691 



pheric tides. Before we ascend to the first 

 causes, we must establish the empirical laws. 

 Those laws comprehend continuity (the want 

 of all irregular interruption), in the movements 

 of ascension, or lowering ; the limit-hours or 

 periods of the maxima and minima ; the dura- 

 tion of time that the barometer is apparently 

 stationary ; the mean extent of the horary vari- 

 ations in different latitudes and at different 

 heights ; the influence of the seasons, or the 

 phases of the moon on the tropical hours, and 

 on the extent of the variations. The observer, 

 who, in any spot on the earth, would throw 

 light on any part of so complicated a phenome- 

 non, must (even in the tropics, where the mean 

 drawn from a small number of statements, fur- 

 nishes results that are sufficiently exact,) relin- 

 quish every other kind occupation. To mark 

 the period and extent of the small successive 

 increase or decrease*, requires continual obser- 

 vation (observatio perpetua). The horary va- 

 riations of the barometer may be compared in 



* During the summer solstice, the equinox of autumn, 

 and the winter solstice of 1806, as well as during the spring 

 equinox and the summer solstice of 1807, I made continued 

 observations at Berlin, conjointly with M. Oltmanns, and 

 furnished with a magnetic glass of Prony, on the horary va- 

 riations of the magnetic inclination, during twenty-nine days, 

 and twenty-nine nights, every half- hour. The limits of the 

 errors were 6" to 8" in arc. 



2 z 2 



