98 
FORMULA FOR FINDING- SPEED OF OBJECTIVE. 
Adopting exactly the same process as in the last example, the speed 
is found to be 13*2 knots per hour. 
These results can be confirmed by calculation. 
A diagram of this description might possibly be useful in a battery 
and if made of reasonable dimensions, almost absolute accuracy could 
be obtained. 
On the small diagram I submit, my own errors in reading speeds 
have not exceeded knots per hour, over results obtained by 
calculation and the average error over ten examples—which were read 
off in less than four minutes—amounted to *08 knots per hour. 
