WE WERE PASSING George Washington University Hospital. The old taxi driver said to me in a voice of some bemusement: "I have been here seven times already today. Usually I would have been seven times to the White House. But that is not where the tourists want to go today. They want come here and stand on the side on the sidewalk." And there indeed they were standing with their cameras.

By that time there was nothing to see outside the hospital. There were still more comings and goings at the White House. If it was Edwin Meese III whom the tourists wished to see -- which seems rather unlikely -- they would have had more chance further down Pennsylvania Avenue. I thought of all the photograph albums in Indiana which now will never have a picture of the White House.

It was only one of the small incidents following the shootings of March 30 which made me contemplate again the nature of the presidency. There is no other office like the American presidency in the world. It has continually baffled the most acute of foreign observers, but they need hardly feel ashamed, because American observers are scarcely more instructive. There is a mystery in this office.

No office in the world is both so durable and so changeable. Each time that Richard Neustadt has brought out a new edition of "Presidental Power," to accomodate the experience of one more incumbent, he is now describing a different office from that in the preceding chapters. After a change of regime, America does not only have a new president, it has a new presidency.

Is it danger of being too weak -- Early Schlesinger -- or threatening to be too strong -- Middle Schlesinger -- and where will the Late Schlesinger stand? This is no dig at Arthur M. Schlesinger Jr. -- or indeed at his father, who once wrote a famous essay based on histrians' ratings of the presidents -- it is a commentary on the nature of the office.

But there is another reason for contemplating the character of the presidency today. The presidential transition has become a crucial phenomenon, not only for America, but for every country in the world which is affected by America. We are today close to the end of the First Hundred Days of President Reagan: one of the most fascinating transitions in recent times is symbolically finished.

The idea of measuring a new president by his First Hundred Days began with Franklin Roosevelt. (There is no evidence that anyone ever thoguht to count the days that Calvin Coolidge had been in office.) It is still electrifying to read of those first three months in 1933. There is still no better place to read of them than in Schlesinger's biography of FDR, unless one has the time to go back to read some of the journalism of the day.

No one had the slightest doubt, almost from the moment of his inaguration, that a transition had taken place. A transition from one era to a new era. Fortune magazine in those days was more than the house organ for corporation executives which it has now become. In its huge folio pages appeared some of the best journalism of the time. Several who then wrote for it have since become eminent in various fields.

In its issue of December 1933 it published an article on FDR of which hardly a word would have to be altered now. There is a completeness about it which is rare in journalism. This may partly be a tribute to the anonymous writer, but the completeness of its judgments, nine months after he took office, is really a commentary on the thoroughness of the transition. FDR had stamped the office unmistakably with his own mark from the start.

To someone who has had and still has deep reservations about the political character and intent of President Reagan, it is nonetheless undeniable that one of the most intriguing attempts to accomplish a genuine transition is today taking place before our very eyes. Whether it will work, whether it should work -- even whether President Reagan in the end will wish enough that it should work -- are still open questions, and really beside the point.We are watching a serious effort to use a new presidency to accomplish a deep change in American politics.

It is interesting to go back to the last significant transition, in 1968, for it has to be admitted, although one dislikes saying it, that all that really happened in 1976 was that pulp was handed over to pulp. But that transition from Lyndon Johnson To Richard Nixon was meaty. The political scientist, moving in after the political journalists have done their job, are for once interesting.

There was a fierce attempt by high officials in the Johnson administration, right up to the inauguration to commit the Nixon administration to the policies of the Great Society. These efforst were not inspired by President Johnson or the White House. On the contrary, he and it opposed them. Some of the brutal fights between LBJ asnd several of his Cabinet secretaries in the closing days of the administration had their occassion here.

The memorandum from his director of the budget, Charels Zwick, to all heads of departments was clear. They were "to be considerate of possible needs of the incoming administration, leaving to them decisions on moves, purchases, and other actions that can be delayed, so that such action can be tailored as clearly as possible to the new administration's policies and programs."

But many of the high officials in the departments, some with the knowledge, and certainly few checked by the veto, of the department heads, used every device they could to "solidify their legacy": a statement of intent attached to the Housing Act of 1968; large sums of money requested in the last Johnson budget for housing programs; the attempt to establish housing subsidy program by firm contracts. . . .

But the more that one reads the accounts of these efforts, the more one agrees with the judgment of one ploitical scientist: "A new president is locked in by his predecessor to the extent that he finds if politically advisable. . . .Nixon had no discernible mandate from the people to move in particular directions [in domestic affairs], and he was forced to work with a Democratically controlled Congress."

