It is time for a mid-summer tuneup, musical evenings to soothe breasts made savage by 100-degree heat.

The coolest thing to do would be to rent the barge on the C & O Canal and let the clip-clopping mule draw you through the water, while Handel's Water Music Suite plays in the background. The Park Service rents the barge out for $400 for 3 hours (7 to 10 p.m.) on Wednesday through Sunday nights until Sept. 7; Friday, Saturday and Sunday nights until the last weekend in October.

The barge, which plies the canal from Great Falls Tavern to Swain's Lock and back, will hold about 65 people, with room for chairs and a center section where you can set up tables for dinner. You provide your own food and liquor. They provide an 8-track tape deck; you provide the Handel. To reserve the barge, call Gregory Donaldson, 299-3613.

Now for a little platter patter. If cool is no problem, you can dance your heart out to a light and sound show put together by the professional disc jockeys at Sound Dynamic.Prices vary according to the size of the dance and the complexity of the light show. For a 3-to 4-hour period, they generally charge anywhere from $150 to $350, with a $35-an-hour charge for additional time. Try, if possible, to book one or two months in advance, so that they can work with you to put together the kind of music you'd like.

They can, however, occasionally come through on short notice; they once stepped in at the last minute to fill in at a wedding where the band failed to show up. Call Lyn Jeffries at 340-8377 or 251-0300.

Most rousing of all would be to have a ceili, with its traditional Irish music and dance. Blackthorn Stick, a group which promotes those arts, will furnish a six-piece ceili band (Irish accordion, flute, violin, drum, antique mandolin and not-so-traditional electric bass) and Irish dancers for around $450. Like the rest of us, they prefer to carouse in cooler times and so perform only from September to May.

For less money, they offer taped music and dancers who will demonstrate or teach Irish dances. They can also occasionally draw on the services of both a bagpiper and a woman who plays the Irish harp. Prices for an evening of Irish dance vary according to what you require. Call Harry Schrecengost, The Blackthorn Stick (474-4641) to work out details of your ceili.

Cheapest of all would be to buy a couple of harmonicas, lay a fire in the backyard and invite friends over to roast hot dogs and sing the songs of long-ago campfires.