THE SPRAT. 137 



line, or edge, which is so distinct as to prove 

 an index to the fisherman in the darkest night. 

 Dr. Parnell informs us, that sprats are found in 

 the Frith of Forth throughout the whole of the 

 year ; and that, like many small animals, they 

 appear to be very susceptible of cold. " During 

 the warm summer months, they are seen sport- 

 ing about in large shoals, in every part of the 

 Frith, occupying a considerable extent of 

 water, and causing a ripple on the surface with 

 their fins, while they become the principal food 

 of many marine birds, which assail them in the 

 water, or prey on them from above. As the 

 cold weather advances, these little fish are no 

 longer seen in the lower part of the estuary, but 

 are found to ascend the Frith to a considerable 

 distance, and to select that part of the river 

 where the fresh and salt waters mingle toge- 

 ther ; for it is a well-known law in chemistry, 

 that when two fluids of different densities come 

 in contact, the temperature of the mixture is 

 elevated for a time in proportion to the differ- 

 ence in density of the two fluids. Owing to 

 mutual penetration and condensation, such a 

 mixture is constantly taking place in rivers 

 that run into the sea, and the temperature of 

 the mixed water is accordingly elevated. In the 

 year 1830, the sprat was remarkably abundant 

 E 2 



