{ 415 ) 



On the Influence of Atmospheric Pressure on the Tidal Waters 

 of Cornwall and Devon. 



While on the subject of the heights in this district above 

 the level of the sea, it may be useful to observe that those whose 

 attention has been directed to the subject of heights of land 

 above tidal seas are by no means agreed as to the time of tide 

 which would best afford a level whence to obtain them. In 

 4iiis district, the north and south coasts are under very different 

 conditions as to tidal levels at the time of high tide. On the 

 north, from the form of the Bristol Channel, the tide wave on 

 the flood is so driven into a gradually diminishing channel, 

 both as regards depth and breadth, that high water-mark at 

 Cape Cornwall is beneath the level of high water-mark at Chep- 

 stow and Bristol ; and consequently intermediate heights, if 

 calculated from high water at the two ends of the channel, 

 would not agree. On the south coast, from its oceanic charac- 

 ter, levels are probably more even at high water ; but a glance 

 at Mr Wheweli's valuable chart of the variable range of tides 

 round the British Islands, wiU shew that this coast forms a por- 

 tion of the general variations in the height of the tide observed 

 among these islands ; and it will be obvious, from the different 

 conditions of the tidal waters on the south and north coast of 

 the district under consideration, that heights taken from high 

 water on one coast would not coincide with those taken from 

 high water on the other. 



It has been generally considered that low water is the best 

 time of tide to take as a level whence to calculate heights on land, 

 it being supposed that the sea is then more equally distributed 

 amid shoals and along shores than when driven over the for- 

 mer, or along the latter, by the force of a flood tide, or during 

 the run of the ebb ; and consequently heights on land, sur- 

 rounded by tidal seas, are generally computed from low water- 

 mark. 



Of late, chiefly in consequence of the observations of Mr 

 Walker, Assistant-Master Attendant in her Ms^esty's Dock- 

 yard, Devonport, and of Captain Denham, R. N., much atten- 

 tion has been called to the subject of half tide as being the level 



