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



are direct. The movement from east to west is only quasi-tidal, being quite different 

 from the manner in which the tides of the ocean are propagated from place to place 

 over the earth's surface. The barometric tides are in truth directly generated by solar 

 and terrestrial radiation in the regions where they occur, and it is in this way that 

 the great variation revealed by observation in the curves of restricted districts com- 

 paratively near each other are to be explained. 



From hourly observations made in June at the base, the top, and two intermediate 

 points on Mount Washington, N.H. (Table IV. p. 48, and Plate I. figs. 16-19), it is 

 seen that the time of occurrence of the morning maximum at the base of the 

 mountain, 2898 feet above the sea, was 8 A.M. ; at 4059 feet, 10 A.M. ; at 5533 feet, 

 11 a.m. ; and at the top, 6285 feet, noon. Hence, as regards this phase of the oscilla- 

 tion, and the steady diminution with height of the afternoon minimum, the influence 

 of an isolated mountain like Mount Washington brings about a result quite similar to 

 what is observed in insular situations. But the analogy of the curves of the two 

 situations is closer still, the morning minimum in both being very strongly pronounced. 

 Similarly in summer on the top of Ben Nevis, the morning maximum is retarded to 

 3 p.m., being seven hours later than the time of occurrence of the same phase at the 

 base of the mountain ; and the morning minimum is very large, whilst the afternoon 

 minimum all but vanishes (Table IV. p. 27). 



The differences offered by the daily curve of pressure at the top as compared with 

 that at the base of a mountain are the simple results of the diurnal march of tempera- 

 ture. As the temperature of the air is lowest in the early morning, and the atmosphere 

 therefore more condensed in the stratum between the top and base of the mountain, a 

 portion of the atmosphere necessarily sinks bodily below the level of the . top, thus 

 lessening the whole pressure there. On the other hand, as temperature rises during 

 the day with the returning sun, the stratum of air above the base of the mountain 

 expands, thus raising more of the atmosphere above the barometer at the top, so that 

 while at the base of the mountain pressure begins to fall about 9 a.m., on the top it 

 continues to rise at the Ben Nevis Observatory till about 3 p.m., simply from the 

 mechanical upheaval of the air owing to its higher temperature. This feature of the 

 curve of pressure at the top of a mountain is thus essentially and immediately a tem- 

 perature effect, resulting from the diurnal contraction and expansion with the changes 

 of temperature of the aerial stratum between the top and bottom of the mountain. 



The above curves of diurnal pressure have all been selected, it will be observed, 

 from places situated in comparatively high latitudes, where the departures of the 

 maxima and minima from the mean pressure are small ; the object in selecting curves 

 of small daily range being to present in as striking prominence as may be done the 

 modifications impressed on the diurnal barometric fluctuations by mere height and by 

 large tracts of land and sheets of water in respect of the extremely different conditions 

 of surface temperature they offer at different hours of the day. 



