416 On the Influence of Atmospheric Pressure on he 



of least variation on tidal coasts, inasmuch as in the same place 

 the high water of any tide rises as much above the line of half 

 tide as low water falls beneath it. It has therefore been sup- 

 posed that the half-tide levels along the coast would coincide 

 with each other, due precaution being taken not to include those 

 estuary tides amongthem, where the low water-mark on the sea 

 coast is often considerably under the low water-level in such 

 situations, the waters running out on the inclined surface of the 

 bottom, in the manner of a common inland river. The greater 

 portion of the Bristol Channel would probably be considered 

 merely as an estuary, the ebb running out in the manner of a 

 j-iver over its bottom. 



In his researches on tides, Mr Whewell has noticed that the 

 mean heightife of the sea at Singapore and Plymouth are nearly 

 constant ; and we understand that, by tidal observations made 

 under his direction at the extremities of the line, levelled at the 

 expense of the British Association for the Advancement of 

 Science, between the English and Bristol Channels, it has been 

 ascertained that the level of mean tide at both ends of the line 

 is the same within a small fractional part of an inch. 



Mr Walker, who has long devoted much time to tidal phe- 

 nomena, considering that half-tide levels on oceanic shores, 

 such as a large part of those of Cornwall and Devon may be 

 regarded, give the equilibrium level of the sea, proposes a simple 

 method* for readily obtaining it, which, whatever opinion may 

 be ascertained of the general value of half tide levels, affords 

 very considerable facility in ascertaining that level at any given 

 place. ^ 



Mr Walker has observed with respect to the influence of 

 the pressure of the atmosphere upon the tidal waters on the 

 shores of Cornwall and Devon, that a fall of one inch of the 



* " Whcu the barometer stands at its mean annual height, and the air is 

 calm and still, set up a tide-pole (or select a rock) in some sheltered corner 

 on the coast. Mark upon it the high and low water-levels, and half-way 

 between these points will be found the mean level of the sea. Under the 

 above condition a single observation will give the mean level very nearly ; 

 but numerous observations are necessary when great accuracy is required.'* 

 — Wallzer's MSS. Mr Walker most obligingly communicated his notes to 

 us to be used in any way which might appear best calculated to promote the 

 progress of science. 



