<)0 DR HUGH ROBERT MILL ON THE 



temperature of the vertical curve is greater by 0°"5 in B than in A. On the hypo- 

 thesis of equal rates of heating and cooling in sunshine and darkness respectively, 

 we find the rate of heating to average 0°'062 per hour of sunshine. This is twice 

 the rate deduced for July, and four times that for June. It is impossible to believe 

 that the heating power of the sun increases so enormously while its angle of incidence 

 diminishes, and hence the hypothesis of assigning to local solar radiation the whole, 

 or even a very large share, of the rise of temperature is obviously wrong. Pro- 

 bably the real determining conditions are very complex. They must indeed ultimately 

 depend on radiation, but we must look for an explanation to the general radiation 

 over the whole area involved, over land whence the heat is carried by wind and 

 surface water as well as over the water itself. Light on the question of the 

 process of thermal change may be looked for rather in the study of mass temperatures 

 than of such linear temperatures as are here noted. 



Figure 21, Plate XXVI. , gives the one case observed of the rate of cooling in a short 

 interval of time, and the interval is as much as five days. In January, of course, the dura- 

 tion of darkness was greatly in excess of that of daylight. In the case in question the time 

 between the soundings contained 51 hours of darkness and only 21|- hours of light. The 

 weather during the interval was usually overcast, with a good deal of rain and some 

 southerly wind. On the 2nd the air-temperature was about 35°, on the 3rd about 44°, 

 and on the 6th and 7th it rose in the afternoon to 49° or 50°. The great warmth of the 

 later days is shown by a slight rise in temperature of the upper 10 fathoms ; but from 20 

 fathoms to the bottom there is a steady fall, the earlier cold weather apparently having 

 produced effects which were steadily and uniformly working clown. The inversion of the 

 climatic change in the air greatly reduced the negative slope of curve B, but the mean 

 temperature was o, 2 lower than that of curve A. Supposing, as before, that cooling by 

 radiation proceeded at the same average rate in the dark as heating by radiation did in 

 sunlight, and also that the total thermal change was due to radiation, the rate of cooling 

 would appear to be 0° - 007 per hour. The disturbing causes are so numerous, however, 

 as to deprive this conjecture of any quantitative value. 



A cross-section was made on September 23rd from Laggan Bay, on the Cantyre side, 

 across the deep trough to Skate Bay, on the Cowal shore. The clay was hazy and dead 

 calm, and there had been no wind to speak of for three clays. The dry-bulb thermometer 

 in air read 49°'2, the wet-bulb 49° 3. The tide was about low water, being at the end 

 of ebb when the Laggan Bay observations were made, and at the beginning of flood 

 when the Skate Bay soundings were taken. It was practically slack water all the time. 

 It is remarkable that the isotherms (fig. 22, Plate XXVI.) dip strongly toward the eastern 

 side throughout the entire deptli of the water. The cause of this is not clear. There 

 was no wind to set up a circulation ; nor was the heating on the eastern side due to the 

 action of the shallow water, for the dip of the isotherms commenced westward of the 

 deepest sounding. There was the appearance of upwelling on the eastern side, and of 

 sinking of surface water on the western. No explanation of the phenomenon presents 



