ON METEOEOLOaiCAL OBSERVATIONS ON BEN NEVIS. 61 



portant of these is the rate of decrease of temperature with height, and 

 the rate of diminution of pressure with height, for different atmospheric 

 temperatures and sea-level pressures. 



In these aspects the double set of observations for the past two-and-a- 

 half years have now been discussed. The decrease of the temperature with 

 height is at the rate of one degree Fahrenheit for every 270 feet of ascent, 

 the lowest rate being one degree for every 284 feet in winter, and the most 

 rapid rate 247 in spring. This rate closely agrees with the results of the 

 most carefully conducted balloon ascents, and of those other pairs of 

 stations over the world which are so situated as to give trustworthy 

 results for the inquiry. Ben Nevis Observatory and Fort William Station 

 are among the very few pairs of stations yet established from which the 

 requisite data can be obtained, the required conditions being great 

 difference in height combined with close proximity, and the position of 

 the thermometers in situations where the effects of solar and terrestrial 

 radiation are minimised. 



The next point, and as regards weather phenomena the most 

 important point, to be determined was the normal differences between 

 atmospheric pressure at the top of the Ben and at Fort William for the 

 different air temperatures and sea-level pressures that occur. These 

 differences, or, as they are technically called, corrections for height, were 

 empirically calculated from the observations, and thereafter the departures 

 from these normals were ascertained for each of the five daily observations 

 since the observatory was opened. The results showed a diminution of 

 pressure from the normals on almost every occasion during the occurrence 

 of high winds at the observatory. In other words, in all cases when 

 high winds (30 miles an hour and upwards) prevailed at the observatory 

 the observations reduced to sea-level showed a less pressure than that 

 actually observed at Fort William. The differences increase with the 

 strength of the wind, and amount not unfrequently to the tenth of an 

 inch, and one day when the winds continued to blow at the rate of 120 

 miles an hour, the five consecutive readings showed differences exceeding 

 a tenth and a half. This diminution is doubtless occasioned by the winds 

 as they brush past the buildings, partially sucking out the air from the 

 interior, thus lowering the pressure. It was therefore necessary to 

 recalculate the table of corrections to sea-level, using in the new calcula- 

 tion only those observations which were made when the wind blew at 

 lower rates than 30 miles an hour. This recalculation has been recently 

 completed, and the inquiry as to the bearing of the Ben Nevis observa- 

 tions on the weather is being pushed forward. 



So far as the investigation has been carried, it is evident that rapid 

 and considerable changes from the normals, but particularly a more rapid 

 decrease of temperature with height than the normal decrease, as shown 

 by the thermometric observations, are frequently a precursor and concomi- 

 tant of storms of wind. This is only what might be expected considering 

 that such observations indicate a disturbance of the equilibrium of the 

 atmosphere. But when with this is conjoined a lower sea-level pressure, as 

 calculated from the Ben Nevis Observatory barometric readings, than what 

 is actually observed at Fort William ; in other words, when the barometric 

 observations indicate a more rapid decrease of temperature with height 

 somewhere in the aerial stratum between sea-level and the top of the Ben than 

 the thermometric observations alone indicate, then the indications of a 

 coming storm become more decided. Conversely the absence of any 



