on Rivers and Canals . 623 
depths of the water, above that limply neceffary to keep 
them afloat, is a thing not at all indifferent. Dr. frank- 
lin has already treated this fubjedt, though perhaps 
not with fufficient accuracy, in a letter to Sir john 
pringle, written in the year 1769. He proves, how- 
ever, that it is univerfally known among people accuf- 
tomed to work boats on canals, that there is a confidera- 
ble difference in the fwiftnefs of their motion according 
to the greater or lefs depth of the water therein; 
and that the water being low is of itfelf fufficient to 
retard the motion of a boat, without the keel thereof 
rubbing againft the bottom of the canal. The reafon he 
afligns for it is evident ; for a boat cannot advance its own 
length in a canal without difplacing a quantity of water 
equal in mafs to the fpace which the boat occupies under 
the furface of the fluid. The water fo difplaced muff re- 
trograde, and pafs under, and to the right and left, of the 
boat : fo that the lefs depth and breadth of water there 
is in the channel, the more in proportion it muff rife up 
and weigh againft the boat, and the more difficulty it 
muft find in palling under and along fide of it, and ne- 
ceflarilymuft retard fo much the more the motion thereof. 
The refult of Dr. franklin’s experiments on this fubje< 5 fc 
may be feen in the letter above mentioned. 
4 M 2 
59. Mr. 
