Halfway through the Edinburgh Festival, the weather - the hottest, driest summer in centuries, experts say - broke, and the Scottish capital turned into the breezy, rainy, exhilarating northern place annual festivalgoers know and love. But before that happened, I underwent, as in a sauna, the two operas brought to the Festival Theatre by the orchestra, chorus, and soloists of the Kirov Opera from St. Petersburg. This was a Russian experience through and through. The composer of both works was Nikolai Rimsky-Korsakov, who not only polished up several classic operas by others to place them permanently in the repertoire, but wrote his own. The two presented here were ''The Legend of the Invisible City of Kitezh'' and ''Sadko.'' The aura was Russian (both pieces are characterized by the disarming, magical-realism atmosphere of folk tales), the balletic interludes were sparklingly Russian (a section of ''Sadko'' set deep down in the ocean), and the words were Russian. Fortunately, the use of ''surtitles'' (the words in English projected above the proscenium) is an established practice now in Edinburgh. But because of Rimsky's demotion of librettos to a subsidiary role in contrast to his music, it is not essential to understand what is being sung. Sometimes, though, the surtitles did seem distractingly literal to the point of laughableness, as when the self-regarding minstrel, Sadko, callously walks out on his wife with the words, ''I do not marvel at woman's reason. Long in hair and short in brain. Farewell, unlucky wife.'' The unluckiness is a matter of opinion, but the good woman's sorrow was one of the few convincing evocations of genuine feeling in this ''opera in seven tableaux'' - more credible than Sadko's joy at rejoining her in the opera's happy ending. Determined amateurism Both operas, as presented - with delightful, traditional scenery, appallingly bad acting, and a very slow pace - seemed like authentic re-creations of late-19th, early-20th century productions. There were no concessions to the current Western approach of giving opera believable dramatic intensity and even lead singers of lithe and youthful proportions. These two Russian operas were more like English pantomimes in their ''popular'' colorfulness and determined amateurism. The final scene in ''Kitezh,'' as Scottish critic Cordelia Oliver remarked, had all the trappings of the final ''transformation scene'' of a pantomime, with the good characters dressed in celestial white, and the lovers once again together, doubtlessly forever. But the orchestral playing, surging or growling, sweeping or luscious as required, was far from amateur, and the singing of even the smallest parts - not to mention the glorious choruses - was superb. The song of the ''Indian Guest'' in ''Sadko,'' sung by Gegam Grigorian, lingers exquisitely in the memory. If these enjoyable but overlong operas put little store in dramatic compulsion, the same cannot be said of two of the plays that have made this particular festival memorable for its drama. 'Entering the Protestant mind' ''Observe the Sons of Ulster Marching Towards the Somme'' is a revival of a play premiered by Dublin's Abbey Theatre (Ireland's national theater) 10 years ago. Significantly, it toured to Northern Ireland: This is a play written by a Republican playwright with a Roman Catholic background, Frank McGuinness. Its revival now is not only to bring it to Edinburgh, but also to mark the one-year-old 1994 cease-fire in Ulster (Northern Ireland). During his visit to Edinburgh, McGuinness said the play was his ''challenge to somehow enter the Protestant mind.'' It became for him ''a massive act of recognition'' of the ''diversity of Ulster Protestant culture.'' In its vividly felt writing, as well as its tense production and taut acting by a nine-man cast, the ''recognition'' comes across. But McGuinness, by choosing as his context the World War I ''blood sacrifice'' of British soldiers (in this case Northern Irish ones) achieves something else. The Northern Irish soldiers fighting alongside the English knowingly sought martyrdom and so started a ''myth'' still potent today. The play becomes a means of eliciting sympathy and, indeed, some admiration for nine human beings caught in an impossible predicament. Simultaneously, though, it stirs an uneasy sense of outrage at the profoundly mindless ''loyalty'' that binds them (to the death) not only to each other, but to their tribal ethos. Deep criticisms do nevertheless lie underneath the playwright's effort to understand. The most unsympathetic criticism - played out by two of the men, one a failed preacher, the other ''half a Catholic'' because his mother ''never converted'' - is aimed at the supposedly arrogant Protestant assumption of individual Godlikeness. It may be the author's intentions that surface when the second man almost brutally challenges the self-tortured preacher to admit he is no different from anyone else. Perhaps becoming a mere soldier had already brought him close to this humiliating but - McGuinness seems to imply - self-saving conclusion. He then goes off, ''like everyone else,'' to die. The play as a whole could be said to generate more heat than light. But one thing is certain: It is finally a remarkable analysis, not of ciphers or stock characters seen through the fog of myth and prejudice, but of nine hard, true, if somewhat messy, individuals. Individual freedom vs. despotism ''Don Carlos'' is both an older and a newer play. Friedrich Schiller wrote it in 1787. Robert David MacDonald's translation for the Citizens Theatre (from Glasgow) is new, and although in blank verse, comes across as modern in the urgent, crisp eloquence of the Citizens cast. This is a play of ideas; at its crux a confrontation of Romantic individual freedom with ruthless despotism. The Protestant-Catholic debate is a part of this absorbing play, too, set historically in 16th-century Spain, but feeling more like 18th-century Germany. The twists and turns of court intrigue, as well as the unstable emotionalism of the young prince, Don Carlos, suggest echoes of Hamlet. But there is little of the Dane about this play's ideas (though the individuals voicing them end tragically). They are idealistic, hopeful - the inevitable wave of the future. Nothing of Hamlet's cynical self-doubt and defeatism here. It is not the prince, but his friend the Marquis of Posa, who really confronts the tyrannical King Phillip II. The king, until then apparently devoid of any human warmth, seems momentarily persuaded by this free-thinking, fearlessly outspoken young man. ''I cannot be the servant of a prince,'' Posa states. The king, who expects his service, looks at him in astonishment. Posa states his reasons: He does not need to serve the king in order to sow happiness. ''This ... I can create alone, and I would feel/ a joy there, and a freedom of decision/ where here it could be nothing more than duty.'' And then he asks the king: ''Can that be what you want?'' The king does not answer this. But later, when Posa asks him, ''Since you have reduced Mankind/ to be an instrument to play on, who/ will share the harmonies with you?'', then the despot is definitely moved. Although dramatically the play was compelling, it was this question that seems quite as relevant today as it must have been to Schiller, that I took home with me. Today it is probably more likely to apply to the manager and the managed in business. But the question of creative individuality smothered by a dutiful, unthinking loyalty is not one that needs to be asked just in historical dramas staged at summer festivals. It has a thought-provoking afterlife.