50 DR HUGH ROBERT MILL ON THE 



Arran Basin at a given time with any approach to accuracy. All we can say definitely, 

 from the two cross-sections available, is that in summer the isothermal sheets are flexed 

 upward in the deepest water, and downward toward the shore, the abruptness of the 

 downward bend being in some inverse proportion to the gradient of the shore. 

 Were it not for the section (fig. 12) being made in a spell of anticyclonic calm, 

 the influence of a west wind might have been credited with producing the isothermal 

 arrangement shown. 



From observations I made previously on a wide sand-beach on the Firth of Forth, it 

 appears that in winter the water resting on the beach is chilled, relatively to that beyond 

 shore influence, in much the same proportion as it is heated by contact with the sand in 

 the summer. What is true of a tidal beach probably holds for shallow water generally, 

 and we may conclude that the isothermal sheets will be curved upward by shallow water in 

 calm weather during winter, when the shore waters are colder than those in mid-channel. 



In discussing the distribution of temperature in a mass of water, it is important to 

 consider isothermal sheets, which are never, except in rare circumstances, horizontal planes. 

 Isothermal lines plotted on a section simply represent sections of the isothermal sheets, 

 the convolutions of which, in some cases, are very complicated. An attempt might be 

 made to construct a model, showing by thin sheets of coloured gelatine the arrangement 

 of water-temperature in three dimensions, but it does not admit of easy diagrammatic 

 representation on paper. 



Observations between Garroch Head and Cumbrae Light. — The tongue of deep 

 water branching off from the East Arran Basin between Bute and Little Cumbrae 

 is the main channel through which the seaward water reaches the Dunoon Basin, 

 and through it the eastern lochs and estuary. Being close to Millport, the head- 

 quarters of the " Medusa," it was found possible to make very frequent observations 

 in a depth of 60 fathoms. 



The fifty-one observations made in this depression are very interesting, and might 

 be discussed in great detail. As, however, it is more important in some ways to study 

 minutely the effects of heat in the deeper water off Skate Island, I shall here treat only 

 of the main features of the Garroch Head observations. 



In form the curves resembled those obtained off Brodick, but the larger number 

 allows one to study the form of the curve in relation to season with some prospect of 

 trustworthy results. 



Positive slope was shown in the vertical curve on twenty-two occasions, zero slope 

 once, and negative slope twenty-eight times. The positive slope was always much 

 greater than the negative ; thus on fifteen occasions the surface layer of 5 fathoms 

 was more than 2°*3 warmer than the bottom layer of the same depth, while only once 

 was it more than 2° '3 colder. The maximum positive slope was 9° 7, the maximum 

 negative slope 3° "2. This shows in an interesting way how heated water keeps to the 

 surface, while cooled water sinks and equalises the temperature, the salinity at surface 

 and bottom not being very different at this station. 



