296 Remarks on some of the disturbing causes [No. 136. 



The purpose for which I have made the statement, is to call atten- 

 tion to the influence of locality as affecting the results of barometric 

 measurements. Different places may have particular times, at which 

 it is unsafe to trust to barometric observations for correct results. 



As there seems reason to suppose, that the principal deranging 

 causes in barometric measurements are connected with the sun and 

 wind, I have often thought that probably the results by night observa- 

 tion may be found to be more consistent than those by day obser- 

 vations. Unless experience should shew a more favorable time, I 

 should prefer observations made about the time of the morning 

 minimum of the tide, because so far as I have observed, the air is then 

 more generally calm than at any other time. This, however, is a 

 matter of fact, on which every one may judge for himself according to 

 his means of knowledge. 



I have been led to the same conclusion by endeavouring to trace 

 the causes of the atmospheric tides, which I am disposed to refer 

 entirely to the direct or remote action of the sun. The following 

 is offered as an attempt to trace this action, and though perhaps not 

 altogether satisfactory, it may lead to something better. 



When the sun rises over China, the atmosphere there getting heat- 

 ed, expands and begins to flow off towards the west, where the sun is 

 exerting no heating power. As the solar heat increases, the western 

 efflux of atmospheric air increases, and goes on increasing till the sun 

 is past the meridian. When the sun rises over India, a similar 

 western efflux is occasioned, but for some considerable time the influx 

 from the eastward being greater than the efflux towards the west, the 

 atmospheric pressure goes on increasing, till by and by, the direct 

 heating effect over India causes a western efflux at first equal to, and 

 then greater than, the influx from the eastward. The atmospheric 

 pressure thenceforward decreases, and it goes on to decrease so long as 

 the heating power of the sun causes the air to expand. At the sur- 

 face of the earth, this effect is greatest about 2 or 3 p. m., but it is not 

 till the heated atmosphere has had time to ascend and dislodge colder 

 air that the total effect is greatest. The atmospheric pressure is then 

 a minimum. When the sun is exerting its greatest power over India, 

 it has ceased to heat the countries to the eastward : the air over these 

 countries being colder, presses on that over the countries to the west- 



