact that the barometer exhibits two maxima and two minima 

 in the course of the twenty-four hours has given rise to much specula- 

 tion as to its cause. The most common explanation is, that it is due to 

 the joint action of the variation of the temperature and moisture. In 

 the morning, the moisture rising into the atmosphere increases the 

 weight of the air more than the heat diminishes it by expansion, while 

 at about ten o'clock, a. m., the effect of heat overbalances that of va- 

 por, and again as the sun begins to dechne, the weight of the latter pre- 

 dominates. Mr. Espy attributes the daily oscillations of the barometer 

 to the upward and downward motion of the particles of air and moist- 

 ure as they are separated or approximated by the change of temper- 

 ature. The particles weigh more while the rate of separation is in- 

 creasing, and less while it is diminishing. This is a true cause, but we 

 are not certain that it is a sufficient one. Whatever may be the cause 

 of the daily oscillations of the barometer, we know the effect is nearly 

 the same in parts of the earth most widely separated. 



The diurnal variation of the pressure of the wind is also intimately 

 connected with the variation of the temperature and weight of the air ; 

 this is shown by the following table, which gives the sums of the pres- 



