'Dorcas the Mannequin, in white undergarments' turned out to be just that: a life-size dummy in a linen shift, quite bald but with nice make-up. When I first saw her she was propped up behind some boxes in one of the Living History Village’s fetching reconstructed seventeenth-century houses. The next day, however, saw us better acquainted. After a memorable evening supervising re-enactors channelling witches at a sabbat – there was dancing near a fire, as promised – the crew set up on a bridge crossing a waterway, somewhere between a river and a stream. Tony Robinson and I were kitted out in full waders that made us look like Cornish fishermen, then we made our way through the rushes to the water’s edge.

There we discussed the ordeal by water, or swimming test, to which suspected witches were once subjected. I tried to paint a picture of the fascinated crowd that once might have lined the banks, all waiting for a signal from heaven to confirm or deny the guilt of the accused. ‘Not too much further’, called an old man from the bridge, his timely arrival stopping me wading beyond a submerged platform installed by the army during the war. ‘Another few inches and you’ll go down six foot’, he warned, at which point, I suppose, Dorcas’s services would no longer have been required. For to illustrate how seventeenth-century villagers would have conducted the test – and here academic purists should remember that television is a visual medium – we put Dorcas, her hands tied to opposing feet, into the river.

She floated like a cork, which suited our purposes. Tony prodded at her with a long stick, just as contemporary accounts describe. The point witnesses once tried to make was that, try as they might to submerge witches, her wickedness was so great that the pure element of water entirely rejected her body. Symbolically, this was an inverted baptism, the opposite of the ritual where water receives the Christian child; here Satan’s handmaids were repelled.

When the camera was rolling – or ‘speeding’ as they say these days – I emphasized that this was not a formal procedure, rather one frowned on by most authorities as disorderly, presumptuous, and sacrilegious: it put God to the test. I also had to do my academic spoilsport bit and put Channel 4’s viewers straight: this was not a Catch-22. The innocent woman did not sink and so drown and die anyway. The results of swimming witches were sometimes fatal – at happened at Tring in Hertfordshire in 1751 – but that was not the intention. The ideal was that the swimming test would galvanize local opinion to back the accusation, so that the witch could be tried at law.

The water ordeal was probably fairly common, but not that frequently heard of in the courts, the reason being that it was legally and religiously dubious. Today we think of it as superstitious, and so it was; but many contemporaries who believed in the power of the devil, and even the existence of witches, thought it was superstitious too.

What we have come to think of as superstitious, religious, rational, and superstitious ideas once co-existed within a single framework of opinion and debate, in tension but not necessarily in conflict. Newton’s interest in alchemy and numerology may strike us as inconsistent with his more lasting scientific achievements; but in his day they were all part of the same spirit of enquiry into the mysteries of God’s creation.

Episode one of ‘Tony Robinson’s Superstitions’, ‘Witches’, will be broadcast on Channel 4 later in the year. Unless good taste triumphs in an edit suite between now and then, viewers will also see me attempt to steal Tony’s soul using a written contract signed in his actual blood. I have it here on my desk: you never know when it might come in handy.

Tomorrow I’m off to the Living History Village at Little Woodham, outside Gosport, where it is forever 1642. Enthusiasts in period costume devote their spare time to this recreated world, a glimpse of rural England in the year civil war broke out. I was last there nearly a decade ago, desperately trying not to spoil the ending for them – all that unpleasantness with King Charles and so on. Back then I was making a history documentary for Channel 4 (about the Witchfinder General), for which this was an ideal film-set. The film company did such a good job of uncovering the story of the East Anglian witch-hunt of 1645-7, I decided to write a book about it: Witchfinders: a Seventeenth-Century English Tragedy (2005).

And so it is a Channel 4 documentary which takes me back there again tomorrow. This time the series is ‘Superstitions’, presented by Tony Robinson. I’ll be required to explain to Tony about the witches’ sabbat– the supposed nocturnal meeting where witches met to pay homage to Satan and plot their evil crimes.

By the end of the sixteenth century, demonological treatises were describing these events in great and lurid detail. Witches bowed down before their dark lord, sometimes administering the ‘unclean kiss’ to his backside; crucifixes were trampled underfoot; and babies were roasted on spits. There was feasting, drinking, dancing (back-to-back, and backwards) and general cavorting. One historian has likened the sabbat to ‘a perverted village fair’.

It seems everyone knew about the sabbat, but there was very little evidence that anyone had ever attended one. Children claimed to have been abducted to secret meetings, where evil women tried to initiate them into their craft. Doubtless such tales were laughed off, but sometimes, at moments of particular anxiety about witchcraft, they could have deadly consequences. In the German state of Augsburg, the testimony of a fourteen-year-old boy led to the deaths of twenty-seven women in 1589.

