THE great Leonid swarm of meteors consists of ortho-Leonids which pursue nearly the same path round the sun, and clino–Leonids which move in orbits sensibly differing from the ortho–orbit. The present investigation is concerned with the ortho–Leonids. They form a dense stream extended along a portion of an immense orbit round which they travel in 33¼ years. This orbit has its perihelion a little inside the Earth's orbit, and its aphelion a little outside the orbit of Uranus. It intersects the orbits of these two planets, but lies in a plane inclined to the ecliptic, so that the meteors which traverse it pass under the intervening planets on their outward journey and over them on the homeward journey.