178 LIEUT.-COLONEL SYKES ON THE ATMOSPHERIC TIDES 



HOY*s hours of observations have been previously noticed. M. Duperrey's observa- 

 tions were made every fifteen minutes, but continued only for two days ; and it is 

 remarkable, although the first day gave a diurnal maximum falling tide of • 1417, the 

 next day gave only '0984 : any deductions, therefore, from observations for short pe- 

 riods of time even in the tropics must be fallacious. The above data unquestionably 

 prove the existence oi nocturnal tides, of which doubts exist, or did exist, in Europe ; 

 although Humboldt says they were observed in Dutch Guiana as far back as 1 722 

 by a naturalist whose name is unknown. 



Unhappily, during 1 830, whilst observing the exact time of the turn of three tides, 

 I was so harassed by public duties, that I omitted to record the barometer at sim- 

 rise, and therefore want the data to assist in determining for any lengthened conti- 

 nuous period the amount of the tide between 10 — 11 p.m. and 4 — 5 a.m. or sunrise. 

 But as there is a remarkable accordance in the absolute height of the barometer at 

 Poona during the monsoons of 1829 and 1830, if I were to adopt the mean height of 

 the barometer at sunrise in 1829 for 1830, and then deduct this amount from the 

 mean height at 10 — 11 p.m. in 1830, we shall have '0332 as the value of the oscilla- 

 tions, which, corrected to 4 a.m., will give '0442 as the value of the falling tide between 

 10 — 11 P.M. and 4 — 5 a.m.; and this amount I have little doubt would be infinitely 

 nearer the truth than my forty or fifty nights' observations taken at different periods. 



From the above short table it will appear that in my observations for four years, 

 the maximum oscillation was between 9 — 10 a.m. and 4 — 5 p.m. ; the next greatest 

 was that between 4 — 5 p.m. and 10 — 11 p.m., amounting to a little more than 75f 

 per cent, of the preceding fall : then follows the rising tide between 4 — 5 a.m. and 

 9 — 10 A.M., amounting to nearly 40 per cent, of the diurnal tide ; and finally comes 

 the falling tide between 10 — 11 p.m. and 4 — 5 a.m., which by the few direct observa- 

 tions I made would not be more than 15 J per cent, of the great tide, but which by 

 the process above noticed, I suppose would be about 38 per cent. ; and this would 

 accord tolerably well with Mr. Prinsep's proportion, which is nearly 28| per cent. 

 I shall not remark upon the discrepancies between the ratio thus eliminated, and that 

 deducible from the observations of the other gentlemen quoted in the Table. One fact, 

 however, appears established by all the observers, that the greatest oscillation is du- 

 ring the day, the least during the night ; the second greatest from 4 — 5 to 10 — 1 1 p.m., 

 and the second least from 4 — 5 to 9 — 10 a.m. ; and that all the irregularities occur 

 within comparatively narrow limit hours. 



My barometrical observations were taken on various levels, excepting for the yearly 

 residence of five or six months at Poona during the monsoon, and for the entire year 

 1830: the statements of the absolute height of the barometer and the annual and 

 monthly changes of pressure of the atmosphere will therefore be comparatively limited; 

 and it may be as well to confine my remarks almost entirely to the year 1830. The 

 maximum height of the barometer, and the mean monthly maximum in that year, 

 both occurred in January, the former being 28*242 inches, thermometer 73°-6, and the 



