294 Remarks on some of the disturbing causes QNo. 136. 



stantaneously through a long column of air, and hence it appears 

 desirable, that the comparative observations should be made about 

 the times of maximum and minimum of the atmospheric tide, when, 

 the variations for a considerable time being almost insensible, it may 

 be supposed that the causes will act with least disturbance. Another 

 practical reason for selecting the times of maximum and minimum 

 is, that perfect simultaneousness being seldom to be expected, it 

 is evidently of advantage to select for observation those times at 

 which the want of this condition will produce the least effect. About 

 the middle of the tide, the barometer generally will vary as much 

 in the course of five minutes, as it will in half an hour from the 

 time of maximum or minimum. 



Though these remarks seem true as far as they go, yet in particular 

 cases, the atmospheric tides may be so affected by circumstances of 

 locality, as to present anomalous results. I am unable at present to 

 quote 'the documents containing the observations which gave occasion 

 to what I am now about to state, but the results were so uniformly 

 and repeatedly observed, that beyond settling the precise numerical 

 amount of discrepancy, the possession of the original observations 

 would add little to the evidence. 



When I was in charge of the Bombay Trigonometrical Survey, I 

 made many barometric observations in the Dekhan and along the 

 Sea Coast. These were compared sometimes with those made at the 

 Engineer Institution in Bombay ; sometimes with the observations 

 made by Colonel (then Major) Sykes at Puna; and sometimes with 

 those of a barometer left in Puna for the purpose. All the observa- 

 tions on the Sea Coast compared with those made inland from the 

 face of the Ghats, as at Puna agreed in one result, but I shall confine 

 myself more particularly to the results of a special comparison for de- 

 termining the height of Puna above the Sea. 



One of my barometers had been repeatedly boiled, I believe up- 

 wards of twenty times, and it was so perfectly free from air, that when 

 set up, the mercury used to adhere to the top of the tube six inches 

 above the level at which it stood when shaken. The tube was full 

 32 inches long, and the mercury adhered to the top at a station up- 

 wards of 4000 feet above the Sea, where the proper height of the mer- 

 cury was about 25.05m. This barometer was compared for several 



