Tidal Waters of Cornwall and Devon. 417 



mercury in the barometer, corresponds with a rise of six- 

 teen inches in the level of the sea, more than would other- 

 wise happen at the same time, under the other general con- 

 ditions ; a rise in the barometer of one inch marking a cor- 

 responding fall in the sea-level of sixteen inches. This he 

 has found to be the usual rate of such alterations in level; but 

 very sudden changes in the pressure of the atmosphere are 

 accompanied by elevations and depressions equal to twenty 

 inches of sea-water for one inch of merdury in the barometer. 

 Regarding the whole pressure of the atmosphere over the globe 

 as a constant quantity, all local changes in its weight merely 

 transfer a part of the whole pressure from one place to another ; 

 and hence he concludes that the subjacent water only flows 

 into, or is displaced from, those areas, where, for the time, the 

 atmospheric pressure is either less or greater than its mean 

 state, in accordance with the laws which would govern the con- 

 ditions of two fluids situated in the manner of the atmosphere 

 and sea. We might account for the difference observed by Mr 

 Walker, in the amount of depression or elevation of sea-level 

 produced by sudden changes in atmospheric pressure, by con- 

 sidering that a sudden impulse given to the particles of water, 

 either by suddenly increased or diminished weight in the at- 

 mosphere, would cause a perpendicular rise or fall in the man- 

 ner of a wave beyond the height or depth strictly due to the 

 mere chanfre of weight itself.* 



* A circumstance connected with this subject, of considerable practical 

 value, has been noticed by Mr Walker, during his long-continued observa- 

 tions. He has found that changes in the height of the water's surface, re- 

 sulting from changes in the pressure of the atmosphere, are often noticed 

 on a good tide-gauge, before the barometer gives notice of any change. 

 Perhaps something may be due, in these cases observed by Mr Walker, to 

 tlie friction of the mercury in the barometer-tube, as it is well known that, 

 in taking careful barometrical observations, it is necessary to tap the in- 

 strument frequently and carefully, to obtain the measure of the true weight 

 of the atmosphere at a given time and place. The practical value of the 

 observation is, however, not the less, be the cause of the phenomenon what 

 it may ; for if tide-gauges at important dock-yards shew that a sudden 

 change of level has taken place, indicative of suddenly-decreased atmo- 

 spheric weight, before the bai-ometer has given notice of the same change, 

 all that time which elapses between the notices given by the tide-gange 

 and barometer, is so much gained ; and those engaged with shipping know 

 the value of even a few minutes before the burst of an approaching hurricane. 



