THE SCOTTISH MARINE STATION AND ITS WORK. 225 



quarters of an hour, when the speed diminishes, and near 

 high water it is the same as that of the rising tide along the 

 shore. The ebb is gradual at first, but when the entrance 

 has been narrowed by the exposure of its banks, it is accele- 

 rated for about an hour and a half. Then it runs out very 

 sloAvly, its exit continuing until the flow recommences. For 

 some five hours, however, the level of the water inside is 

 practically unchanged. The depth of water in the quarry 

 is fi'om five to eight fathoms at low water. Temperatures 

 were taken of the air and of the water, both at the surface 

 and the bottom, at as short intervals as circumstances 

 allowed, in some cases every half hour for thirty-six hours 

 consecutively. The results of these observations are thus 

 summarised by Dr. Mill : 



'' (1) During daylight the air was always at a higher 

 temperature than the water, but after sunset the water was 

 warmer than the air ; and taking an average for the whole 

 period, the mean temperature of the air was the higher. 



" (2) The surface temperature followed that of the air, 

 and was little affected by tidal changes. 



" (3) The bottom temperature followed that of the air, 

 but the crest of the heat wave was retarded by several hours, 

 and the curve was profoundly modified by the tides. 



" (4) The temperature was higher at the surface than 

 at the bottom during the day ; but, as a rule, it was higher 

 at the bottom than at the surface by night. 



'' (5) When the tide flowed in the early morning it exer- 

 cised a cooling effect on the bottom thermometers, but when 

 it flowed at other times it produced a warming effect. ^^ 



This preliminary piece of work naturally led to an inquiry 

 into the physical conditions of the Firth of Forth, with 

 reference, in the first place, to the temperature and salinity 

 of the water at vai'ious times and states of the tide. For 

 the former purpose Negretti aud Zambra's deep-sea thermo- 

 meter, which registers by inverting, has been used. It is 

 shown in Fig. 5. The neck of the bulb has a contraction 

 at A, beyond which is a reservoir, b, whilst a small receptacle, 

 c, is provided at the other end of the tube. When the 

 instrument is placed bulb downwards the mercury contracts 



