Semi-diurnal Fluctuations oftlie Barometer. 167 



portion of the semi-diurnal movements of the barometer which 

 occur in that country; but these causes are not sufficient to 

 produce the diurnal fluctuations in other places, such as Bom- 

 bay, Calcutta and La Guayra. And there can be little doubt 

 that the real causes, whatever they may be, which give rise 

 to the double undulations in these tropical parts, produce them 

 in places where they are less extensive, although the opera- 

 tion of the causes in the latter places may be weaker and 

 more difficult to trace. 



I have shown in my " Atmospheric Changes" that there is 

 no reason to believe that the daily warming of the atmo- 

 spheric gases by the direct influence of the sun produces any 

 appreciable alteration in their pressure on the mercury of the 

 barometer, as the effect of that warming on the whole column 

 in the locality is so small, as to prevent much disturbance of 

 atmospheric pressure; yet great influence has been attributed 

 to solar heating near the surface in producing the semi-diurnal 

 fluctuations that take place. 



Colonel Sabine, in his Report on the Meteorology of 

 Toronto at the meeting of the British Association in 1844, 

 gives an explanation of the daily oscillations. He says, " As 

 the temperatureof the day increases, the earth becomes warmed 

 and imparts heat to the air in contact with it, and causes it to 

 ascend. The column of air over the place of observation 

 thus warmed rises, and a portion of it diffuses itself in the 

 higher regions of the atmosphere, where the temperature at 

 the surface is less. Hence the statical pressure of the column 

 is diminished. On the other hand, as the temperature falls, 

 the column contracts, and receives in its turn a portion of air 

 which passes over in the higher regions from spaces where a 

 higher temperature prevails; and thus the statical pressure is 

 augmented." 



In the Athenaeum of July 5, 1845, the Colonel is repre- 

 sented as having said at the then recent meeting of the British 

 Association, that in Dr. Buist's Meteorological Report from 

 Bombay, " the explanations thereby afforded of the diurnal 

 variations of the gaseous pressure at Bombay, which, although 

 at first sight more complex than at the stations of Toronto, 

 Prague or Greenwich, he conceives to be equally traceable to 

 variations of temperatures." Colonel Sabine therefore, after 

 having examined Dr. Buist's meteorological registers, retains 

 the opinion that the semi-diurnal alterations of the gaseous 

 pressure are produced by alterations of temperature, as that 

 temperature is shown by the thermometer. 



As I propose to examine this theory and to compare it with 

 another, it will be convenient to designate the two by distinct 



