POPE LEO XIII USED TO go through the streets of Rome in the 19th century in a gilded horse-drawn carriage. Pius XI tooled around the Eternal City in a 1930 Mercedes Nurberg. His namesake, Pius XII, toured in an open car with a papal throne in the back seat.
But because the times have grown more lethal, John Paul II is confined to his popemobile.
No pope in the 2,000-year history of the Roman Catholic Church has traveled so far and wide as John Paul II, nor behind so much bolted-down armor plate and bullet-proofed protection.
And for good reason: People have tried to kill him.
In 1981, a Turkish terrorist shot John Paul II in St. Peter's Square. He survived that, but a year later a deranged Spanish priest tried to stab him at the shrine at Fatima, in Portugal. A year and a half ago in Seoul, South Korea, a disturbed youth fired an air pistol at him.
The popemobile - so-called by the Vatican - was introduced after the 1981 shooting as a defense against assaults. The innovation permitted the pontiff to continue to put himself among the faithful and at the same time provide him with a degree of safety.
Since his election in 1978, John Paul II has proved to be the most gregarious of pontiffs. More than any of his recent predecessors, he wants to see his people up close.
Just as the face of John Paul II probably is familiar to more people on Earth than any other, so this strange white vehicle, with the glass bubble on top, diminutive wheels (to facilitate rolling it on and off transport planes) and papal crests, probably is the most familiar conveyance.
John Paul will ride in the popemobile after arriving Sunday morning at Oriole Park at Camden Yards to celebrate Mass. Afterward, the popemobile will carry him through the downtown, bringing him to the residence of Cardinal William H. Keeler at Charles and Mulberry streets.
The rest of his time here - during his trips to the Cathedral of Mary Our Queen and St. Mary's Seminary - the pope will travel in other "presidential" vehicles, said Stephen W. Mason, special agent in charge of the Secret Service in Baltimore.
The first two popemobiles were built by Mercedes-Benz after the 1981 assassination attempt. They are white sedans wrapped in armor and bulletproof glass. Since then, according to a count made two years ago, they have been joined in the Vatican garage by four others.
Fourteen other popemobiles await the arrival of the pontiff in different parts of the world. Two years ago, during John Paul II's visit to Denver, he rode in a popemobile manufactured in and brought up from Mexico.
No popemobiles are kept in the United States, said Mr. Mason, who said he did not know the source of the one the pope will use here. For security reasons, such information is not freely given. However, a spokeswoman at the Mercedes-Benz headquarters in Montvale, N.J., said that during this visit, the pope would be going around in a Mercedes Gelanderwagen.
In 1984, in anticipation of a papal visit, the Canadian government had Thibeault Inc. - which builds 60 percent of the world's firetrucks - construct two armored limousines. They cost $130,000 each.
Countries which can't afford such extravagance do what they can. During the pope's 1987 visit to his native Poland, John Paul rode atop a flatbed Polska truck, surrounded not by plate steel or bullet-proof glass, but flowers.
Poor countries usually improvise, welding metal sheeting to the sides of vans and utility vehicles, adding a coat of white paint to suggest the gleaming vehicles of richer lands. The version jerry-rigged last year in Papua, New Guinea, resembled a bank truck with glass four inches thick.
In most countries, popemobiles transform themselves into white elephants the moment the pope departs. But a little improvisation can help recoup the initial investment. For instance, after the pope left the Philippines last year, the Manila city government began to sell rides, to permit people to tour the stadium where the pope said Mass, and to have their pictures taken inside the glass bubble which, it is said, could withstand grenade blasts and machine gun fire.