NATURAL HISTORY OF THE BANKS OF THE TAY. 89 
rulo, the water has the higher temperature, which is the reverse of 
what we found in our analyses of the water during the month of 
January. 
There is still a third table, showing the composition of the water 
supply for the city j but, before examining it, we wish to state our 
reasons for coming to the conclusion that the Tay tidal water, or sea 
water, passes in larger or smaller quantity to a point at least loo 
yards above the lower filter-bed :—• 
1. In the first table the sample from Perth water supply (No. 7 in 
table I.) gives total solids 5-2 and chlorine *9 parts per 100,000. The 
river was very low before Tuesday, 31^1- January, and it was nearly 
full moon, giving us high tides. The water drawn from the service 
pipe on Tuesday must have passed in on the Sunday or Monday 
afternoon when we were having very high tide. In table II. the same 
thing occurs with samples VI. and VII. 
2. Six samples were drawn from the Tay on two different days at 
full tide, one sample being taken from the top or surface water, and 
the other from the bottom, and it was found on each occasion that 
the amount of chlorine in the surface water was '75 100,000, 
and in the bottom water ’85 per 100,000. These six samples were 
drawn on 23rd and 24th November, full moon, and therefore high 
tide being on 23rd about 3 p.m. This gives 1*22 parts of common 
salt in the surface water to i‘4o parts in the bottom water per 
100,000 of the water, or an increase in the bottom water of about 
15% of common salt. This proves what was suggested in the first 
paper on the physiography of the Tay, viz,, that the heavier water 
remains at the bottom, and contains a perceptible amount of sea 
water. 
3. The action of Perth water upon the inside of the iron pipes of 
the service supply also indicates the presence of sea water. This 
action will be shown and explained, we understand, by Mr. John 
Young, civil engineer, and is, we expect, due to the presence of chlo¬ 
rine or chloride of sodium in the water. It is found more or less in 
all pipes, and the amount of it indicates the amount of chlorides 
present. From the immense deposit in the Perth pipes, we are 
warranted, we think, in saying that more chloride must pass through 
these pipes than the chlorides found in the Tay water above Perth, 
and, further, that this excess of chlorides is got from the tidal water. 
Let us now examine this third table, which gives a number of 
analyses by at least four different analysts of the Tay water after filtra¬ 
tion as supplied to the city. We forbear to give names, but every¬ 
thing has been done to ensure accuracy and to endeavour to get at 
the truth concerning our water supply. Some of these samples were 
drawn and sealed up by two independent gentlemen, one sample, as 
