on Rivers and Canals. 627 
thofe in the real canals and bilanders in the Low Coun- 
tries, an inch in the one anfwering to a foot in the 
other. 
Clofe to the canal, and out of the way of all wind, was 
fufpended a pendulum of fine waxed thread, to prevent 
the variations of the atmofphere from altering its lengthy 
which from the point of fufpenfion to the center of gra- 
vity in the lead was 39} Englifli inches, fo that its ifo- 
chronic vibrations were exactly leconds of time. 
60. It was necefiary, in order to render the experi- 
ments exact, that they fhould be made at a time when 
the air was perfectly calm ; for the leafi: breath of wind, 
during the motion of the boat, caufed great variations 
and irregularities in them, which it was abfolutely necef- 
fary to prevent, in order to be able to deduce any exact re- 
fults from them. On the contrary, in a perfect calm, the 
times of the paflage of the boat, from one end of tire ca- 
nal to the other, were exceedingly regular, as may be feen 
* 
from the table of experiments which I give below. 
By means of the pendulum I was able to meafure 
the times of paflage of the boat along the canal, in all 
the different depths of water, to a third or even to one 
quarter of a fecond. The boat being held fail againft 
the back end of the canal by the hand of an afliftant, and 
then let go, it was eafy for me to perceive the precife 
inftant 