It is true that Nixon had made the usual Republican noises about cutting the budget and the size of the federal government. But that was not really his commitment nor really the concern of his supporters, and so not really his mandate. It is prescisely all those that are different this time. That perpetually difficult word in democracies -- what is a mandate, where is its source? -- this time has very different answers.

Whether it is President Reagan's own convictions, of the depth of which one cannot yet be sure; or the activity of David Stockman, whose endurance has yet to be proved; or the right wing keeping at the administration like a terrier; a change in public mood which has altered the character even of the Democratic opposition: there is every sign now of a new president acting as if there is a mandate.

One often idles through the first volume of Dwight D. Eisenhower's memoirs. They are persistently interesting, as subtle as he was, and will cool any fevered brow. If you are feeling out of sorts, if a fine love has taken an unexpected turning, pick up Eisenhower's memoirs. But then you turn back to the title of the first volume. "Mandate for Change." To change what? He wanted to, and so did, change very little.

But is feels different now. And why should one be asked to say more? It just does not feel the same. It is like wearing a new suit. One does not know whether it is one's body or the suit which must change shape. But it has to be one or the other. It can be put emphatically: Through the office of the presidency, Ronald Reagan has what, through the office of prime minister, Margaret Thatcher does not have.

Let us ask the taxi driver to go back. Being one of Washington's wonderful old taxi drivers, he will not mind and will not overcharge. Let us pause and gaze at the tourists outside the hospital. There is the mystery; there is the secret. Something gathers round the presidency which gathers round no other institution in this country and no other in the world. This feeling of gathering is the clue.

This is not affection for a monarchy, as in Britain, for the monarchy has no power. This is not a crowd outside No. 10 Downing Street in a crisis, because the prime minister is not the focus of a nation's affection. It is not a tribute to solitary grandeur, as to Charles de Gaulle, for grandeur is not a property of American presidents. The presidency of Giscard d'Estaing attracts none of the same feeling.

What is indefinable in the American presidency is its relationship with the people. That relationship may be formed, but it is not defined, in the ballot box. Beyond the votes, for or against, something else happens. A vital presidential election is about to take place in France. Its results could well alter much. But no one has the same feel as in America of a possible new gathering and redefinition of a people.

It is almost a heresy for someone of my beliefs to say that this transition takes my mind back to 1933. Yet how can one deny it? This is not Truman to Eisenhower, or Eisenhower to Kennedy, or Johnson to Nixon, or Ford to Carter, this is a feeling of a nation with its own mandate. What is extraordinary about those First Hundred Days of FDR is that he did so little in fact: Yet he with the nation did everything. They spoke to each other.

Some key to America lies in the absence of ceremony from its public life. It is a subject to which I often return, because I am certain it is key, but I find no way to turn it to unlock the door. So few ceremonies attend a president when he is well that it almost takes a heart attack or an assassination or an attempted assassination by a nut to attract any out-pouring of dutiful attendance to him. Yet there is some deeper and continuing ceremony of the people in their attitude to the office; and it is democratic to the core.

Tocqueville did not understand it. He totally missed the importance of Andrew Jackson, the president when he was here, treating him as some upstart at whom he looked through a monocle. This famed observer of "Democracy in America" could not see the president who was accomplishing the transformation of the Revolution into a Democracy.

In a still startling introduction to an anthology of English and American verse of that time, W. H. Auden proclaimed that it was Jackson who did the trick in the end and altered everything. It is for good reason that Andy Jackson rides on his rearing horse in Lafayette Park in front of the White House. Burrow into the writings of Irving Kristol, that most interesting of the neoconservatives, and you will find that his unforgiven enemy is Andrew Jackson.

So the presidency is made. So it changes. So it remains the same. The constant is that the American people gather themselves slowly, as do any free people with all their private concerns, but when they at last gather they make the presidency strong. And their ceremony is always on sidewalks.

It will be so sad if as a result of the shootings last month, the president is kept further from the sidewalks. The risk goes with the job, as Julius Caesar could have told them. (Fifteen days earlier and it would have been the Ides of March.) The fact is that most Americans shoot their presidents only with cameras. They receive the presidential touch by Polaroid.

It is the only office in the world where the incumbent is likely to turn up in the album of family photographs. For I promise you this. Comb through any family album in the Dordogne. You will not find a picture of a mere hospital.