f 
NATURAL HISTORY OF THE BANKS OF THE TAY. 39 
But if temperature data regarding the riYer proper are scanty, 
I this is by no means the case as regards the estuary, where a series 
I of the most minute and elaborate observations was commenced at the 
I 
I Abertay Lightship in June, 1889, and is still continuing. These 
I results are published in full in the Report of the Fishery Board for 
; Scotland for 1891, and an abstract is given in the Report already 
referred to. We may here recount some of the more interesting 
phenomena which they reveal. The Abertay Lightship is anchored 
at the mouth of the estuary. In summer time, if low tide occurs in 
the afternoon, the water, as would naturally be expected, has warmed 
since the morning. But if high tide occurs in the afternoon, the water 
is found to have cooled since the morning in spite of the heating 
influence of the sun. In the winter months on the other hand, these 
conditions are exactly reversed. A low morning tide warms up to a 
high afternoon tide, but a high morning tide cools down to a low 
afternoon tide. The explanation of these phenomena may be given 
in the words of the Report itself;—“It thus appears that the tidal 
effect on temperature is stronger than the solar. In summer, no 
j matter how hot the day may be, the water at the Abertay lightship 
: cools steadily until the hour of high tide; in winter, no matter how 
j cold the night may have been, the water warms steadily until the 
II hour of high tide. The explanation is simple and sufficient. The 
i temperature of the water of the Tay is always higher in summer and 
lower in winter than that of the sea, and putting the case generally, 
I the Abertay light-vessel floats in Tay water at low tide, in North Sea 
' water at high tide.” 
CONCLUSION. 
Such a river system as that of the Tay requires for its full study 
the conjoined labours of many specialists. The present paper 
attempts to give an idea of the river system as it is to-day, the 
I channel by which the drainage of a large tract of country reaches 
, ' the sea. From this point of view a river basin is to be regarded as 
; a complex system of sloping surfaces, the lower edges of which meet, 
; one and one, along a sloping line which represents the bed of a 
! stream. These bed-lines and their related slopes are in turn tributary 
to longer and more gently sloping bed-lines formed by the meeting of 
wider and less steep valley slopes. At the end where the slope of the 
; bed-line disappears by reaching sea level, the free flow onward of the 
water is retarded by meeting the sea which already occupies the great 
hollow beyond the land. The amount of this retardation and the 
manner of mixing depend on the saltness of the sea-water and the 
strength of the tides. So far as available data go this paper has tried 
to describe the Tay with respect to these features. A great agent of 
