63 
into practice the rules connecting the resistance of the full- 
sized vessel with that of an exact model to scale. 
By means of a tank 200 feet long, and models on scales 
of 1 to 50, or 1 to 20, the resistance and rolling qualities of 
all Her Majesty’s ships have since been verified before they 
are constructed. And the same is now done by manufac- 
turers of mercantile vessels, like Mr. Denny, who have tanks 
of their own. The qualities of ships thus tested were 
originally limited to those of resistance and of rolling, and 
so far as I know, no extension has taken place; for although 
in 1876 it was pointed out by the author before Section 9 
of the British Association, that by constructing models of 
our war ships on a scale large enough to enable them to be 
used as launches, say 1 to 16, and supplying these launches 
with power as the cube of their dimensions — then the 
manoeuvering qualities would be similiar if conducted on 
scales proportional to their lengths, the time occupied by 
the launches in executing a particular evolution, ns compar- 
ed with that occupied by the ships, being as the scjuare root 
of their lengths. So that with such models, the officers and 
seamen could be instructed in the handling of their ships 
without cost of risk. This has not been done. The Admiralty 
replying, so far as they did reply, that their officers were 
continually experimenting with the launches — disregarding 
the fact that the launches in use were in no sense models 
of the ships, and were supplied with power five or six times 
too great in proportion — thus ignoring the point of the 
suggestion, namely, that the experience gained by the models 
might be applicable to the ships, which with their present 
launches it is not, and only tends to mislead those who 
attempt a comparison. 
Since making this suggestion, I have been much engaged 
in experiments with water, which have enabled me 
to extend this law of similarity, until I find it is possible 
now to lay down the conditions under which to test the sea- 
worthy qualities of a vessel from those of its model. 
