THE UNDERGROUND WATERS OF NORTH-WEST YORKSHIRE. 25 
2. The cause of the adoption of the several routes. 
3. The conditions of underground flow of water, with special 
reference to the retention of reagents for long periods, and the 
apparently capricious effects of the Tarn flushes upon the outflow 
at Malham Cove. 
4. The nature of the underground spaces in which the 
water is contained during its transmission. 
The second is the main problem, viz. : What is the cause 
which has determined the route taken by each flow after its 
absorption through the swallow-holes? In dealing with this, the 
other points named will receive some explanation. 
The history of the early speculations upon this subject has 
shown how prone observers have been to assume that, where a 
dry valley, obviously the result of stream erosion, connects a sink 
with the place of emergence of a stream, the flow will follow 
the same general course under ground that it used to do above. 
This very natural assumption has been made in the case of the 
Tarn Water Sinks and Malham Cove. It seemed in some 
measure supported by Messrs. Morrison and Tate's experiments, 
when all the Tarn flushes afl'ected botli Aire Head and Malham 
Cove. Moreover it seemed diflicult at one time to account on 
any other hypothesis for two circumstances : — 
1. That the water issuing at Aire Head had crossed the 
track of the Cove stream unless the two stream courses were 
coincident. 
2. That the Cove stream enormously exceeded in volume 
the Smelt Mill stream, which was not large enough to account 
for the former, as suggested by Dr. Whitaker and local tradition ; 
besides which the hardness of the Cove water was less than that 
of the Smelt Mill, notwithstanding the limestone strata between. 
The experiments of the Committee, however, have shown 
that (at least in June, 1899) no appreciable quantity of water 
flowed in the direction usually assumed. 
The explanation which here follows, suggested in the first 
instance by Mr. Kendall from certain a j^riori considerations, 
was confirmed by an inspection of the Ordnance six-inch maps 
