LAST MONDAY a disturbing news story appeared in The Times of London. The water which the fountains in Trafalgar Square are now tossing into the air is no longer "clean, sparkling," but an "insalubrious green." It was in such ominous occurrences that the Romans used to read the portents of their coming fall. The sun must be setting even on London if there are algae in the fountains of its own Forum.

The story, in fact, is interesting. The 100,000 gallons of water in the fountain system are now being cleaned only once every three weeks instead of every two weeks because of government economies. That sounds familiar here. The sanitation services can no longer use chlorine in their work on the fountains because of health and safety regulations. That also sounds familiar here.

But then there is the third reason. People are now throwing their trash and even garbage into the fountains: "unwanted hamburgers, ice cream and bread, together with a wide variety of food wrappings and tin cans." Trafalgar Square has always been a place where large crowds gather for political demonstrations or simply for some wild celebration. Never in the past were even cigarette ends thrown in the fountains.

When one turns back from this story to the reports of the riots in Britain, both seem to tell of the erosion of a civil society. Britain has rightly been renowned for its civility. The habit of forming a line. instead of elbowing to the front; the common tribute of visitors, "Your policemen are wonderful"; the cab drivers with whom you did not need to check the fare or count the change: all the marks of civility used to be there.

Of the two reactions to the riots which I have met in Americans, the strong feeling of shock that "it" can happen in Britain is quite common. If in Germany, if in Italy, if in France . . . only to be expected. But in the land of the All England Lawn Tennis Club . . . the very sky must be falling in! The past civility grew out of what was then a homogeneous society.

The other reaction of some Americans -- the eager sneer that Britain is, after all, no different from anywhere else -- is not really very interesting. It is tiresome only when it comes from an Australian cartoonist. The fact is that Britain has taken in the West Indians and the Asians, whereas the other white nations of the Commonwealth, Canada and New Zealand and Australia, have followed immigration policies of studied discrimination against colored peoples.

One must be careful of comparisons between the riots in Britain now and those in America in the 1960s. But there are some similarities. The most important is that the migration of blacks from the South to the northern cities in America is parallelled by the migration of West Indians and Asians to the great industrial cities of Britain. In this endeavor. America and Britian are joined, alone of all predominantly white nations.

Whether it is the internal migration in American or the external migration to Britain, these countries have color problems because they have been willing to receive colored peoples. Not merely to receive them, but to receive them as citizens: with votes and civil rights, and the full protection of the law. This is very different from the transient foreign labor on which much of Europe today builds its prosperity.

In a magnificent, almost three-column, editorial on July 10, The Times of London said: "Our society is composed of a patchwork of different races and cultures making it inevitably less homogeneous than a generation ago."

The ideal in this situation is not integration but 'equal opportunity, accompanied by cultural diversity, in an atmosphere of mutual tolerance,' to use the words of Mr. Roy Jenkins."

Let us lift our eyes from the gutters of the torn cities if we are to talk of this problem. It really is no surprise that it is Britain and America. With the practical idealism of their liberty-loving histories in their blood, which rise to this challenge and this opportunity. Here are the crucibles of the multiracial societies which, in our world now shrunk to the size of a television screen, are this globe's only promising and adventurous hope for the future generations.

One quickly grows restless at the eulogies to our common heritage in after-dinner speeches at the English Speaking Union. Yet the fact is that there is an Anglo-Saxon concept of citzenship under the law, unique in itself, of which America and Britain are the exemplars, even under the stress which the pursuit of their tradition invites. It is when that tradition gets into the hands of unappreciative "WASPS" that one is disturbed.

The Times of London followed its first extraordinary editorial -- titled "The Soiled Coin" -- with a second and shorter one, titled "Where Are We Going?" It is hard to read its strictures on Prime Minister Thatcher -- against whom it has no axe to grind -- without wondering how soon the same strictures will have to be addressed to President Reagan. This is The Times with the recovered voice of "The Thunderer":

"She failed to raise the tone of her remarks to the level of events. Not for the first time she was unable to strike the right note when a broad sense of social understanding was required."

"The prime minister curiously made no serious attempt to speak to the younger generation, white or black, who make up the vast majority of the rioters."

