Appendix F. ATMOSPHERIC TIDES. 417 



elevations, as the period of ebb and flow : the mercury falls slowly 

 from 9° 50 a.m. (when it is at its highest) till noon, then rapidly 

 till 3 p.m., and slowly again till 4 p.m. ; after which there is little 

 change until sunset ; it rises rapidly between 7 and 9 p.m., and a 

 little more till 10 p.m. ; thence till 4 a.m. the fall is inconsiderable, 

 and the great rise occurs between 7 and 9 a.m. 



It is well known that these fluctuations of the barometer are 

 due to the expansion and contraction by heat and moisture of 

 the column of atmosphere that presses on the mercury in the 

 cistern of the instrument : were the air dry, the effect would be 

 a single rise and fall ; # the barometer would stand highest at the 

 hottest of the twenty-four hours, and lowest at the coldest ; and 

 such is the case in arid continental regions which are perennially 

 dry. That such would also be the case at Calcutta and throughout 

 the Himalaya of Sikkim, is theoretically self-evident, and proved 

 by my horary observations taken during the rainy months of 1848. 

 An inspection of these at the end of this section (where a column 

 contains the pressure of dry air) shows but one maximum of 

 pressure, which occurs at the coldest time of the twenty-four hours 

 (early in the morning), and one minimum in the afternoon. In the 

 table of mean temperatures of the months, also appended to this 

 section, will also be found a column showing the pressure of dry 

 air, whence it will be seen that there is but one maximum of the 

 pressure of dry air, occurring at the coldest season in December, 

 and one minimum, in July. The effect of the vapour is the same on 

 the annual as upon the diurnal march of the pressure, producing a 

 double maximum and minimum in the year in one case, and in the 

 twenty-four hours in the other. 



I append a meteorological register of the separate months, but at 

 the same time must remind the reader that it does not pretend to 

 strict accuracy. It is founded upon observations made at Dorjiling 

 by Dr. Chapman in the year 1837, for pressure temperature and wet- 

 bulb only ; the other data and some modifications of the above are 

 supplied from observations of my own. Those for terrestrial and 



This law, for which we are indebted to Professor Dove, has been clearly 

 explained by Colonel Sabine in the appendix to his translation of Humboldt's 

 " Cosmos," vol. i. p. 457. 



vol. ir. E E 



