Chase.] 29 '^ [January. 



It will be seen that the purely theoretical height A corresponds 

 more nearly with the observations than the mixed height B. It is 

 therefore evident that there is a slight disturbance (which may per- 

 haps be owing either to variations of temperature, or to a resisting 

 medium), which follows a different law from the principal disturbance. 



The changes are least near the times of high and low tide, and 

 greatest midway between the two tides. If we compare the average 

 high and low tides, we see that the observed height is somewhat less 

 at high tide, and somewhat greater at low tide, than theory would 

 give. These results would naturally follow from the combined fluid- 

 ity and gravitation of the air. 



From Ih. to 15h. inclusive (during most of which time the radius 

 vector of each particle of air is increasing), the observed height of 

 the barometer is less than the theoretical height. 



From 16h. to Oh. inclusive (radius vector diminishing), the ob- 

 served height is greater than the theoretical height. 



The greater pressure before noon than before midnight, is precisely 

 the result which would follow from the passage of the earth through 

 a resisting medium, but it is directly opposed to the supposed ten- 

 dencies of varying temperature. 



The apparent difference in the laws that govern the aerial and 

 ocean tides may be partially, if not wholly, accounted for by con- 

 sidering the difference of constitution in the two media, and the re- 

 lative positions of the observer. The air is highly elastic and com- 

 pressible, while water is cohesive and incompressible; the observer 

 is placed underneath the atmosphere, but above the ocean. The air 

 can therefore readily yield to any expanding or condensing force, 

 without much perceptible motion, while a similar force applied to 

 water would produce motion in the direction of least resistance; any 

 force that tends to throw fluids away from any given portion of the 

 earth, produces a high aerial tide, but a low barometric tide, and 

 after some interval a high oceanic tide. 



The frequent coincidence of high water with a low barometer, has 

 been noticed by many observers, and it is strikingly presented in the 

 comparative drawings given by Lubbock, in his Theory of the Tides. 

 The prompt effect of rotation, combined with the retardation of the 

 cumulative action which produces the lunar tides, may perhaps ac- 

 count for the errors of theory in Lubbock's Table of the 