"She did not acknowledge the role and responsibility of government, including her government, as a central source of authority which helps to set the tone within which social relationships are conducted."

"No British government has had an effective strategy for our inner cities."

"In Britain the upper ranks of the civil service, local authorities, the military, the police, teaching (and journalism) are almost entirely white."

"In her television broadcast she went out to emphasize that governments have only a limited role to play: a good many other have responsibilites as well. So indeed they have. . . . It would be foolish to deny, however, that in times of national aunguish people naturally look to their government to give a lead, to set the tone for the country's response. . . . In the words of the song, it should accentuate the positive." r

So one could go one quoting. And it is difficult to read sentences like that here and not wonder, without predicting that America will endure another "long, hot summer" this year, how the present administration would react, with its own addiction to the diminished role of government, if riots like those in Miami last year spread across the nation. How can a government act positively if it thinks that all the most important decisions are private?

There are many important comparisons and contrasts which could be drawn between the American and British riots; and indeed between different riots in Britain, as between differently motivated riots here. The violence in Southall in London was that of an established and lawabiding Asian community, defending itself against the invasions of white punk "Skinheads" with affiliations to the far right. That was not true elsewhere.

Much working-class punk music in Britain is directed against the middle and upper classes. "Skinheads" vs. "Mods." Here are the words from one of the Skins songs: Kick him in the head, Boot him in the balls, Chop off all his hair. O what fun it is to kick A Mod, until he's dead! The Daily Mail of London has called the pop magazine which published this stuff "the Skinheads bible of hate." When that depth of class hatred is then expressed in racial hatred, a society is indeed hard pressed to preserve its homogeneity and so its civility.

The Republican Party and the present administration are doing a fine joy in separating the blue-collar worker for the more disadvantaged and often colored people who are below him on the social and economic scale. But it will not be at all surprising if the policies of President Reagan force the equivalent of the Mods here -- the more affluent and liberal middle class -- back into alliance with the oppressed wretched of the inner cities, with whom they sometimes share a feeling of alienation from the unregarding mainstream of society. The mainstream has a habit of leaving people stranded at the top as well as the bottom.

The British riots are a reminder of how, in multiracial societies, class and color are inextricably mixed. Class is very different in the two countries, and one hesitates to contemplate the recurrence of riots here.But I would not be at all surprised if, within his four years, President Reagan reassembles the vast rally of whites and blacks which, in the fall of 1963, gathered before the Lincoln Memorial to echo: "I have a dream."

For the superficial in Britain are, as here, now bending their attention to blame television. Of course, television is a carrier; but of what? Most obviously it is the carrier of the evidence of affluence elsewhere int he society. Poor people in the past say only other poor people in the same conditions as them. But night by night now, they watch the commercials, all of which say: "You too can have all this."

Whether it is in Liverpool or Manchester, in Detroit or Newark, television commercials are a revolutionary goad. As an American said to me in the past week, talking of the riots in Britain, "Imagine what a poor mother of children in the inner city feels, when a commercial says that one can borrow $10,000 tomorrow, and she knows that she could not even cash a check at the local Safeway?" That is the impact of television.

If we are to draw parallels between the riots in Britain and those here, then we should push to the heart of the matter as the violence itself does. What we are speaking of is citizenship. Proudly, both America and Britain have said to its colored peoples that they are citizens. Citizenship is the gift of governments. We do not go to Exxon -- at least, not yet -- to get our passports.

The private sector can do much for people, better than any government, but it cannot grant or protect their citizenship. America and Britain work wonderfully to grant, extend and protect the political and civil rights of citizenship. But citizenship is defined in economic and social terms as well; and the irony is that this is the work of the private sector and its seductions.

The thing can be put very simply. The private sector cannot invite the poor into its supermarkets, with all that abundance open to the eyes and to touch, without the poor being affected by the fact that it is so near and yet so far. It is not within the power or even function of the private sector to cope with that disparity. The citizenship in the supermarket and in the commericals can be granted only by government. Otherwise, people will smash store windows and steal.

Can you get that, Mrs. Thatcher? Can you get that, President Reagan? If you do not, though you do not intend it, your gutters will one day run with blood.