of the Fishery Board for Scotland. 
365 
It appears from this table that the average change of temperature 
on the surface between morning and afternoon is about a degree, and 
the daily range of temperature on the bottom something more than 
half a degree. The changes are sometimes in one direction, sometimes 
in the other, and the mean of the whole series shows a slight warming 
in the afternoon. 
The tidal relation of the water is remarkably simple, and affects the 
bottom almost equally with the surface. It may be summed up in 
the following statement. 
In the warm months when high tide occurs in the morning, and 
low tide in the afternoon, the water in the afternoon is nearly a degree 
warmer than in the morning — the average of the month of June shows 
even 2° -25 of increased warmth. But in the warm months, when low 
tide occurs in the morning, and high tide in the afternoon, the water 
is about three-quarters of a degree colder in the afternoon than in the 
morning — the average of the month of June shows this afternoon 
cooling to be as much as 1°'8. In the cold months the phenomena are 
reversed. When the tide is high in the morning and low in the after- 
noon, the water is about half a degree colder in the afternoon than in the 
morning — in December as much as 1°'0. When the tide is low in the 
morning and high in the afternoon, the water is about three-quarters 
of a degree warmer in the afternoon — in December as much as l°-6. 
It thus appears that the tidal effect on temperature is stronger than 
the solar. In summer, no 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 cold the night may have been, the water warms 
steadily until the hour of high tide. The explanation is simple and 
sufhcient. The 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, the Abertay light-vessel floats in Tay water at low tide, in 
North Sea water at high tide. 
The changes of temperature are very marked and must affect St 
Andrews Bay to a considerable extent. The period of spring tides, 
which here occur in the afternoon, is that at which the effect of the 
warming in the forenoon and chilling in the afternoon is most apparent 
in summer, the ordinary rise of temperature being checked. At spring 
tides in winter, the low tide phase, occurring in the forenoon^ is marked 
by great chilling of the water due to the cold outflow of the Tay, 
and the high water in the afternoon is marked by a rise of temperature 
much greater than that due to the feeble effect of sun heating. 
This suggests a method by which it may be possible to detect whether 
fluctuations of temperature and salinity (as will be presently explained), 
such as actually occur, are sufficient to produce any effect on the move- 
ment of fishes. The fishermen about St Andrews Bay and the mouth of 
the Tay might be interrogated as to whether they have noticed any 
regular variation in the catch according to the state of the tide. The 
trawling statistics of the ' Garland ' might also be discussed with regard 
to the number and species of the catch at different hours and tidal phases. 
Even although no definite information is afforded by such a discussion, it 
can hardly fail to suggest further matter for investigation. 
The specific gravity observations at Abertay were taken with the same 
regularity and accuracy as the temperature, and they have been discussed 
with equal detail. On account of the slight difference of salinity with 
depth, it was considered sufficient to take account of the intermediate ob- 
servation at 4 fathoms only, and instructions have been given to the 
observers that in future the three sets of observation — at surface, 4 
fathoms, and bottom — will be sufficient. 
