J 4 A Synopsis of the principal 



the barometer being 29-92 inches, the refraction at the ho- 

 rizon is 39' ."14", 7." It would be only 30' 24'",1 it" its den- 

 sity diminished in arithmetical progression; and would be 

 nothinof at the surface. Tlie horizontal refraction, which 

 we observe about 33' ()",0, is a mean betvveen these 

 limits. 



When the apparent height of a star upon the horizon 

 does not exceed eleven degrees, its sensible refraction de- 

 pends only on the state of the barometer and thermometer 

 m the place of observation ; and it is nearly proportional to 

 the tangent of the apparent distance of the star from the 

 zenith, diminislud by 3| times the corresponding refraction 

 at this distance, the thermometer being considered as at the 

 freezing point and the barometer at 29"92 inches. It is 

 from these principles that have been formed the Tables of 

 Refraction, corresponding to the several variations in the 

 scale of the thermometer and barometer. 



The action of the Sun and Moon has a considerable ef- 

 fect on the water of the ocean, and produces the phaeno- 

 iTiena of the tides. 



The sea rises and falls twice in each interval of time 

 comprised between the consecutive returns of the Moon to 

 the same meridian. The mean interval of these returns is 

 !•* 0'' 50' 28", 3 : consequently the mean interval between 

 two followinii periods of high water is 12*' 25' 14", 3. So 

 that the rerardation in the time of high water, from one 

 day to another, is 50' '2 S" in its mean state: and it is af- 

 fected by all those causes which intluence the Moon's mo- 

 tion . 



This retardation varies with the phases of the Moon. It 

 is at its minimum towards the syzigies when the tides are 

 at their maxinium, and it is then only 38' 57"jl. But, 

 when the tides are at their minimum, or towards the qua- 

 dratures, it is then the greatest possible; and amounts to 

 I'' 14' 5S",8, 



The variation in the distances of the Sun and Moon from 

 the Earth (and particularly the Moon) has an influence 

 also on this retardation. Each minute in the increase or 

 diminution of the apparent diameter ot the Moon augments 

 or diminishes this retardation 3' 42", 9 towards the syzigies : 

 but towards the quadratures the efliect is three times less. 



The daily retardation of the tides varies likewise with 

 the declination of the Sun and Moon. In the syzigies, at 

 the lime of the salstices, it is about 2' 13",Q greater than 

 in its mean state : and it is diminished in the same propor- 

 tion, at the time of the equino.\es. On the contrary, in 



the 



