26 DR HUGH ROBERT MILL ON THE 



Trip XIII., September 1887. — With its central day 47 days after that of August, 

 this trip lasted from September 20th to 30th, with the exception of the 25th, 26th, and 

 27th, when a break-down of the "Medusa's" machinery and other causes prevented observa- 

 tions from being made. This was the last trip on which I made the observations 

 personally, and the last also on which the density of the water was determined. (Part 

 II. pp. G 8 4-6 8 5.) 



The temperature for August had been normal over the Clyde Sea Area, and that for 

 September about 1° below the seasonal average. The early part of September was stormy, 

 with frequent south-westerly gales ; but during the earlier part of the cruise (19th to 

 24th) there was an anticyclonic calm, with very light breezes and haze. The 26th and 

 27th were characterised by strong winds from N.W. or S.W. ; the 28th, when Loch 

 Strivan was visited, was calm, with low barometer and a northerly air ; while on the 29th 

 and 30th, the Gareloch and Dunoon Basin were studied amidst heavy squalls from the 

 north-east. 



During this trip special attention was given to the condition of the water in the East 

 and Central Arran Basins, several cross-sections being run in order to bring out the effect 

 of the proximity of land. 



The surface water was everywhere beginning to cool, although heat was still being 

 propagated downward in all parts of the Area. Observations by Dr Murray from the 

 " Medusa " in the Sound of Mull, and off Ardnamurchan Point in the open Atlantic, showed 

 a uniform temperature of 57° '3 from surface to bottom, where the depth exceeded 100 

 fathoms, on September 4th. In the Channel the average temperature throughout the whole 

 depth was 56° on September 21st ; over the Great Plateau itself it was 55° ; and practically 

 the whole mass of the Area was over 50°. The only tracts where the temperature was 

 lower were the deep loch basins (Loch Fyne and Loch Goil) and the deepest part of the 

 Central and Eastern Arran Basins below 60 fathoms. The deep water of the Area was 

 at no previous time, nor on any subsequent occasion, so warm as during this trip, which 

 may be taken as representing the maximum storage of heat. The observations early 

 in the month on Loch Ness and Loch Morar showed that the minimum temperature at 

 the bottom of these fresh-water lakes was 42°*1. 



From October 1887 to October 1888 the "Medusa" was at work on the Clyde Sea 

 Area or in the northern lochs almost continuously, and results of great interest were 

 obtained in each of the separate divisions of the Area. The continuous method of work- 

 ing, although of the utmost service in elucidating the changes in progress in different 

 basins (under the head of which they will be considered), was not so well adapted to bring 

 out the general character of the Area as a whole at definite periods, separated by approxi- 

 mately equal intervals of time. These observations form the subject of a special discussion 

 by Dr Murray on the effect of wind in producing temperature changes in sea and fresh- 

 water lochs, published in the Scottish Geographical Magazine for 1888, vol. iv. p. 345. 

 They are grouped together in what follows into a series of "trips" more or less com- 



