54 
MR. AIRY ON THE LAWS OF INDIVIDUAL TIDES. 
greater age inferred from the heights * is certain, as it is one given by observations 
which (here according with theory) show that the rise of tide occupies a shorter time 
at springs than at neaps. Some other cause must therefore exist, of such a kind as 
to make the tide later in springs than in neaps, and of such a magnitude as com- 
pletely to overrule that already assigned. I conceive that this cause may be the 
increased range of the tide in horizontal extent up the river, at springs ; by which 
the tide is changed from a gulf-tide to a continuous-canal-tide, which travels more 
slowly. But without more rigorous theory, I dare not pronounce upon this point. 
Royal Observatory , Greenwich , 
February 10 th, 1843. 
* Philosophical Transactions, 1842, p. 8. 
