Have you noticed that many outdoor writers regularly preface statements i fishing articles with 'usually", "generally", "frequently", "often", or, "as a rule"? They have learned not to use words like "invariably", always" or never" when talking about fishing.

Fly fishing, especially, is a game of ever-changing variables. Basic rules stay the same for the fisher, but the fish can't be taken for granted. I is an exceedingly inexact science.

However, one general rule does seem to hold true. That is that the dog-days of July and August find stream trout fishing at low-ebb.

Northern brook trout waters, usually blessed with ample shade, are still capable of producing impressive catches, but fishing pressure is practically nil because of deer flies and mosquitoes. Here, long sleeves, head nets and insect repellants enable a few hardy souls to conduct business as usual.

Farther south, it's a different story. Brown trout streams are literally trampled to death by now and, more often than not, are low and clear. The combination of hot weather and low water is bad news for the stream angler.

The chances are, tactics that produced well in early season will bring only frustration now. A few minor changes in strategy however, often can revive fishing action.

Remember, as thermometer readings rise, trout gravitate towards the areas best supplied with oxygen. They remain active at higher temperature if the water is well oxygenated.

Aerated water does for them what air conditioning does for us. It makes them happy and comfortable. Their metabolism speeds up and they are hungry. Broken water like rapids and riffles are their air conditioned restaurants.

The circling eddies where rapids empty into pools often hold good trout even in midday and adequate oxygen means they are alert and hungry.

Work riffles and rapids with gray or tan nymphs (remember, light colored flies in warm weather and darker ones in cold) by casting down and across and letting your fly or flies swing across the current. Do this several times before extending line a foot or two and trying new water. Work all the broken water thoroughly.

A little weight on the fly or leader is often beneficial. There are good holding spots all through a rapids, many of them above or below stones that are not visible on the surface. Weight will help your fly probe these places.

Nymphs can be weighted when constructed by wrapping lead wire around the hook before the body is made. Use a pliers to flatten the wound lead so hook clearance isn't lost.

Lead wire is also good for taking a leader down. Use fine wire and make single overhand knots around the leader and pinch tight. Usually finger pressure alone is sufficient. Be sure to break off projecting ends so the current can't spin the lead like a propeller and twist the leader. With light wire several of these can be spaced along a leader without materially affecting casting.

Keep flies on the small side. To my way of thinking, size 14 is about right for daytime fishing. A.007 inch diameter tippet should work fine.

When downstream fishing, hooks with offset points usually hook more surely than those with regular bends. A trout almost always takes a fly with its lips, which of course turns the hook on the side. When taken going away as is usually the case downstream in fast water, a conventional hook is more easily pulled away from a fish than is one with an offset point.

Points on most hooks can be offset a little by merely pulling on the shank while the fly is still in the vise. Otherwise use a pliers and move the point sidewise slightly.

Bringing a good fish upstream that has taken a fly in rapids or other heavy water is different. He has all the odds. He can hang straight downstream, brace against the current and rest. He doesn't have to exert himself at all.

The best way is to keep the fish working and swimming across and again the current by extending the rod tip first off to one side then the other and zig-zagging him upstream.

Now is the time when staying on the stream an hour or so after dark is apt to give you a chance at a lunker brown trout.

This is the witching hour. Much mid and late summer insect activity peaks after sun down. Adult caddis flies flutter along the stream edges and across its surface. Their larvae move around in the stream flow and large burrowing mayfly nymphs leave their daytime homes to forage for food.

The combination of higher daytime temperatures, bright sunshine, and the availability of food has converted the large, wiser fish from diurnal to nocturnal feeding.

Then too, they feel vulnerable in the gin-clear water and are reluctant to leave their hiding places while the sun is high.

Use bigger nymphs now. Gray and tan are still good colors. Size 12 and even 10 will produce well and hold better than the small ones you used in the daylight.

Leader tippets can be heavier with the larger flies. This is good because nighttime casting will find you catching on stream side grass and bushes more often than expected.

Fish around snags and under-cut banks where you would expect the large trout to rest out the day. As the night progresses, they often move into shallow riffles to feed, so cover these carefully. Fish them just as you did in the day time.

Be sure to prospect in daylight the part of the stream you intend to fish at night. Drop-offs, bogs, barbed wire, and muskrat holes that are hazardous anytime are many times more so after dark.

If your first try isn't successful, try another. Remember fishing is an inexact science. We follow the rules but we can't be sure the fish will. This much is sure, your chances of connecting with a lunker brown at this time of the year are much better after dark.

Here are a couple of tips on playing big trout. If one heads for a snag and it looks like you would be likely to break off trying to stop him, just release the pressure. Often he will change his mind.

If he does go under a snag, lay the rod down and leave for 15 minutes or so. Almost always he will come out of his own accord and you can resume playing him when you return.