IT would be interesting if any of the readers of NATURE could give some information respecting the usual height of cirrus clouds. Mr. Clement Ley, in his work, “The Laws of the Winds,” states—“The time occupied by these clouds in passing from the zenith to 45°, or the contrary, furnishes us with a standard of measurement which is both convenient for simultaneous observations, and also possesses this obvious advantage, that whenever the altitude of the cloud station is at all determinable, none but the simplest of calculations is required in deducing the actual from the apparent velocity.” Granted; but it would have been advantageous had he shown by an example what he means. For, he goes on to say, “The ordinary range of the actual rapidity of this current is about twice as great as that of the rapidity of the surface winds, for while the latter, at stations most fully exposed to their violence, rarely attain, in Europe, a velocity of 60 or 70 miles an hour, the most elevated clouds not uncommonly traverse a distance of 120 miles an hour, and occasionally much more.” Coupling this with the next statement—“I have only once or twice observed an actually motionless cirrus cloud, and it is on rare occasions that an hour is occupied in passing from the zenith to 45°,” let me ask, what would be the vertical height of such a cloud?