Stories of the sabbat are virtually non-existent in English witchcraft trials, at least the full-blown version with witches flying in on broomsticks. Here, gatherings of witches were typically more homely affairs: the devil was a generous, if rather shifty, host; there was bread and beer; sometimes those present were offered sixpence, or some other paltry sum. In 1673 a Northumbrian maidservant named Anne Armstrong told magistrates that she had been forced to attend a sabbat where she had seen thirteen witches dance for Satan; they had been served cheese and mutton, also a capon poached in plum broth. Such tall stories revealed not some foul conspiracy against Christendom, but the fantasies of poor women. No roasted babies: coq au vin.

I’m looking forward to tomorrow night, when, it seems, the sabbat is to be recreated in deepest darkest Hampshire. The director has promised me it will be ‘interesting’, in that way people say when they mean ‘eye-popping’. The Health & Safety briefing notes that there is likely to be some ‘dancing near a fire’, and the props list includes the following: two crucifixes (for breaking), four black candles, a pyrex dish, phallic sausages, tinned insects, antibacterial handwash, 250 communion wafers, latex gloves, a mossy hearth, high-vis jackets, and some images of the devil. The call sheet also mentions 'Dorcas the Mannequin, in white undergarments'. I will report back.

A BBC documentary about Abraham Lincoln  screened last night, told how certain Southerners, mostly descendents of Confederate soldiers killed or injured in the Civil War, remember the great president as a mass murderer. At the moment, I'm deep in research about seventeenth-century English America, and the look of indignation and hurt on these people's faces, all these generations later, reminded me of the passions stirred by the Salem witch-trials of 1692.

There is something about the unreality of witchcraft to Western minds that fuels a sense of injustice in descendants of those executed for it. From time to time, they petition governments and even go to court seeking redress. Critics of such actions sometimes cite the legal concept of ‘fairness’, which argues that what was deemed fair in its own time cannot be overturned later on. For this reason, in 1998 the Home Secretary turned down a request that the Lancashire witches of 1612 be exonerated. Penal codes in past ages were so severe by modern standards that almost everything now seems unfair, and therefore theoretically worthy of judicial review. Children hanged for petty theft, religious dissenters burned alive, libellers mutilated on the stocks – does posterity not owe them justice too? And if it does, where will our penitence end?

The executions for witchcraft at Salem, however, are unique in a number of respects, but one stands out here. The trials were officially ruled to have been unfair not just hundreds of years later, but very quickly after they happened. This occurred not because suddenly witchcraft was believed to be impossible, but because the basis upon which suspects were found guilty of the crime was utterly discredited once the hysteria had died down.

Between the halting of the trials in 1693 and 1711 when an act to reverse the attainders against the convicted was passed, a number of extraordinary events took place - events overshadowed in popular memory by the wailing, hallucinating, and ‘pointy reckonings’ in Arthur Miller’s play The Crucible. The courts admitted that mistakes had been made, especially giving credence to unreliable witnesses and so-called ‘spectral evidence’. Jurors recanted; one of the judges, Samuel Sewall, his head hung low, apologized publicly. Petitions were filed for restitution of good name and the return of property; compensation claims were handled by an administrative body set up for that purpose; and ministers appealed for sentences to be overturned to appease an angry God. Finally, in the statute mentioned above, it was declared that the devil had not been in the accused all along, but rather in the accusers. All in all, a startling turnaround.

But things did not end there. Poignantly, six of the alleged witches executed in 1692 were not named in the act to reverse the convictions, apparently because no one ever petitioned the court on their behalf. One, Ann Pudeator, was exonerated in 1957, the rest in 2001. 'We've had an awful lot of descendants that have been out there working for it’, Shari Kelley Worrell of Barrington, Illinois, told reporters in Boston; she is an eighth great-granddaughter of one of those hanged. Acting governor of Massachusetts Jane Swift chose Halloween to ratify the pardons, thus, according to the editor of Records of the Salem Witch-Hunt (2009), ‘reflecting a cultural conflation of innocent individuals imprisoned, and in some cases executed, with the celebratory festivities of a fantasy world inhabited by real witches, ghosts and goblins’.

Today Salem, with its tawdry museums and souvenir shops, does not afford much dignity to the dead and their devastated families. A twilight stroll in Danvers (the modern name for Salem Village where most of the accused lived), up the lane towards the former homestead of Rebecca Nurse, hanged on 19 July 1692, is a more effective memorial because, like the best memorials, it’s something that happens in your head in a moment of silence.

The insensitive time-tabling of the Salem pardons does not seem to have bothered Ms. Worrell and her co-campaigners. They all cheered as Governor Swift signed the act, surely oblivious to the fact that the terrorist attacks of six earlier had unleashed a modern American witch-hunt. Once again, innocent individuals were imprisoned, paranoia got the better of reason and justice, and civil liberties were aggressively sacrificed to defend Christian civilization. And once again, within a decade the panic was followed by a period of sober reflection when, most decent people decided, things had got just a bit out of hand.

