WHEN I began to study physics - I am, I believe, one of the few political writers with an A-level in the subject - I was introduced to a small object with a hemispherical bottom and a pointed top. If pushed over, it immediately returned to the vertical position. It did this because the bottom was weighted with lead. It was meant to illustrate the principle of the centre of gravity.

The physics master, one John Andrew Owen, called it a "kelly". This was a word I had never heard before, except as a surname (it is now used as a girl's first name as well). I have not heard it used since in this connection. My several dictionaries fail to provide any illumination. Nevertheless I shall follow the usage of John Ann, as the master was perhaps inevitably called, and assume him to have been correct. Mr Tony Blair is New Labour's kelly.

These objects were, I think, popular toys also in a former age. In their diversionary rather than their instructional role, they would be painted with smiling faces above striped waistcoats and spotted bow ties. With suitable modifications to the clothing depicted, and some sticking-out ears (which would do little to affect the physics), a New Labour entrepreneur might make a tidy sum by selling Tony Blair kellys to constituency parties. The local MPs could be roped in as sales reps, which would at least give them something useful to do.

Mr Blair's invulnerability - to Mr Bernie Ecclestone, Mr Geoffrey Robinson and, above all, Mr William Hague - places him in a powerful position when it comes to shifting his colleagues around. He is certainly more powerful in this respect than Harold Wilson or James Callaghan ever was. He is so on account of his kelly-like quality and of the huge majority at his disposal, which only Wilson after 1966 came near to equalling.

Besides, Wilson and Callaghan were cautious men, forever balancing left and right, intellectuals and trade unionists, north and south, England and the Celtic nations. Today the first two categories no longer exist, or not in the same way. There are few trade unionists, even fewer intellectuals, certainly no one in the class of Richard Crossman and Anthony Crosland. As groups, left and right are forgotten. As adjectives they are virtually meaningless.

The balancing acts performed in the Wilson and Callaghan years rarely produced good cabinets. Even those which looked impressive on paper failed to knock down the coconuts and win the china shepherdess or the imitation cut-glass sugar bowl. In theory Wilson's cabinet of March 1974 was one of the most gifted of any of the post-war era: Callaghan at the Foreign Office, Denis Healey at the Treasury, Roy Jenkins at the Home Office, Crosland at Environment, and Barbara Castle at Health and Social Security. This talented lot duly led us straight to the IMF crisis of 1976.

Whether things would have been happier, then or earlier, if Healey had been at the Foreign Office and Crosland at the Treasury is a matter of speculation. In two periods of Labour government totalling 11 years neither was allowed to occupy the post which he wanted and for which he had consciously trained himself. If they had, I suspect that matters would not have turned out very differently. Still, it is a pity that, of a talented trio, only Lord Jenkins was given the chance to shine as he should have done, though to some extent he made his own luck.

I have written before about the last argument I had with the late Peter Jenkins. I maintained that politicians were becoming not only less talented but, what is not quite the same thing, less interesting. Jenkins replied that neither was true. It was just that we were both getting older. If we were in our early thirties again we should soon discover politicians as gifted and fascinating as Crossman, Crosland and, from the Conservatives, Iain Macleod.

I doubted it then (for Jenkins died too young in 1992) and I doubt it now. What I do recognise is that Mr Blair's government has so far been more successful than any Labour government since 1964. But it is success of a very odd kind. Waiting lists in hospitals continue to grow. There has been little progress in reducing classes in junior schools to under 30, which again the manifesto promised: ministers say it will be 2001 before the figure is attained. The success is in presentation rather than in policy.

It has happened before, though on nothing like the same scale. Wilson could do little wrong in the eyes of the public between the victory of 1964 and the devaluation of 1967. Between 1976 and 1979 the consensus among the worldly wisemen was that Margaret Thatcher, a woman, could never defeat solid old Jim.

Despite their caution and their conservatism, neither of the Labour prime ministers of 1964-79 was as constrained in his choice of colleagues as Mr Blair last May. For in 1980 the parliamentary party had, in a change to its standing orders, decreed that all the elected members of the parliamentary committee, more commonly known as the shadow cabinet, should be accommodated in the real cabinet on that happy occasion when one came to be formed. I had always thought that this change was imposed on the party by Mr Tony Benn and his acolytes, who were then at the height of their influence. Mr Gerald Kaufman assures me that, though he deplored the change himself, this was not so.

At all events, Mr Blair almost kept to the rules, excluding only one elected member, my old adversary in the law courts Mr Michael Meacher, who is Minister for the Environment outside the Cabinet under the superintendence of Mr John Prescott. I must say it is a little hard that Mr Meacher is not in the Cabinet while Dr Gavin Strang is in as Minister for Transport, also a part of Mr Prescott's dominion. But then, people say Dr Strang is for the heave-ho. Indeed, they have been saying it weekly since last June, as they have of Dr David Clark likewise.

Dr Clark is Chancellor of the Duchy of Lancashire, in charge of the Freedom of Information Bill. Mr Peter Mandelson is his theoretical inferior as Minister without Portfolio in the Cabinet Office, again under the omnipresent Mr Prescott. It would be a simple device to move Mr Mandelson into Dr Clark's job, which would give him the cabinet post he has always coveted but might be less good news for those with freedom of information at heart.

Those ministers who are called successes include Mr Gordon Brown of course, Dr Mo Mowlam naturally, Mr Alistair Darling inevitably and Mrs Margaret Beckett surprisingly. She is a former pillar of the Old Left with form as long as your arm, the hardest of hard women, who is now equally tough with trade unionists and manufacturers of television sets and washing machines, which she promises to reduce in price. I thought Sir Edward Heath had done away with price-fixing when he abolished resale price maintenance, but apparently not.

Mr Blair's greatest difficulty remains the embarrassing Mr Robin Cook, a clever man in the wrong job. He probably cannot shift him yet for reasons of prime ministerial prestige. But the trouble is that all the other big jobs are filled, partly because of Mr Prescott's ministerial ubiquity. I should get Mr Cook to use his gigantic brain to sort out the social services. I expect he will end up as Leader of the House. Whatever else he may be, Mr Cook is no kelly.