Lets get a few things straight: Pre-unification, the ranks of Naval Cadet, Midshipman, Acting Sub-Lieutenant and Sub-Lieutenant were in use in the R.C.N.
The actual rank worn on the sleeve by A/SLt's and SLt's was the same and they were BOTH referred to as Sub-Lieutenant, except in official correspondence.
The progression referred to their training level:
Officer candidates came in the service as Naval Cadet (indicated by a "pin" on the coat lapel) and, upon graduating from the then equivalent of "Basic", became Midshipman (a "pin" over a square of broadcloth on the lapel) to do their true naval apprenticeship.
Once they graduated and commissioned, they went to their first ship as A/SLt's (one broad stripe with executive curl), a rank they retained until they were ready to stand their own watch. At that point they went before the board to obtain their watch keeping certificate AND, as a result of passing it, the substantive rank of SLt, which however was still denoted by a single broad stripe with curl .
As for the term "second lieutenant", it would not work in the Navy, as it would be considered to refer to the Combat Officer. Why, you ask?
In the older days of the Navy, ships had a Captain and every other officer's position onboard, other than the masters, were referred to as lieutenants, with the First Lieutenant being the second in command to the captain, the Second Lieutenant being the next most senior, and so forth down to the very junior Sixth Lieutenant (Merchant ships still use a similar system, with captain, first mate, second mate, etc.). So in the Navy, being Second Lieutenant means you are second in line to become captain, an elevated position, not that you are second to all lieutenants, a low position (which we keep for the army
).
This is why, even to this day, the X.O. of a ship is often referred to as the First Lieutenant, or Number One, or Jimmy the one.
And BTW, Lieutenant-Commanders used to be (a long time ago) considered part of the "lieutenants". They just happen to be lieutenants that were senior enough that, when the need arose, you could temporarily appoint them in command of a third rate or fourth rate ship of the line - until they could be properly promoted to commander or relieved by a commander/captain that became available for the job. As a result, Lieutenant-Commanders were part of the "junior" officers and, usually referred to as Lieutenant by their seniors in command of the ship, as Lieutenant-Commanders by their peers and juniors under normal circumstances, and as Commanders by all while in a command of their own.
When I joined (many moons ago and perhaps a few eons), junior officers often addressed LCDR's as Commanders onboard the steamers, a practice that had disappeared by the time I myself reached that rank (Damit !)
