230 THE SCOTTISH MARINE STATION AND ITS WORK. 



earlier than farther seaward, and, conversely, that as the sea 

 is approached the i-ange becomes less and the date of the 

 maximum is retarded. At Alloa the annual range would 

 appear to be about 35° F., at Queensferry about 20°, while 

 at the Isle of May it probably does not greatly exceed 10°. 

 The extreme temperatures observed in this last locality were 

 55° in August and 43° in December. These results and 

 certain others are very ingeniously exhibited by Dr. Mill in 

 a diagram constructed by means of polar co-ordinates.* 



As regards the admixture of sea- water, it is found that the 

 density increases at first very rapidly, and then more gradu- 

 ally as the sea is approached. The mean density at Alloa 

 for the period during which observations were carried on was 

 1-00042, whilst at the Isle of May it was 1'02511. When 

 the tide rises in the upper part of the estuary the salt water 

 comes up underneath the fresh, damming it back and gradu- 

 ally mixing with it. The influence of the smaller rivers is 

 not perceptible in the centre of the Firth ; each freshens a 

 tract along the shore apparently not more than a mile wide. 



An interesting phenomenon observed was a slight fall in 

 the density of the water just at the mouth of the Firth, 

 which was subsequently shown to be due to the fresher water 

 of the Tay carried southward by the flood tide. 



From the Firth of Forth it was only natural to pass to the 

 Firth of Clyde, and the examination of this region presented 

 a variety of questions of great interest owing to the uneven 

 condition of its bed, whilst the investigation is facilitated 

 by its accessibility at all times of the year. A broad sub- 

 marine plateau stretches across the mouth of the Firth 

 between the Mull of Cantyre and the Ayrshire Coast, and 

 this, in conjunction with the fact that the opening is to the 

 southward into the Irish Sea, diminishes the effect of the 

 ocean water of the Atlantic. A deeper channel runs up on 

 either side of the Island of Arran, that on the east extending 

 directly up into Loch Fyne, where in the neighbourhood of 

 Tarbert it attains a maximum depth of over one hundred 

 fathoms. Between Cumbrae and Bute there is a branch of 

 this depression, whilst a third commences north of the 

 * ' Proc. Roy. Soc. Edin.,' xiii, pi. vi, fig. 2. 



