A horn Mozart might have recognized--without all the valves and tubing of the modern double horn. In order to play in different keys, you'd plug in different crooks. From Stanford University's collection.
Early orchestras had a basic wind component of two oboes, two horns, and two bassoons. Flutes were less common, and clarinets didn't start showing up as "standard equipment" until Mozart's heyday. The symphony itself derived from the early three-part Italian opera overture (called "sinfonia"). Horns have often figured prominently in operas. However, until the "hand stopping" technique was developed in the Classical period, allowing for smoothly connected melodies, the horns could only play the notes of the natural harmonic series, as bugles do.
A simple way to think of it is that until Mozart's time, you could expect to hear horns in fanfares for gods and royalty or for rousing hunting music. In the Romantic period (after 1815), horn makers developed valves that allowed horns to play in different keys (without plugging in tubing called "crooks"). By the end of the 19th century, horns could be played pretty well in tune on any note of the scale, allowing composers to write music of great complexity for the instrument. Wagner, of course, developed his "Wagner tubas" to supplement his horn section. With Wagner and Strauss, the horn section grew from the four of Weber's Romantic orchestra to as many as 12 instruments!
So going chronologically:
Most Melting. Mozart's Così fan tutte: "Per pietà," Fiordiligi's achingly exquisite aria in which she agonizes over whether to remain faithful to Guglielmo or transfer her affection to the dashing newcomer (Ferrando disguised as an Albanian). The horn echoes and multiplies Fiordiligi's tender sentiments and questionings. (Act II; the horn part starts at 4:00) Soile Isokoski sings "Per pieta"
Most Determined. Beethoven's Fidelio: "Abscheulicher, wo eilst du hin" is Leonora's call to battle as she vows to rescue her husband Florestan from certain death as a political prisoner. Horns are great throughout but the end is fun starting at 6:58 (Act III). Karita Mattila sings "Abscheulicher" Karita Mattila (dressed as "Fidelio," a man)
Most German. Weber's Der Freischütz is chock full of fab horn music. Two places stand out: the joyous hunter's chorus in Act III and the magical moment in the "Wolf's Glen" scene when the Black Hunter's ghostly hunt is summoned up from the fires of Hell at the casting of the 5th magic bullet (Act II finale about 8 minutes in). Weber writes a dissonant parody of the hunting music to let us know that something evil is happening!http://www.youtube.com/watch?v=aHFzg9WGot4
Most Suicidal. In "Fra poco a me ricovero" from Lucia di Lammermoor, Donizetti features the horns in the introduction to set the mood of arch-Romantic self-pity as Edgardo prepares to kill himself in the graveyard of his ancestors. A lot of audience members miss this aria because they think the show's over with the "Mad Scene," but I think this is the best music in the opera. The aria proper begins at 4:40 with more of them beautiful horns. Pavarotti at the top of his form in 1982!
Most Naughty. Strauss's Der Rosenkavalier: the opening orchestral prelude depicts a passionate bout of lovemaking between young Octavian and his older married lover, the Marschallin. The music bounds forth with panting, ever-increasing energy until three tremendous horn "rips" alert us to the climax at about 45 seconds in. And I mean "climax" literally! Wagner's musical climax in the "Liebestod" (Love-Death) at the end of Tristan und Isolde is significantly multi-orgasmic but we'll discuss that another time. Carlos Kleiber leads a stunning performance.
By the way, even though we Americans learned to call the instrument a "French horn" in music appreciation class, to the rest of the world it's just a horn. "Cor" in French, "corno" in Italian, "Horn" in German. Ironically, the American "French" horn is based on a German model! Anyway, Happy Birthday, Deb, and a tip of the spit valve to all the great horn playing in opera!
A cool video of a woman playing Siegfried's horn call in a beautiful European palace.