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NATURE 



[August 3, 1893 



In the " Monthly Report of the Maryland State Weather Ser- 

 vice" for May, 1893, Prof. W. B. Clark again refers to " The 

 Leading Features of Maryland Climate" (see Nature, vol. 

 xlvii. p. 585), giving tables of temperature, rainfall, &c. The 

 «ame parallels of latitude show great variations in climate due 

 to the complexity of the surface configuration. 



In the same Report Prof. Clark describes " The Available 

 Water-power of Maryland," only a small portion of which 

 is at present utilised. Most of this occurs in the Piedmont 

 Plateau, the central area of Maryland bounded by the Coastal 

 Plain and the Appalachian Region. The north fork of the 

 Potomac, draining an area of about 1300 square miles, has a 

 maximum discharge of over 700 times its minimum. This great 

 variability, which is nearly fatal to the extensive use of water- 

 power on this river, is attributed to the absence of lakes, the 

 steepness of the mountain-sides, and the narrowness of the val- 

 leys. Some of the tributaries of the north fork are fairly 

 constant in flow. 



We learn from the Botanical Gazette that the University of 

 Minnesota has established an inland biological station at Gall 

 Lake, in Minnesota. The laboratory of marine biology of the 

 University of Pennsylvania, at Sea Island City, New Jersey, is 

 now open for its third summer session. The same journal 

 informs us that Baron von Midler is intending to publish a 

 volume which shall complete Bentham's " Flora Australiensis." 



Messrs. Krigar Menzel and Raps have contributed an- 

 other instalment of their work on the motion of vibrating strings 

 to the Prussian Academy of Sciences. Their beautiful experi. 

 ments on the continued vibrations of bowed strings have been 

 supplemented by the photographic study of the peculiar motions 

 exhibited by plucked strings. To confine the vibrations strictly 

 to one plane, and also to control the instant of exposure, 

 a special plucking apparatus was designed. The string was kept 

 resting against a small plate in the vertical plane by means of a 

 hook which could be released by pressing upon a lever. The 

 motion of the lever also closed a circuit which released the 

 instantaneous shutter of the camera. The wire vibrated in front 

 of a slit illuminated by an arc light, an image of the slit being 

 projected upon the wire so that the screen of the camera showed 

 a well-defined bright slit interrupted by a dark spot where it was 

 crossed by the wire. This dark spot would vibrate during the 

 oscillation of the string, and a trace of its motion was obtained 

 by receiving the image upon a revolving drum covered with 

 bromide emulsion paper. The point at which the string was 

 plucked was determined by obierving the interval between the 

 sounds emitted by the two parts on either side of the hojk. 

 Different vibrating points along the string were photographed, 

 and beautiful white-on-black traces were obtained. The general 

 type of these is represented by a zig-zag line with straight flat 

 portions at the top and bottom of each wave. All the component 

 lines are straight, showing that the point of the string moved from 

 one extreme of displacement to the other at constant velocity, 

 then had a period of complete rest, and afterwards returned to 

 the first position, again at constant velocity. As the vibrations 

 succeeded each other, the top and bottom portions gradually 

 slanted towards the middle, some of them showed ripples, and 

 the up and down lines exhibited a slight convexity towards the 

 left, i.e. the past. The authors further showed that all these 

 observations are to be explained by the accepted theory of the 

 vibration of strings, as worked out by Kundt and others. 



The last number of the fournal of the Institute of Electrical 

 Engineers contains an important paper by Mr. W. B. Sayers on 

 the prevention and control of sparking ; continuous-current 

 dynamos without winding on the field magnets, and co nstant- 



NO. 1 240, VOL. 48] 



pressure dynamos without series winding. Both Mr. Swinburn 

 and Mr. Esson have given expressions for the maximum 

 load, which can be carried without sparking, in terms of the 

 ampere-turns upon the armature, the length of the air-space, the 

 angle subtended by the polar surfaces of the field magnets, and 

 the forward induction. Thus in ordinary ring and drum arma- 

 ture machines the considerations of sparking limit some of the 

 most important elements in the design of the machine. So that 

 the lightening of machines by putting the conductors in tunnels, 

 reducing the air-space to a mere clearance, which is the condition 

 in which minimum exciting force is required, has not been 

 hitherto practicable. In order to secure the sparkless reversal 

 of the commutator section under the collecting brush at any 

 desired place between the horns of the pole-pieces, the author 

 has designed a machine whose chief peculiarities are as follows: — 

 The air-space is a mere clearance — one millimetre. The re- 

 versal of the sections is effected by inductors, or coils, which 

 he calls commutator coils, and are independent of the winding. 

 These commutator coils are not inserted in the closed or 

 re-entrant circuit of the ring or drum, but are inserted in 

 the connections that run at intervals from the re-entrant 

 winding to the several bars of the commutator. The function 

 of these coils is to furnish electromotive forces that will balance 

 those due to back-induction and self-induction in the sections as 

 they are successively reversed. These commutator coils are 

 so arranged as to be acted on by the pole-tip which is 

 strengthened by the armature current, and the brushes of the 

 machine when run as a generator are set with a backward lead 

 instead of a forward one. These auxiliary coils also permit of 

 the reversal of the armature sections just after they have emerged 

 from under the strengthened pole, the result being that those 

 turns of the armature which have hitherto been called back turns 

 become forward turns, and the effect of the cross induction is to 

 increase the reversing field instead of to diminish it. The 

 machine is self-exciting by means of the armature windings only, 

 that is, it generates a current without any winding on the field 

 magnets, which may, therefore, more properly be called keejiers, 

 and runs absolutely without sparking at the brushes. 



Apparently Humboldt's description of the combats 

 designedly brought about between wild horses and electric 

 eels, in order to effect the capture of the latter, has to go 

 the way of many others. A writer in the Spectator, who 

 has travelled on the llanos of Caraccas — the scene of 

 Humboldt's account — says that he failed to find any con- 

 firmation of this method of capture. He adds that those 

 who have investigated the matter have come to the conclusion 

 that trembladores, as the eels are termed, could not be taken 

 with the help of horses. The method of capture usually 

 adopted is by nets, and it is found that by wearing indiarul)ber 

 gloves, the fish can be handled with impunity. 



The Photographic Annual for 1893, edited by Mr. Henry 

 Sturmey, has been published by Messrs. Iliffe and Son. It 

 is a remarkably fine production, and contains a vast store of in- 

 formation of interest to all concerned with photography and its 

 various applications. Among articles of bibliographical im- 

 portance we note one on the progress of photographic chemistry 

 during 1892, by Mr. C. H. Bothamley, and Mr. Albert Taylor's 

 concise description of all that was done in astronomical photo- 

 graphy during the same year. Photography in relation to 

 meteorology is the work of the late Mr. G. M. Whipple ; and 

 Mr. Chapman Jones is responsible for a portion of the volume 

 devoted to photographic optics. In addition to this section on 

 the making of photographic history, there is one containmg 

 articles on " Practical Subjects by Practical Men," which consists 

 chiefly of ' ' dodges " devised by devotees of the art. Numerous 

 excellent specimens of half-tone engravings embellish the pages 



