40 THE VOYAGE OF H.M.S. CHALLENGER. 



a totally different diurnal barometric curve from that of a place situated on a plateau, 

 though its height and geographical position be otherwise similar. 



Correction for Height. — Table V. gives the corrections for height which have been 

 employed in reducing to sea level the barometric means of Table VI. This table is 

 based on the formula given by Laplace in his Mecanique Celeste, which is published in 

 Mr. Scott's Instructions in the Use of Meteorological Instruments, p. 80 ; modified by 

 the results obtained from four years' observations at the Ben Nevis Observatory, 4406 

 feet high, as determined by levelling, and those at its low level station, near the sea at 

 Fort William. 



The four years' observations ending with 1887, give a decrease of temperature with 

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

 has been adopted in arriving at the approximate mean temperature of the intervening 

 stratum of air between the stations, the barometric means of which are being reduced to 

 sea level. Since, in this discussion, the monthly means based on series of years' obser- 

 vation are alone dealt with, these approximate means may be regarded as sufficiently 

 close to the true means for the purpose on hand. The mean of the intervening stratum 

 of air being assumed to be the arithmetical mean of the temperature at the station 

 and that of the sea level to which the reduction is made, the temperature of the 

 intervening stratum was, in practice, found by adding to the station temperature a 

 correction, at the rate of one degree Fahrenheit for every 540 feet in height. 



The Ben Nevis Observatory and the Fort William stations are perhaps the best pair 

 of stations yet established from which the requisite data can be obtained in connection 

 with the inquiry as to the rate of the diminution of pressure with height ; these two 

 stations affording the conditions of great difference in height, combined with close 

 proximity, and the positions of the thermometers in situations where the effects of solar 

 and terrestrial radiation are minimised. 



The corrections for height, for the Ben Nevis Observatory, for different sea level 

 pressures and different air temperatures were empirically calculated from the observa- 

 tions. In applying the first results thus calculated, it became evident that it would be 

 necessary to employ only those observations which were made when the wind blew at 

 lower rates than thirty miles an hour, the reason being that the winds of higher veloci- 

 ties, as they brush past the buildings of the Observatory, suck the air out from the room 

 where the barometer is hung, thus lowering the pressure ; and the higher the velocity> 

 the greater is the effect on the pressure thus produced. 



A table of corrections for a height of 4406 feet was prepared in this way for sea 

 level pressures, varying from 27"500 inches to 30'800 inches, and for air temperatures 

 varying from 15° to 66°. For these same temperatures and sea level pressures, a 

 similar table of corrections for Ben Nevis Observatory was constructed from Laplace's 

 formula. 



