Editor's Note: (Harry Mount is a British journalist and author. He is the current editor of The Oldie Magazine. The opinions in this article belong to the author. This article has been updated to reflect latest news. )
(CNN) The news that the Duchess of Cambridge has given birth to a third child won't just delight the happy parents. It will also please all British monarchists (of which I am one). The direct succession of the royal family is now thoroughly secured.
The rule of thumb for the monarchy -- and aristocratic families -- was to produce an heir and a spare.
That meant two sons traditionally -- although it is the case in the royal family that daughters of the monarch, or the heir apparent's oldest son (i.e Prince William), inherit ahead of their uncles, or their male cousins. So the new baby will automatically become fifth in line to the throne.
Republicanism has not been a very popular cause in Britain since the Civil War in the 17th century, when Charles I was executed and Oliver Cromwell ruled the country.
For the last 81 years -- ever since the abdication crisis of 1936, when Edward VIII gave up his throne to marry the American divorcée, Wallis Simpson -- getting rid of the monarchy has been a non-starter. Support for the royals is usually extremely strong whenever the public is polled.
The only other major crisis came in the aftermath of the death of Princess Diana -- which we were reminded of last week, as the country marked the 20th anniversary of that tragedy.
Then, support for the monarchy momentarily dipped, as some people -- wrongly, in my opinion -- thought the Queen hadn't behaved ideally in staying at Balmoral immediately after Diana's death; instead of rushing down to Buckingham Palace and lowering the palace flag in honor of the late princess. But support for the family soon returned.
You might have thought the modern age would temper what might seem like an old-fashioned affection for the Queen. In fact, in the age of social media, news about the royal family is consumed more voraciously, and on an even wider scale.
Of course, if you look at it objectively, monarchy seems like a ludicrous system -- the idea that someone should inherit the right to rule over a nation. But, in fact, logically speaking, you can see why people are attracted to the concept.
No one under the age of 70 in the United Kingdom can remember anyone but the Queen being in charge. For 65 years, we have been bombarded with images, audio and video, not just of her as Queen, but also of her children, grandchildren and great-grandchildren -- and the deaths of her daughter-in-law, her sister and her mother.
Of course, some people understandably react against that massive coverage. But most of the rest of us absorb it, and the royal family becomes the universal background to our lives. They are a huge part of our mental furniture -- meaning that a big proportion of the British population actually dream about the Queen.
Compare her to someone like Mick Jagger, who has been around for almost as long. A huge star since the early 60s, admittedly -- but do you know the name of any of his first cousins or his grandchildren?
Imagine the power and prestige an American president holds -- and imagine that president holding not just one term, but more than 16. When the Queen came to the throne, Harry Truman was president.
The Duke and Duchess of Cambridge's new son will have similar wall-to-wall coverage as he grows up -- and most of us will develop a similar attachment to him as he matures.
The new arrival will also provide the family with another top-tier royal. In recent years, the royal family's advisers have been promoting a slimmed-down family, which relies on the Queen, Prince Philip, Prince Charles and the Duchess of Cornwall, Princes William and Harry, and the Duchess of Cambridge.
In time, that core group will encompass increased appearances by Prince George, Princess Charlotte and the new arrival.
Slowly, the other members of the royal family will become increasingly marginalized. Princess Beatrice and Princess Eugenie -- the daughters of Prince Andrew -- have been struggling to find royal roles in recent years.
This new arrival will not help their cause much, but he or she will massively boost the cause of the monarchy.
Correction: An earlier version of this article incorrectly described Prince Andrew as Queen Elizabeth's youngest son.