42 John Eliot — On, the occasional Inversion of the Temperature [No. 1, 



clear weather. It is not necessary to give data for these statements, as. 

 a reference to the Tables I to IV will confirm them. We shall therefore 

 assume these two figures, viz., 18° and 36°, as representing approximately 

 the daily ranges of temperature of the air at the hill stations and adjacent 

 plains in Upper India in ordinary fine clear anti-cyclonic weather in 

 January. If there were absolutety no motion of the air, vertical or 

 horizontal, an increase of temperature of 36° of the lowest strata of air 

 over the plains would cause pressure, as measured by the barometer, to 

 increase about two inches. No such increase actually occurs. The 

 only large barometric movement in such weather is the diurnal oscilla- 

 tion (slightly exceeding in amount a tenth of an inch), which goes on 

 with great regularity. Again, as no such large increase of pressure oc- 

 curs, it is evident that it is counterbalanced by the subsequent changes 

 of pressure due to air motion of expansion, convection currents and 

 horizontal movement. The cooling of the air takes place most 

 rapidly for some hours after sunset when the air movement is ap- 

 parently least. The adjustment of pressure to the changing temperature 

 conditions during night is frequently not accompanied by any per- 

 ceptible or measurable air movement (vide data of Table, p. 41). The 

 slightest observation of the way in which the smoke of the evening 

 fires in an Indian town in Upper India lies over it motionless indicates 

 clearly that the only important air movement which occurs in the 

 evening during the rapid cooling of the air, can only be one of compres- 

 sion due to descent of the air above the lowest stratum, and that this is 

 so extremely slow a process as to be imperceptible even by its action 

 on mist and smoke. Considering the first 1000 feet thickness of the 

 atmosphere to be homogeneous, the upper surface would have to descend 

 about 60 feet in order to produce the compression required to maintain 

 pressure at the same amount. This motion may appear to be consi- 

 derable, but if it occurs as an accompaniment to the cooling it will 

 take several hours to be completed. A total downward movement 

 of the air at a height of 1000 feet through sixty or seventy feet 

 spread over several hours is exceedingly small and cannot be detected by 

 any of the ordinary methods of measuring air motion. The assump- 

 tion of this slow motion of compression is hence in accordance with 

 facts and competent to explain them. The cooling by night hence 

 takes place in a nearly quiescent atmosphere, and if there be any con- 

 vection currents, they are so feeble, more especially when compared 

 with those which accompany heating dui'ing the day, as to be of no 

 importance and negligible. Hence the motion of the air at night 

 in Upper India during fine clear weather in January may be assumed 

 +o be a very small general downward movement producing the 



