236 HYDROGRAPHICAL SURVEYING [chap, ix 



Effect of In a harbour such as Portsmouth, where extensive mud-flats 



5?"f" cover or uncover at about the same time when the water reaches 

 Flats. 



one particular level, its rise or fall will be arrested for a short 



interval whilst the water is overflowing or flowing off the flats. 



Directions The following method, taken from a pamplilet issued by the 



forreduc- Hydrograpliic Department, is that adopted at the Admiralty 



Observa- in discussing the observations, and deducing therefrom the 



tions. ^^^g hour or establishment, marked on the chart as H. W. F. 



and C. Also the mode of determining the mean spring range 



or rise, the mean neap range, and the mean neap rise : 



1. The time and height of high and low water should be 

 observed successively both by day and night ; this is absolutely 

 necessary to make the observations valuable ; and the watch 

 or clock by which the times are taken should always show 

 mean time at the place. 



2. The greater the number of daily results thus obtained, 

 the more accurate will be the value deduced from them ; but 

 six months' observations under normal conditions are absolutely 

 necessary for a definite determination. 



Where the conditions are varied at certain seasons, such as 

 obtains in different monsoons at different periods, a year's 

 observations are required. 



If, however, circumstances will not admit of a continued 

 series of tidal observations being made, the high and low 

 waters that take place on those days when the moon's 

 transit occurs between 11^ and 2' and &" and 8*^ should be 

 preferred. From the former (11'' and 2'') the tide hour or 

 establishment and the mean spring range are obtained, and 

 from the latter (6'' and 8') the mean neap range is known.* 



3. The times and heights of high and low water must be com- 

 pared with the moon's upper and lower transit, in order to 

 obtain the mean intervals and the mean heights (see Table I.). 

 The transit in the Nautical Almanack being given for the meri- 

 dian of Greenwich, the transit for any other meridian must 

 be corrected for longitude, a table for wliich is given in most 

 works on navigation. 



4. The times and heights are to be classified according to 

 the moon's transit, beginning with the high and low water 



* With regard to this, it is considered that the remark only applies to 

 the abnormal tides of Europe, and not to the Pacific tides, etc. 