Two recent news stories serve as a reminder of the reality of witchcraft in the modern world. The first comes from Kiganda, a region of Burundi in eastern Africa. A twelve-year-old albino boy was murdered on 30 December by hunters who supply the demand for body parts used for magical purposes. He was the fourteenth albino to be killed in Burundi since 2008. It’s believed that this grisly trade is sponsored by witch-doctors across the border in Tanzania, where perpetrators, when caught, are treated harshly by the authorities. A man has been sentenced to death for the unimaginably dreadful crime of cutting off the legs of a five-year-old girl and drinking her blood. Demonologies of the fifteenth and sixteen centuries alleged that witches engaged in this sort of activity, but no evidence ever came to light that they did.

The other story this week also relates to the age-old struggle of legislators, administrators and judges to control witch-beliefs in their populations. It comes from Romania, where the government has just passed laws to compel witches to pay tax on income received for their services. The witches are retaliating by casting a spell on President Traian Basescu using the mandrake root – a mystically powerful ingredient well-known to medieval herbalists.

In the first half of the twentieth century, campaigners for freedom for Spiritualists in Britain appealed for genuine mediums to be given licences to prevent them being tarred with the same brush as fraudulent practitioners. Some mediums, prosecuted under the 1824 Vagrancy Act or the 1735 Witchcraft Act, protested their bona fides as law-abiding members of society, which included payment of income tax. The position of the government and police, in contrast to Romania today, was that they should protect members of the public against the exploitation of their grief for financial ends.

Yesterday I received an email from Steve Brooker, so-called ‘Mud God, who spends his spare time scouring the shores of the Thames picking up all sorts of treasures from prehistoric harpoons to Roman shoes, medieval cannon balls to Georgian shoe buckles. The History Channel is making a series, ‘Mudmen’, to be broadcast in the New Year. The premise is that Steve takes mudlarking novice (and TV and radio presenter) Johnny Vaughan out on the river, then they pursue the story behind artefacts they discover. One such object is part of a Bellarmine jug, a seventeenth-century stoneware bottle once buried with hair, urine and nail clippings as a charm against witches. Last month I helped make the related programme, which delved deeper into the history of witch-beliefs and witchcraft trials.

Steve was emailing to say that he has seen a rough-cut of the film while doing the voice-overs. The tone is light-hearted and the content accessible, so it’s likely to appeal to older children, but the producer was keen to preserve the seriousness of the subject matter, namely that people once suffered and died because of the belief in witches. The Bellarmine charms that turn up from time in old houses and archaeological sites are constant reminders of just how pervasive and serious these ideas were. An extraordinary example, complete with its counter-magical contents, was uncovered by workmen in Greenwich in 2004. I mention it in my book Witchcraft: a Very Short Introduction (OUP, 2010), and you can read about the chemical analysis here (apparently the urine came from a smoker).

I was interviewed by Jenni Murray on Radio 4’s Woman’s Hour this morning. The notebooks of a seventeenth-century puritan, Nehemiah Wallington, have been digitized by the John Rylands Library in Manchester, and have attracted attention in the press. In the mid-1640s Wallington recorded some episodes from the East Anglian witch-trials, notably the confession of an adolescent girl in Essex, Rebecca West, and the subsequent discovery of an alleged coven. ‘Imps, warts and sex with the devil’ screamed the Daily Mail headline; ‘Journal of the witchfinder general opened up’ was how the Daily Telegraph introduced the story.

Inevitably, newspapers have to simplify things, but it’s striking how much many people still want to know the mind of the evil bogeyman Matthew Hopkins. When I was approached by Radio 5 Live some weeks ago, the text message I received from the Oxford University Press press office, said they wanted to talk about the discovery of Hopkins’s diary. I knew it was too good to be true, although I’ll admit I felt a momentary pang of panic. If such a diary ever were found – it’s extraordinarily unlikely – I’d have to write my book Witchfinders all over again.

As it is, Wallington adds little to the story, because for his information he relied on printed newsbooks, the content of which is still freely available. What is most interesting is the way he added small embellishments, possibly drawing on rumours and reports he had heard, and how he introduced small errors, for example saying Rebecca West came from Colchester rather than Lawford.

Watch this extraordinary Channel 4 documentary about witch-hunting in the Central African Republic. These alleged witches are not just lynched by angry neighbours, but formally tried by due process of law. There are some striking parallels here with early modern European witchcraft trials, especially the problems faced by the judiciary - problems of defining the crime and deciding what constitutes admissible evidence.