706 DR HUGH ROBERT MILL ON THE 



The observations quoted above serve to suggest an explanation of the action of the 

 tide. The cross section of the sill at Otter Ferry, over which the tide flows, is almost 

 exactly 0*01 square sea miles, the opening in fact is so large that if the whole loch basin 

 were pumped dry, and water from the Arran Basin admitted as a current running at 5 

 knots past Otter Spit, ten hours would be required to fill Loch Fyne. The mean tidal 

 increment of Loch Fyne is, however, only x^th of the volume of the loch, being repre- 

 sented by an average rise of 8 feet over a surface of 22 square sea miles in area, i.e., mea- 

 suring 0*029 cubic sea miles. Now if we were to suppose that a solid mass of water the 

 width of the entrance to the loch, and 8 feet thick, were to be inserted, sliding over the 

 surface, until in six hours the whole surface was raised by that amount, the average velocity 

 of the current at the Spit would be very nearly 5 miles an hour. Such a velocity is found 

 at spring tides but only for a short time, and this fact alone is sufficient proof that the 

 tidal current is not entirely superficial. That the tide sweeps across the bar at Otter 

 to the full depth is proved from the salinity observations already referred to, and if the 

 whole tidal increment entered in this way its average velocity would be about half a 

 knot. 



We might suppose that the flood tide entered in this way, carrying in 0*029 cubic 

 mile of water, containing 95 per cent, of pure sea water, and that a superficial stratum, 

 8 feet deep, from the whole surface of the loch poured out at ebb tide. This, judging 

 from our observations, would contain 88 per cent, of pure sea water. The amount of 

 fresh water carried in by each tide would be 0*001465 cubic mile — that carried out 

 0*003516, so that at each tide 0'002051 cubic mile of fresh water would be removed 

 from the loch. But to supply such an outflow of fresh water, and maintain the average 

 salinity, the effective inflow of rain water would require to be equal to 618 inches per 

 annum over the whole area. This demonstrates that the outflowing tide is not a mere 

 skimming of the loch. If we suppose that an amount of rain equivalent to a fall of 30 

 inches per annum over the whole area reaches the loch, the nett effluent of fresh water 

 at each tide would be 0*0001 cubic mile ; the ebb tide would then contain 94*6 per cent, 

 of sea water, while the flood tide contained 95 per cent. This is probably the order of 

 magnitude of the difference in salinity of the inflowing and outflowing tides. It 

 corresponds to a difference of 0*00010 in density at 60° F., and is a quantity which could 

 be demonstrated with perfect certainty by the hydrometer employed. The current of 

 ebb tide evidently affects the water to a considerable depth, at any rate in the Gortans 

 Basin. In Part III. of this memoir temperature observations will be adduced to show 

 that in all probability the currents of the Upper Basin of Loch Fyne are not produced 

 nor much affected by the tides. 



If, as we assume above, each tide brings in 0*0290 cubic mile of water, it appears 

 that the whole Loch Fyne Basin might have its contents changed in eighteen tides or 

 nine days. This is the minimum which can be assigned on the assumption of thorough 

 mixture of the tidal water throughout the mass, and it is probably not far from the 



