498 PRESSURE AND TEMPERATURE AT THE BEN NEVIS OBSERVATORIES. 



is 0*047 inch higher than at Fort-William ; and on the other hand, during the occur- 

 rence of cyclonic weather, when the temperature at the top is very greatly lower than 

 the average as compared with that at Fort- William, the pressure at the top, reduced to 

 sea-level, is 0"029 inch lower than that of Fort- William. There is, therefore, a mean 

 •difference of pressure for these distinct extreme types of weather of 0*076 inch. 



A part of this difference is undoubtedly due to the time of the day when the 

 extreme differences most frequently occur. Since the excess of the sea-level pressure 

 at the top occurs generally when the lowering of the temperature from the effects of 

 terrestrial radiation is greatest, it results that the mean temperature of the upper and 

 the lower thermometers is somewhat less than that of the true mean temperature of the 

 aerial stratum between them, and consequently a larger correction is used in reducing 

 the upper barometer to sea-level. On the other hand, since the maximum temperature 

 differences between the two thermometers takes place most frequently during the times 

 of the day when solar radiation is most powerful, it follows that the mean of the upper 

 and lower thermometers is higher than the true mean temperature of the aerial stratum 

 intervening, and consequently a less correction for height in reducing to sea-level is 

 used than is really due. This consideration, however, can only account for a small part 

 •of the large difference between the two sea-level pressures stated above. 



The broad result is this, and it is clear and explicit : when the higher Observatory 

 has the higher temperature, and when the differences of temperature at the two 

 Observatories are small, then the reduced pressure at the top of the mountain is the 

 greater of the two ; but when the differences of temperature are very large, then the 

 reduced pressure at the top is the less of the two. The regular progression of the 

 figures in the two tables shows that what is substantially a true mean has been 

 arrived at. 



The result, which is altogether unexpected, raises questions of the greatest import- 

 ance affecting the theory of storms, the effect of vertical movements of ascending or 

 descending masses of air on the barometric pressure which accompany anti-cyclones and 

 cyclones, and the urgent necessity there is for a more exact knowledge than we at 

 present possess of the absolute amounts of aqueous vapour at different heights in the 

 atmosphere under different weather conditions, and how this knowledge may be arrived 

 at from the readings of the dry and wet bulb hygrometer. 



