SECT. 3] APPLICATIONS OF THE GYROPENDULUM 339 



loading can be assessed and eliminated and those associated with the astro- 

 nomical tide are small (Table II), it seems probable that slopes related to 

 seasonal or longer period influences may be observed in relatively pure form. 

 These arise, it is thought, from the regional average stress of wind on the sea 

 surface or the climatological factors influencing the distribution of water 

 densities within the body of the ocean. 



Except near coasts, where set-up may occur, the stress of the mean 

 wind should be accompanied by an equilibrium slope transverse to the average 

 wind direction. Slopes associated with horizontal gradients of water density 

 should parallel these gradients when motional equilibrium has been attained. 

 The time required to establish an equilibrium circulation and associated sea- 

 surface slope may range from pendulum days to many years depending upon 

 the latitude and on whether the response of the ocean is of a barotropic or 

 baroclinic nature (Veronis, 1956; Veronis and Stommel, 1956). 



As can be seen from Table II, the departures of the mean sea surface from 

 the horizontal are so small that in good weather it has been useful to consider 

 the average horizon as the boundary of a level surface on the earth. But the 

 circle of the horizon does not lie at 90° to the zenith, being in excess of this due 



AIR COOL 



'GEOMETRICAL 

 HORIZON 



-GEOMETRICAL 

 HORIZON 



Fig. 12. Effects of thermal lapse rate on the range to the visible horizon from a given 

 height of eye. (After Minnaert, 1954.) 



to dip or occasionally less than one right angle because of the effects of meteoro- 

 logical refraction (Brocks, 1950). The surface joining the circle of the horizon 

 with the observer's eye usually tends to be a very flat cone, apex up (or occa- 

 sionally down), that is apparently buckled by horizontal gradients in the lapse 

 rate of temperature in the air and sea, actually buckled by waves, tilted by 

 regional gradients of atmospheric pressure, and the steric or dynamic gradients 

 of surface elevation associated with fluid motion. 



The horizontal distance to the optical horizon varies with the air-sea tem- 

 perature contrast. When the air over the sea surface is colder than the water 

 the visible horizon tends to move closer to the observer than the locus of 

 tangent rays, and be pushed beyond the geometrical horizon when the air is 

 warm and the sea is cool, as suggested in Fig. 12. Altogether these effects tend 



