148 POPULAR LECTURES AND ADDRESSES. 



hydrostatic equilibrium causes the surface of the 

 water to be lower, where it is pushed down by the 

 greater weight of air, and to be higher where there 

 is less weight over it. It does not follow that in 

 every case it is lower, because there may not be 

 time to produce the effect, but there is this tendency. 

 It is very well known that two or three days of low 

 barometer make higher tides on our coast. In 

 Scotland and England and Ireland, two or three 

 days of low barometer generally produce all round 

 the shore higher water than when the barometer is 

 high ; and this effect is chiefly noticed at the time 

 of tidal high water, because people take less 

 notice of low water as at Portland where they 

 think nothing of the double low water. Hence 

 we hear continually of very high tides very 

 high water noticed at the time of high tides 

 when the barometer is low. We have not 

 always, however, in this effect of barometric 

 pressure really great tidal rise and fall. On the 

 contrary we have the curious phenomenon that 

 sometimes when the barometer is very low, and 

 there are gales in the neighbourhood, there is very 



