CLYDE SEA AREA. 11 



degree, or occasionally each half degree, are inserted ; the spaces between being coloured 

 as in the vertical sections. By this means the depth to which any particular tempera- 

 ture penetrates, and the date at which it reaches that depth, are shown at a glance. 

 The completeness of such a diagram obviously depends on the frequency with which 

 observations were made, and the coincidence of the periods of maximum and minimum 

 penetration with a date of observation. Characteristic deep-water stations were selected 

 for this treatment, including the Channel, Skate Island, and Garroch Head in the Arran 

 Basin, Gantock and Dog Rock in the Dunoon Basin, Stuckbeg in Loch Goil, Strachur in 

 Loch Fyne, and Shandon in the Gareloch. 



The rate of seasonal descent of the maximum temperature was worked out from these 

 sections and exhibited in the form of a curve. Special functions of temperature-change 

 were selected for graphic treatment in certain cases, and these will be described in their 

 proper place. 



The regional distribution of temperature was worked out for each cruise by the 

 use of charts, on which the temperature at the surface, at 5, 15, 30, and 50 fathoms, as 

 ascertained from the vertical curves, were laid down. These charts, however, it has not 

 been considered necessary to publish, two specimens only being given. 



Terminology Employed. — In order to refer concisely to the different temperature 

 conditions indicated by curves and sections, it is necessary to define certain expressions 

 which I venture to use with special meanings. A mass of water at uniform 

 temperature throughout is termed homothermic, and the curve of vertical temperature 

 corresponding to this condition (a straight vertical line) is called a homothermic curve. 

 Similarly, a mass of water, the temperature of which varies from point to point, 

 is said to be heterothermic, and the corresponding curve expressing vertical distribution 

 of temperature at any point is called a heterothermic curve. The heterothermicity 

 of a mass of water from surface to bottom (this direction is the only one here 

 considered) may be of several kinds, each typical arrangement giving a curve of 

 special character. Thus, when the temperature is highest on the surface and lowest 

 at the bottom, the prevailing character in summer, the resulting curve is said to have 

 positive slope ; when the temperature is lowest at the surface and highest at the bottom, 

 the curve is said to have negative slope. The slope of a curve is arbitrarily measured 

 by the difference between the mean temperature of the superficial layer of five fathoms 

 and that of the bottom layer of five fathoms. It is thus practically the same as 

 vertical range of temperature between these 5 -fathom layers. When the curve 

 is of one curvature throughout, it may be (l) straight when the rate of change 

 is uniform all the way from surface to bottom ; (2) 'paraboloid when the rate of 

 change is greatest in the superficial layers, and diminishes downward ; and (3) inverted 

 when the rate of change of temperature becomes greater as the depth increases. 

 These typical curves frequently occur in combination : thus a large mass of water 

 may be homothermic, while an upper or lower layer may exhibit any one of the varieties 

 of heterothermicity. Of these mixed types, two at least may be mentioned — the 



