BY IAN NIALL IT is raining at the moment and, as usual, I can .hear, through the open door of my room, the steady beat of the downpour on the sky- light. There is something rather nostalgic about that sound for me because I remember wet summers more than those supposedly endless summers of broiling sun of the long ago. The fact is, I wasn't born long enough ago to know them, and I am fond of summer rain. It puts new life into things, revives the scorched grass, freshens the air so that after it the swifts and swallows are more buoyant. It has become the habit this summer to say of rain, 'Dear me, it didn't come a minute too soon,' or 'There wasn't enough of it. It didn't go down an inch into the soil.' Until this downpour today I could hear a tractor at work trying to make a belated hay harvest of a poor crop of grass. The tractor is silent now and the driver is probably busy drying his shirt and cursing the rain that should have come a month ago.