If the night also be calm, the small portion of air contiguous to any of these surfaces will become cooled below the dew-point, and its moisture deposited on the surface in the form of dew. If this chilled temperature be below 32° Fahr., the dew becomes frozen and is called _hoar-frost_. The above two phenomena, though both expressed in our language by the word dew, which perhaps helps to give rise to a confusion of ideas on the subject, are not necessarily expressed by the same word. For instance, in French, the first phenomenon--the falling evening-dew--is expressed by the word _serein_; while the latter--the dew seen in the morning gathered in drops by the leaves of plants, or other cool surfaces--is expressed by the word _rosée_. The merit of the discovery of the “Theory of Dew” has been commonly ascribed to Dr. William Charles Wells, who published in 1814 his _Essay on Dew_, which obtained great popularity. The merit should, however, be divided between him and several others. M. Le Roi of Montpellier, M. Pictet of Geneva, and especially Professor Alexander Wilson of Glasgow, largely contributed by experiment and inducement to its formation. THE AURORA BOREALIS --RICHARD A. PROCTOR The aurora is one of those phenomena of nature which are characterized by exceeding beauty, and sometimes by an imposing grandeur, but are unaccompanied by any danger, and indeed, so far as can be determined, by any influence whatever upon the conditions which affect our well-being. Comparing the aurora with a phenomenon akin to it in origin--lightning--we find in this respect the most marked contrast. Both phenomena are caused by electrical discharges; both are exceedingly beautiful. It is doubtful which is the more imposing so far as visible effects are concerned. When the auroral crown is fully formed, and the vault of heaven is covered with the auroral banners, waving hither and thither silently, now fading from view, anon glowing with more intense splendor, the mind is not less impressed with a sense of the wondrous powers which surround us than when, as the forked lightnings leap from the thundercloud, the whole heavens glow with violet light, and then sink suddenly into darkness. The solemn stillness of the auroral display is as impressive in its kind as the crashing peal of the thunderbolt. The reader is no doubt aware that auroras or polar streamers, as they are sometimes called, are appearances seen not around the true poles of the earth, but around the magnetic poles which lie very far away from those geographical poles which our Arctic and Antarctic seamen have in vain attempted to reach. The formation of auroral streamers around the magnetic poles of the earth shows that these lights are due to electrical discharges of electricity, which, though only visible at night, take place in reality in the daytime also.