8 MR. AIRY ON THE LAWS OF THE TIDES 



The results for height in the subsequent sections of this paper are all referred to a 

 point thirty feet below the bolt at Buckingham Lock, Dublin. 



At each of the stations the course of observation was as follows : — The observer 

 adopted, as the tide which was to be completely observed, either the interval from 

 high water to high water, or that from low water to low water, according to the con- 

 venience of the hours. Thus, having begun, for instance, with commencing a tide 

 at the morning high water, when the high water occurred at convenient hours both 

 in the morning and the evening ; as the tides in the succession of days fell later and 

 later every day, the termination of the tide at last fell inconveniently late in the 

 evening, and the observer then began his observations about six hours earlier in the 

 morning, so as to commence with low water and to terminate with low water. After a 

 time it became necessary, in consequence of the evening low water occurring incon- 

 veniently late, to commence again with high water ; and thus there was in every few 

 days a change in the arrangement of observations. 



The observations were generally commenced about half an hour before the com- 

 mencing high water or low water, and were generally continued about half an 

 hour after the terminating high water or low water. Thus, of the four principal 

 phases which occur in each day (two high waters and two low waters), three were 

 effectually observed in the day series of observations. As there were at each station 

 at least two observers, one of these persons made observations for an hour or more 

 in the night, partly before and partly after the remaining high water or low water ; 

 and thus all the high waters and low waters were observed. This system had the 

 advantage of giving all the phenomena of diurnal tide, and giving one semidiurnal 

 tide completely observed in each day, with little distress to the observer. Its only 

 disadvantage is, that the observations at different stations do not always apply to the 

 same portions of corresponding tides ; but there appears to be no method of securing 

 this precise correspondence of observations except by incessant observations day and 

 night, or by self-registering tide-gauges. Each observer registered the height of the 

 water on his tide-pole at every five minutes by his watch. 



The watches were for the most part chronometers or lever watches. An officer 

 visited each station at least three times, and the greater number of the stations four 

 times, carrying a good pocket chronometer whose error on Greenwich time was 

 known. Two itinerant chronometers were thus employed. The error of each of 

 the observers' watches was afterwards computed for every day of observation from, 

 these comparisons, and this error was applied to form the corrected Greenwich time 

 of every observation, in a column purposely left in the sheets of observations. 



At two stations only, Ballycastle and Glenarm, the means for registering the time 

 proved imperfect. At the former, in consequence of the failure of the watch, the 

 time was taken from the town-clock, and corrected for the longitude of the place; 

 it is supposed that this time may be sometimes ten minutes in error. I much regret 

 that the extraordinary phenomena of the tides at Ballycastle are thus developed with 



