May 2nd, 1SS7.] 



SCIENTIFIC NEWS. 



67 



ARTIFICIAL WHIRLWINDS. 



AT a recent meeting of the Academic des Sciences, M. 

 Mascart gave an account of some experiments made 

 by M. Weyher to illustrate the action of whirlwinds. The 

 accompanying illustration will assist our readers to under- 

 stand the effects produced. The drum overhead was about 

 one metre in diameter, and was placed about three metres 

 above the water in the tank below. The drum was closed 

 on the top and open at the bottom, and inside of it there 

 were several blades, as in an ordinary rotary fan. When 

 the drum was rotated the surface of the water was covered 

 with spirals, all converging to a common centre. On the 



APPARATUS lOR PRODUCING -ARTIFICIAL WI^RLW1^DS. 



speed of the drum being increased to about thirty to forty 

 metres per second at its circumference, the water was raised 

 in the form of a cone, about twenty centimetres in diameter 

 at the base, and ten to twelve centimetres high. Another 

 cone, inverted, was also formed above the first one, and fine 

 drops of water and spray were carried up higher and thrown 

 out on all sides. 



These experiments were made in the open air, and were 

 certainly interesting, as the effects produced so closely re- 

 sembled those caused by whirlwinds on a larger scale. Any 

 one wishing to try the effects himself can, however, do so 

 with greater ease by using a small apparatus, enclosed 

 in a glass cylinder, so as to avoid cross currents of air and 

 wind, which are troublesome. We are indebted to our 

 contemporary. La Nature, for the illustration. 



In America, in order to save the postman the trouble of knock- 

 ing at the door when he places a letter in the box, an arrange- 

 ment is being introduced whereby the raising of the llap of the 

 box closes an electric circuit and rings a bell. 



FORMATION OF DEW. 



FROM the days of Aristotle, observant men have sup- 

 posed that the moon and stars had an important 

 influence on the formation of dew. They saw that it 

 appeared only in calm and clear nights, and concluded that 

 moisture was detached in very small particles from the chill 

 air. The earliest speculators thought that dew fell from 

 the heavens. But in the beginning of the seventeenth cen- 

 tury, Nardius, of Florence, made the bold assertion that 

 dew was an exhalation from the earth. After much wrang- 

 ling, this principle was accepted by scientific men for a 

 century and a half. The experiments of Wilson, of Glas- 

 gow, and Sisc, of Canterbury, conflicted with this view, 

 and it was left to Wells to propound the theory which, until 

 a year ago, had been adopted by scientific men. 



According to this distinguished observer, dew was con- 

 densed out of the air near the surface of the earth, and the 

 great advance he made was to show that the amount of dew in 

 any night on any body depended on the power of that body 

 to radiate heat at the time; that the more a substance was 

 cooled by radiation, the more dew it collected, and that a body 

 before it got dewed, was cooled by radiation to a temperature 

 below the dew-point at the place. As to the source of the 

 water vapour, he considered it was in the air during the heat 

 of the day; but he admitted that some dew might rise from 

 the ground at night. He, however, failed to prove that 

 dew did or did not rise from the ground, and this discovery 

 was left to another observer of our own daj'. 



Mr. John Aitken, of Darroch, after minute observations, 

 found that he could not accept the theory of Wells, that 

 most of the dew was condensed out of the air near the 

 earth's surface, and he doubted the truth of the latter's 

 opinion as to the source of the vapour which forms the dew. 

 He attacked the established theory in two essential 

 points, viz., as to the formation, and as to the nature of 

 dew. We will confine ourselves, for the present, to an 

 explanation of the first of these points. 



Mr. Aitken was aware that Le Roi, of Montpelier, had, in 

 his treatise " Sur la Rost'r," noticed that dew was formed 

 under an inverted bell-glass as much as in any other situa- 

 tion. He learned from Gersten, that a plate of metal laid 

 upon bare earth on a dewy night, remained dry on its upper 

 surface, while being quite moist below. He studied Du 

 Fay's perplexities, that dew appeared earlier upon bodies 

 near the earth than upon those which were at a greater 

 height. He noted Webster's dreamy speculations on the 

 rising instead of the falling of dew, from the fact that the 

 lower part of a metal plate (in consequence of its upper 

 surface being in contact with the air and exposed to a clear 

 sky) is colder than the earth a little below the surface, and 

 therefore condenses the vapour. He was struck with the 

 unvarying fact that the ground a little below the surface 

 was warmer than the air over it, sometimes to the extent of 

 18"^ Fahr. And he concluded that, so long as the surface 

 of the ground is above the dew-point, vapour must rise 

 from the ground ; the moist air will mingle with the 

 superincumbent air ; and its moisture will be condensed 

 and form dew, whenever it comes in contact with a surface 

 cooled below the dew-point. 



His experiments were thus conducted. He placed a thin 

 metal tray over grass after sunset. On dewy nights the 

 inside was wet, and the turf under it was wetter than that 

 not under its cover. On some nights, the dew was formed 

 only inside ; and on all nights the deposit on the inside was 

 heavier than that on the outside. From this he concluded that 

 far more vapour rose out of the ground during the night than 

 condensed as dew on the grass, and that this vapour from 

 the ground was trapped by the tray. Next, he cut from the 



