Mr T. Tredgold cm Steam-Boats. 247 
cal power required to keep the boat in motion at the velocity v. 
will be 
a v 
u 2 
Whence it appears, that the mechanical power, or the power 
of a steam-engine to impel a boat in still water, must be as the 
cube of its velocity. Therefore, if an engine of twelve horses’ 
power will impel a boat at the rate of seven miles an hour in 
still water, and it be required to know what power will move 
the same boat at ten miles per hour, it will be 7 3 : 10 3 : : 152 : 
10 3 x 12 
: — ^3 = 35 ; or an engine of thirty-five horses’ power. 
This immense increase of power to obtain so small an increase 
of velocity, ought to have its influence in fixing upon the speed 
of a boat for a long voyage, and its proportions ought to be 
adapted for that speed, with a proper excess of power for emer- 
gencies. A low velocity should be chosen, where goods as well 
as passengers are to be conveyed. Our example places this in a 
striking point of view, for to increase the velocity of the same 
boat from seven to ten miles per hour, requires very nearly three 
times the power, and, of course, three times the quantity of 
fuel, and three times the space for stowing it, besides the addi- 
tional space occupied by a larger engine ; consequently, if seven 
miles per hour will answer the purposes of the trade the vessel 
is to conduct, the advantages of the lesser speed must be evi- 
dent. 
According to the principles we have calculated upon, the 
power required to give a boat different velocities in still water will 
be as follows : 
Miles per Hour. 
3 miles, 
4 
5 
6 
7 
8 
9 
10 
Horses’ Power. 
5| horses’ power. 
13 
25 
43 
69 
102 
146 
200 
In short voyages, the extra quantity of engine-room, and tori- 
nage for fuel, is not so objectionable ; but, in a long voyage, it 
reduces the useful tonnage to so small a proportion as to render 
it doubtful whether such vessels will answer or not. The con- 
