CLYDE SEA AREA. 55 



the distribution of temperature in time and depth, the general history of the warmth 

 cycle at the Garroch Head station appears at a glance. The section on Plate III., fig. 3, 

 is interesting when compared with that for the Channel. The isotherms have no longer 

 the simple form and uniform perpendicular direction, but the lagging of the deeper water 

 is a conspicuous feature. The colours indicative of high temperature are widest at the 

 top, but taper to a narrow band at the bottom, where the colours showing cooler water 

 occupy proportionally more space. Just after the maximum and after the minimum 

 temperature of the whole mass of water has been reached, the bands of colour lie 

 vertically as in the Channel section. In the course of heating it appears by the section 

 that the surface reached 50° on 15th June 1886, and remained above that temperature until 

 the 19th November. The isotherm of 50° reached the depth of 30 fathoms on August 

 27th, and the bottom (65 fathoms) on September 14th; so that the bottom was three 

 months behind the surface in warming to a given extent. But temperature at the bottom 

 only remained above 50° until December 10th, or only 20 days later than the surface. So 

 that in cooling the lag of the season at the bottom was only one quarter as much as in 

 heating. On this occasion the isotherm of 54° reached its deepest point (15 fathoms) 

 on September 14th. 



In 1887 the surface was at 50° on May 12th : that isotherm had worked its way down 

 to 30 fathoms by August 4th, and to the bottom by August 17th. By November 14th, 

 or after six months, the surface temperature was again 50°, and the same temperature 

 was reached at 30 fathoms on November 18th, and at the bottom by November 20th. 

 Thus, while the warming at the bottom lagged more than three months behind the 

 surface, the cooling was only six days behind. The rate of warming was interrupted by 

 a comparatively cold spell early in July, but the isotherm of 54° reached 35 fathoms, its 

 deepest point, on September 27th. 



The observations for the maximum of 1888 are not complete, but the isotherm of 50° 

 never seemed to reach the bottom at all, and 54° only penetrated to 7 J fathoms. 



Comparing 1886 and 1887, we see that the warm period (denoting by this expression 

 the time when the water-temperature was over 50°) lasted at the surface respectively for 

 5 and 6 months, while the warm period at the bottom lasted respectively for barely 3 

 and scarcely more than 3 months ; in other words, for just half the time. 



The slow penetration of heat giving a great inclination to the isotherms, scarcely 

 extended below 35 fathoms, or half way down. In the lower half the temperature 

 changed nearly homothermically ; as it did throughout for the greater part of the year. 



The difference from the Channel is shown by the reduction of the range of temperature 

 in the lower layers, and its retardation in date. This is evidently due to reduced facility 

 for mixing the deeper and the superficial layers of water. The deeper half of the water 

 — that lying near or below the level of the bar separating the north-eastern branch from 

 the main East Arran Basin — behaves very similarly to the Channel water, heating and 

 cooling on the whole nearly simultaneously throughout its whole extent. The sharp 

 contrast lies between this mass and the surface layers. The greater range of salinity is 



