SUMMER seems to have arrived, as you probably noticed, and Washington has undergone its annual tranformation into Calcutta on the Potomac. This is a foolish local myth to the effect that Washington summers used to be more pleasant in the old days. On the contrary, anyone who could possibly flee the town did so, usually by this time in June, and did not return until well in September. It was not only miserably uncomfortable. In a mid-19th century city in a hot climate, summers were positively dangerous.

It's not a bad moment to reflect, with gratitude, on the acommplishments of the people who have made Washington summers survivable: the people who built, and today run -- you probably aren't ready for this -- the water and sewer systems. Everyone takes that immense network of pipes utterly for granted. But can you imagine, as the temperature rises, what life was like without them?

Here we take down from the shelf Constance McLaughlin Green's fine histories of this city: In the 1850s, the water supply for nearly everyone was either a cistern holding stagnant rainwater, or the public pumps in the squares. Most people threw their garbage into the unpaved streets; there the roving pigs might dispose of it, or might not. There were a few rudimentary sewers draining into nearby streams like Rock Creek. But few houses were connected. The sewage from the White House washed out into the swampy field that is now the Ellipse.

How do you get rid of dead horses? The method of choice was to load the carcasses onto a barge, float the barge down the Potomac out of sight and discreetly dump the horses into the river. The sentimental recollections of an idyllic and pristine 19th-century river are not entirely accurate.

The city's first adequate water supply finally arrrived with the completion of the 10 miles of aqueduct from Great Falls, just before and during the Civil War. The construction of the sewers, along with the paving of the streets, came a little later, getting well under way in the 1870s.

One bad year shortly before the pipes were laid, the mortality rate here reached 35 per 1,000 people. That's four times the current level. The average life span then appears to have been about half the present one of about 73 years. Nationally, the infant mortality rate is now down to 12 per thousand births. In the 1850s here, it fluctuated probably between 10 and 20 times as high. Summer was a bad time for small children. They were especially at hazard to the hot weather diseases.

Good old-fashioned summertime. Good old-fashioned dysentery, cholera and typhoid. The practice of medicine is a noble profession. But the people who have saved lives on a grand scale, over the past century, are the civil engineers and the plumbers.