Semi-diurnal Fluctuations of the Barometer. 169 



The first column exhibits the fluctuations at the equator in 

 the great ocean. The range extends beyond two millimetres, 

 and the descent from 9 in the morning till 4 in the afternoon 

 gives the whole extent of the range. 



The two next columns show the alterations at Cumana and 

 La Guayra, both above 1 0° north latitude, and the ranges are 

 nearly equal to that at the equator. 



The fourth column shows the changes at Calcutta to be 

 nearly as great as in the preceding places, but both this and 

 the La Guayra columns exhibit singular irregularities in the 

 earlier parts of the mid-day descents. 



In the Padua column, 45° north, the fluctuation is much 

 reduced in the extent of its range, but retains the same general 

 character. 



In Halle, in latitude 54°, the alterations do not differ ma- 

 terially from those at Padua. 



The changes are very small in Abo and Petersburg, and 

 in the former place the second rise attains a greater height 

 than the first. 



To these it is desirable that we should add the following 

 table (p. 170) of the height of the dry- and wet-bulb thermo- 

 meters, and the difference between the two, — with the dew- 

 point and the height of the barometer at Plymouth for three 

 years, as furnished by Mr. S. Harris, and published in the 

 Ninth Report of the British Association (p. 167). 



In all these places the temperature shows only a single 

 fluctuation, such as is seen in the table of the thermometer at 

 Plymouth, namely one rise generally from about 5 a.m. to 

 1 or 2 p.m., and one fall from that time until 5 the following 

 morning. Now, if the temperature of the atmosphere, as 

 marked by the thermometer, caused the diurnal fluctuations 

 in the way supposed, we ought to have in all these places one 

 undulation in the twenty-four hours instead of two, — the rise 

 of temperature causing a decline of the barometer during the 

 hotter part of the day, and the fall of temperature producing 

 a rise of the barometer in the colder part. Yet Colonel 

 Sabine himself says that at Bombay, where there is only one 

 rise and one fall of temperature, there are two risings and 

 two fallings of the barometer ! And these movements of the 

 barometer take place not only when that instrument is taken 

 as the measure of the whole pressure of the atmosphere, but 

 also when the vapour pressure is deducted, and the mercury 

 of the barometer is taken as the measure of the gaseous pres- 

 sure alone. These facts are opposed to, and are irreconcile- 

 able with, the temperature theory. 



