296 



- KNOWLEDGE 



[May 18, 1883. 



portions of the iron framework K K'. The wire is coiled 

 round the cores so as to make the adjacent pole-pieces, 

 N N' and S S', of similar polarity, and is also continuous 

 from B' round both field magnets to B- of the commutator 

 (C Figs. 1 and 4), of which more anon. 



The armature (A, Figs. 1 and 4) is the most important 

 feature in the Brush dynamo. It consists of a core of soft 

 iron in one piece, and ha^^ng wound round it, in specially- 

 constructed grooves, a number of coils of copper wire. 

 Fig. 2 represents a portion of the circular core, the grooves, 

 G G, having parallel sides, so that the coils are of equal 

 dimensions throughout. The coils are made sufficiently 

 thick to be flush with the projections, F F, and so present 

 almost perfectly level surfaces to the field magnets during 

 the process of rotation. Roimd the core is a groove, C, which 

 extends almost to the inner surface, while on both faces of 

 the projecting portions are series of concentric grooves, H H. 



The armature of the 16-light machine is twenty inches 

 in diameter, and contains eight coils of No. 1-t B. W. G. 

 cotton-covered wire, each coil weighing about 20 lb., and 

 containing about 900 feet of wire. All are wound in the 

 same direction, the inner end of each coil beiny con- 

 nected to the inner end of the coil, exactly opposite to it 

 (there being in all machines an even number of coils). 

 The outer ends all pass along the shaft (S S, Fig. 4) to 

 the commutator, C^ C-. 



The commutator is that portion of the machine where 

 the various currents generated in the coils of the armature 

 are collected and made to pass through the external circuit 

 in one common direction. It is very unique, difl'ering from 

 all its competitors in more than one important feature. 

 It is, of course, fixed on to the shaft, but instead of being 

 in the form of a series of discs radiating from the centre 

 of rotation, it consists of a number of copper rings or 

 flat cylinders, there being a ring to each pair of armature 

 coils. Each ring (Fig. 3) is divided into three parts, of 

 which A is connected to one free end of a pair of arma- 

 ture coils, and B to the other end. C is quite isolated 

 from the other segments, and is practically an insulator. 

 Each of the three parts is separated from the shaft 

 S by means of an insulating substance, and from each 

 other by air-spaces. B', B-, &c., are flat strips of 

 copper, with a number of longitudinal cuts. They are 

 aljout two inches wide, and are called brushes, their function 

 being, by pressing against the copper rings as they revolve, 

 to collect the current from the coils, or rather to connect 

 the machine with the external circuit As there are four 

 pairs of coils in the armature of the machine being de- 

 ' scribed, there are four rings, for which two pairs of brushes 

 are provided — one pair for two rings. One of each pair 

 presses above and the other below the rings, the points of 

 contact being on the same diameter of the ring. 



The consideration of the principles upon which this 

 interesting machine is constructed we are compelled, by 

 want of space, to defer. 



THE MOON AND MAY FROSTS. 



ALL gardeners dread the frosts which often come in 

 May, after the vegetation has been stimulated into 

 activity, and the plants are easily injured by a check. In 

 France, the cold is popularly attributed to the moon which 

 is new in April, and reaches its full at the end of that 

 month or early in May. This moon is termed la lune 

 rousse, because, during the clear nights it illuminates, young 

 leaves, and buds freeze and turn red, although the air may 

 be several degrees above the freezing point. 



Louis XVIII. is said to have bewildered Laplace when 

 he visited the Bureau des Longitudes, by asking him to 

 explain how the lune rousse acted upon the crops. The 

 great astronomer looked in confusion, and finally told the 

 king that this moon had no place in astronomical theories, 

 and in consequence the Bureau was unable to answer his 

 question. 



M. Mabillon, in an article on this subject, in Ciel et 

 Terre, says that this story led Arago to investigate the 

 subject, but he does not give the date of his inquiries. 

 Wells explained such incidents in his celebrated " Essay 

 on Dew," of which the first edition was published in 1814. 



M. Mabillon quotes the following from a work of Plan- 

 tin's, " Ephemerides Perpetuelles de I'Air," published at 

 Antwerp in 1556: — "Si du vingt-et-cinquieme d'Apvril 

 jusques au vingt-et-huitieme d'Apvril la pleine lune 

 advenait, trouvat nuicts sereines et belles, sans aucun vent 

 (auquel temps la rosee ha coutume tumber a grade plante) 

 les anciens de longue experience tenaient tout certain que 

 les grains de la terre seraient endommagez." The reader 

 will be amused with this old French. It means that when 

 full moon comes between April 25 and 28, with clear nights 

 and no wind blowing, " Dew has the custom of falling in 

 great plenty, and long experience makes it certain that the 

 crops of the earth will be damaged." If dew forms on nights 

 when the clear air enables plants to lose much heat by 

 radiation, they will receive it as hoar-frost ; and if the air 

 is too dry for much dew, the radiation may be stronger 

 and their juices frozen, although the air is considerably 

 warmer. " The formation of dew indeed," said Wells, 

 " not only does not produce cold, but, like every other pre- 

 cipitation of water from the atmosphere, produces heat." 

 Wells found that bodies on the surface of the earth become, 

 during a still and serene night, colder than the atmo- 

 sphere," and then he saw that the gardeners he had smiled 

 at " in the pride of half-knowledge " were right in using a 

 " thin mat or any such flimsy substance," to protect their 

 plants. " On a clear night," he says, " in an open part of 

 the country, nothing almost can be returned to us from 

 above in place of the heat which we radiate upwards." 

 The plants which suffer on the clear May nights 

 receive no warmth to compensate for the heat , 

 they radiate. A cloudy sky is a protection, and so 

 is a damp or misty one. The old writer already cited 

 says that if the conditions he mentioned occur " on the 

 seventh day of the Ides of May," corresponding with the 

 9th of our calendar, "or within three or four days later, 

 vines and olives are in great danger." M. ^Mabillon states 

 that in " the vine-growing districts, the people make 

 artificial clouds by heaping up to the windward dry leaves, 

 with green wood and resinous products, which they ignite 

 some hours before sunset, and thus make a great smoke. 



