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ON ATMOSPHERIC WAVES 



[Sect. X 



" The barometer, at the level of the sea, does not indicate 

 a mean atmospheric pressure of equal amount in all parts 

 of the earth ; but on the contrary the equatorial pressure 

 is uniformly less in its mean amount than at and beyond 

 the tropics/' Vessels that are outward bound should 

 upon passing 40** north latitude, commence the series of 



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three-hourly observations, with an especial reference to 

 the equatorial depression. These three-hourly observa- 

 tions should be continued until the latitude of 40" south 

 has been passed : the whole series will then include the 

 minimum of the depression and the two maxima or apices 

 forming its boundaries. (See Daniell's ' Metcorol 

 Essays,' 3rd edition.) In passages across the equator 

 should the ships be delayed by calms, opportunities should 

 be embraced for observing this depression with greater 

 precision by means of hourli/ readings ; and these readings 

 will not only be valuable as respects the depression nere 

 spoken of, but will go far to indicate the character of any 

 disturbance that may arise, and point out, as nearly a 

 such observations will allow, the precise time when such 

 disturbance produced its effects in the neighbourhood of 

 tlie ships. In point of fact they will clearly illustrate the 

 diversion of the tendency to rise, spoken of in the Report 

 before aJbided to, as resulting in ascending columns and 



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sheets, between which wind flaws, capricious in their 

 direction and intensity, and often amounting to sharp 

 squalls, mark out the course of tlieir feeders and the 

 indraft of cooler tiir from a distance to supply their void. 

 Hourly observations, with especial reference to this and the 

 following head of enquiry, should also be made off the west- 

 ern coast of Africa during the homeward-bound V0y..2;e. 



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