166 LIEUT.-COLONEL SYKES ON THE ATMOSPHERIC TIDES 



just within the inner doors of a field officer's tent, having a third canopy or extra fly 

 to it ; and during the hot months commonly pitched under the shade of lofty trees. 

 For the remaining, or monsoon months of the year, the instruments were kept in a 

 room at Hay Cottage, Poona, through which there was a constant draft of air by two 

 windows opposite to each other in the line east and west. In using the hygrometers, 

 they were always taken to the door of the tent or to an open window. Whether in 

 determining ordinary pressure, atmospheric tides, temperature, moisture, or heights, 

 by the barometer or boiling-water process, I have invariably deemed it necessary to 

 guard my observations from error by the employment of instruments in pairs. I have 

 been thus minute in the description of my instruments and my manner of using 

 them, not less to supply the means for a just estimate of the value of my meteorolo- 

 gical observations, than to enable meteorologists who may tread in my steps to benefit 

 by my experience and disasters. 



The barometrical means have been reduced to32°FAHR. by Professor Schumacher's 

 tables, with corrections for the expansion of the brass scale ; and the monthly means 

 for 1830 were obtained by the ingenious process recommended by Professor Forbes. 



In regard to the following barometrical observations, I must premise, that my 

 three best barometers, although precisely of the same construction and placed under 

 precisely similar circumstances, would occasionally differ slightly from each other, 

 not only in the amount of the oscillations, but in the period at which the several tides 

 turned ; and this fact is of some importance to those who may be disposed to rely 

 too confidently upon the indications of a single instrument. 



My erratic life necessarily disabled me from determining the mean absolute height 

 of the barometer at any one place for a period exceeding five or six consecutive 

 months, excepting for the year 1830. I cannot, therefore, state the annual range of 

 the barometer for several years successively ; but repeated returns to Poona in vari- 

 ous months of the year would have supplied the materials for tolerably just estimates, 

 even had I not one entire year's observations made at Poona. The monthly range is 

 recorded for many complete months, and the diurnal range for six years with few 

 omissions ; but I propose to confine my deductions to my observations for the last 

 four years, as the instruments I had in use at first (Jones's) did not admit of a 

 delicate adjustment of the lower level of the mercury. 



The great features in the barometrical indications are the diurnal and nocturnal 

 tides, embracing two maxima and two minima in the twenty-four hours ; the former 

 occurring with occasional exceptions between 9 and 10 a.m. and 10 and 11 p.m., and 

 the latter between 4 and 5 p.m. and 4 and 5 a.m. The same hours obtain at Calcutta 

 and Madras at the level of the sea ; at Kotgheriy on the Neelgherry mountains at 

 6407 feet; in South America at 12,000 feet; and in London and Edinburgh, and 

 other places in Europe where careful observations have been made. Hitherto little 

 has been known respecting the nocturnal atmospheric tides, but the existence of the 

 diurnal tides is now established beyond doubt in most parts of the world. Hum- 



