The wave of Middle East revolutions continued over the weekend on three main fronts. In Syria, president Bashar al-Assad’s violent crackdown on opposition entered its second week, with tanks being used to storm a mosque in the southern city of Deraa. Fighting also continues in Libya, and in Yemen the proposed agreement between president Ali Abdullah Saleh and his opponents is now in doubt.

The agreement was to form a national unity government with the opposition and for Saleh to resign within 30 days of the agreement, allowing democratic elections within a further 60 days. The opposition leadership, after some hesitation, agreed in principle to the deal and to granting the president and his adherents immunity from prosecution. Yesterday, however, Saleh apparently refused to sign and “asked a senior aide to represent him at the signing ceremony in the Saudi capital”.

Many protesters were already unhappy about the immunity and the 30-day delay (although the protests have already been going on for twice that long without being able to force Saleh out), and there’s little doubt that the latest developments will increase the pressure from the streets. And as examples around the world have shown, promises of immunity are often a necessary evil that can be unwound later if the political will is there.

It’s unclear whether Libya’s colonel Gaddafi will last another 30 days, but it may not be much more than that. Although there’s a view that he has recovered some degree of popular support — the BBC’s Christian Fraser Christian Fraser reported that last week’s pro-Gaddafi demonstrations were “seemingly more spontaneous than those we have witnessed so far” — it is unlikely to be significant, and the Western allies have committed themsleves much too far to be able to back out without securing his removal. Most likely he and Saleh will be the third and fourth casualties of the revolutionary movement.

Four dictators in about as many months would be not a bad rate. In addition to Syria, the next group of regimes under threat includes principally Bahrain, but also Algeria, Morocco and now Mauritania (not geographically in the Middle East, but a member of the Arab League). Only a handful of countries in the region seem to have been so far free from unrest — Saudi Arabia and the United Arab Emirates come to mind, both partly shielded by their oil wealth, but their turn may nonetheless come.

It’s also interesting to see the way the region’s revolutionaries are trying to consolidate the gains made so far. In Egypt, there are persistent calls to put Hosni Mubarak (currently in hospital) on trial; his interior minister is already facing charges. Even Mubarak’s name is being erased from public places.

This shows up an important difference between this year’s events and the revolutions of 1848 in Europe, in which popular uprisings scored remarkable early success but were eventually defeated.

Although the comparison is often made, unlike Egypt and the rest the European revolutions (France was the main exception) did not actually involve the removal of autocratic leaders: rather the autocrats were forced to make concessions, but remained at least nominally in power and were later able to reassert their authority.

The reason is that those autocrats were hereditary rulers — kings, dukes and emperors. Overthrowing a hereditary monarch is a more serious step, and was widely thought to be unnecessary; all that was required was for them to become constitutional rulers, giving up real power while remaining as figureheads in the way that the British monarchy had. (The rulers, of course, turned out not to be so keen on this plan.)

There’s really no such thing, however, as a constitutional dictator. No one would keep on a Mubarak or a Gaddafi as a Queen Elizabeth-like figurehead. That’s why their position rapidly became untenable, whereas in monarchies such as Bahrain and Morocco the protests (at least initially) were about demanding reforms rather than wholesale regime change.

When and if the revolutionary wave reaches the more well-entrenched monarchs, especially that of Saudi Arabia, it’s to be hoped that their opponents will have learnt the lesson of 1848 and realised that there’s no substitute for a complete change at the top.