CLYDE SEA AREA. 119 



As these tables show, the loch was in an almost perfectly homothermic state at the 

 period of annual minimum in March or April, and returned to a similar condition in 

 September or October, just after the annual maximum. At other times there was a con- 

 siderable range of temperature between surface and bottom, and also between the mean 

 vertical temperatures at the various stations, the range from one to another being 1° or 

 2°. The change of temperature between contiguous stations seems to be due as much to 

 the influence of wind as to that of season. 



In order to allow an exact comparison to be made with the other natural divisions, the 

 temperature of the surface layer 5 fathoms deep was calculated, and the result is expressed 

 together with the mean in the curves of fig. 47, Plate XXXI. , on which the average 

 monthly temperature of the air at Helensburgh is also shown. For the whole period 

 under observation the average temperature of the superficial slice of water was identical 

 with that of the mass, being a fraction of a degree higher during the season of heating, 

 and a fraction of a degree lower during the season of cooling. The two curves coincided 

 at the annual maximum and minimum. It is consequently sufficient to consider the 

 variations of the temperature of the mass as a whole. 



Starting from the minimum of 41° "6 on about April 15th, 1886 (about half a degree 

 lower than that of any of the other divisions), the Gareloch reached its maximum of 

 54°'l on September 15th, a gain of 12°"5 in 153 days, or at the average rate of 0°*081 

 per day. The period and amount of heating were practically identical with those of the 

 Channel, and the rate of gain of temperature was twice that of Loch Fyne. The mini- 

 mum of 42°*5 was reached on February 28th, 1887, indicating the loss of 11 0, 6 in 166 

 days, the rate being o, 070 per day. Here again the rate of change of temperature was 

 twice that in Loch Fyne, and slightly greater than in the Channel. The next maximum, 

 58°*0, occurred on July 23rd, the duration of heating being only 145 days, more than a 

 month less than the time of heating at any other place. The rise of temperature was 

 15° "5, and the average rate of heating as much as 0° - 107 per day, being the greatest 

 observed in any mass of water in the Area. The shallow Gareloch, on this occasion, was 

 naturally enough the division to respond most rapidly to the exceptional solar radiation 

 of 1887. The following fall of temperature was prolonged until March 31st, 1888, when 

 it reached a minimum of 41 0, 9 after 251 days, the longest period of cooling recorded 

 in any division. The total loss of temperature was 16°'l, and the rate of cooling o, 064 

 per day, the same as the rate in the Channel. The last maximum observed probably 

 occurred on September 15th, and reached 54°-8, a gain of 12 0> 9 in 168 days, again 

 the shortest period of heating in the Area. The rate of heating was 0°'079 per day. 



Table XLV. gives concisely the comparative statistics of heating and cooling in the 

 four typical divisions of the Area and in Loch Goil. 



The mean duration of heating for the Gareloch, as a whole for the three years 

 observed, was 162 days, and for the two periods of cooling 208 days. Taking into 

 account the two years for which comparable observations exist, the ratio of the time of 

 heating to that of cooling was 160 to 208 days, or 100 : 130. This may be compared 



