The entry on her ended with the observation that even sooner than most of her predecessors “May had demonstrated the precarious and transitory nature of a tenancy at 10 Downing Street”.
And yet she now seems safer from eviction than she did last autumn when she advertised her vulnerability by grinding repeatedly to a halt during her party conference speech, overcome by fits of coughing.
While Jeremy Corbyn struggles to persuade his own MPs that he is a fit and proper person to be Prime Minister, the Conservatives seem to have decided that May is after all capable of getting Brexit through while maintaining some semblance of unity on her own benches.
She aspired from an early age to be the first woman Prime Minister. Margaret Thatcher beat her to it, and offers a dreadful warning. For once Thatcher’s own MPs had decided that with her policies on Europe and the poll tax she was leading them to perdition, they had no hesitation in throwing her overboard. And the same will happen to May, when she too has become a liability to her own party.
A Prime Minister has to be able to scrape together a majority in the House of Commons by some means. And this has a number of beneficial consequences. In the first place, it is difficult for a criminal, a demagogue or a dimwit to become PM.
Someone like Donald Trump could not get to the top here. When he stood up to speak in the Commons, MPs would find him simply too embarrassing to choose as their leader.
Many PMs have been failures, in the sense that in the end they were overthrown, and left office without having achieved what they hoped to achieve.
Throughout the 20th century, our leaders strove to avert the relative decline of British power, and could not do so.
Even Winston Churchill, saviour of the British nation in 1940, could not prevent, within his own lifetime, the decline and fall of the British Empire to which he was so devoted.
But to fail is not the same as to be contemptible. When I started writing about the PMs, I expected to find quite a few who were complete duds.
After all, we have a long tradition of treating our politicians as idiots and fools. The 18th-century caricaturists gave brilliant expression to that contempt, and so does Martin Rowson, who has drawn the PMs for my book.
Yet two thirds of the way through writing about these 54 people, I had to admit that very few of them were fools. The average length of time which they spent in office, not always in a single stint, was five and a half years, which means they were weak enough to be overthrown.
But in a free country, that is a fitting arrangement. We do not want to be ordered about by a succession of elected dictators. As soon as something goes wrong, we ditch whoever we hold responsible. An essential function of the PM is to serve as the scapegoat for national humiliation.
So in the late 18th century Lord North took the blame for losing the American colonies. He was Prime Minister, so it was his fault. That is how the British constitution works.
But he was in a position to lose the American colonies because he was so popular, such a brilliant manager of men, and so willing to do what the King and Parliament wanted, that he stayed in office for a dozen years. When one looks at his life, one finds he was not just able, but loveable.
A single anecdote is sufficient to substantiate this astonishing claim. North was one night at the opera at Covent Garden when he was asked by a distant acquaintance to identify “that plain-looking lady in the box opposite”.
He replied in an amiable tone that she was his wife, whereupon the questioner attempted to recover by saying: “No, no, I meant the dreadful monster sitting next to her.”
“That, sir, is my daughter,” North replied. “We are considered to be three of the ugliest people in London.”
In more modern times, Neville Chamberlain was a great domestic reformer, who planned to improve health services so that Labour would never get the opportunity to introduce the National Health Service.
But in the late 1930s, he tried and failed to appease Hitler, and it is unlikely his reputation will ever recover from that failure.
It is too early to say whether Tony Blair will ever be forgiven for the Iraq war, or David Cameron for calling and losing the EU referendum. But in both cases, the PM took the blame, and by doing so performed a service for British democracy, though not quite the one they had hoped when they entered Downing Street.
Andrew Gimson is a Parliamentary sketch writer and contributor to the Conservative Home website. Gimson’s Prime Ministers: Brief Lives from Walpole to Blair is published by Square Peg (£10.99).