March 31, 1899.] 



SCIENCE. 



493 



making a brief exposure. In this way each 

 plate coutains its owu data for orientation. 

 The author thinks that the somewhat large dis- 

 crepancies between this method and that by 

 meridian circle observations is due to the jai-ring 

 ■of the plate by stopping and starting the clock. 

 Its value as an independent method, however, 

 is recognized. 



THE SOLAE ECLIPSE OF MAY 28, 1900. 



The committee appointed at the recent con- 

 ference of astronomers and astrophysicists to 

 consider the observations to be made at this 

 eclipse has issued a circular letter asking for 

 opinions as to the observations deemed advisable 

 and what cooperation our American astrono- 

 mers can render. The eclipse path extends 

 from the Gulf coast to the Atlantic, but the 

 duration of totality is short, only 1" 13' near 

 New Orleans and 1" 40* near Norfolk, Va., ac- 

 cording to the circular. The figures given by 

 the circular of the English Nautical Almanac 

 are a few seconds larger than these, 1™ 17'.8 

 west of New Orleans, and 1"° 45". 6 south of Cape 

 Henry, Va. Some excellent points of observation 

 may be found in Portugal and Spain, where the 

 totality will range from 1"' 34'' to 1'" 19*. Euro- 

 pean astronomers are likely to locate at this 

 end of the line. American observers should 

 cover thoroughly the path through the United 

 States, which includes many places readily ac- 

 cessible. The U. S. Weather Bureau has is- 

 sued a second bulletin upon the probable 

 weather to be expected. This is based upon 

 special reports made in May, 1898, the former 

 report including those of 1897. A third report 

 for 1899 is promised. The conclusion thus far 

 is that the most unfavorable weather is to be 

 expected on the Gulf and Atlantic coasts, and 

 that the most favorable locations are in the 

 northern parts of Georgia and Alabama, upon 

 the southern end of the Appalachian Mountains. 

 WiNSLOw Upton. 



Providence, E. I., March 15, 1899. 



NOTES ON PHYSICS. 



THE EFFECT OF COMMUTATION ON THE FIELD 



OF DYNAMOS AND MOTORS. 



Messes. Everett and Peake, in a paper on 

 ' The Effect of Commutation on the Field of 



Dynamos and Motors' in the London Elec- 

 trician of December 30, 1898, find, by means of 

 an exploring coil and instantaneous contact 

 maker, that the effect of commutation is to pro- 

 duce somewhat regularly recurring ripples in 

 the curve connecting E. M. P. and position of 

 the exploring coil, the maximum of the ripples 

 occurring at intervals equal to the width of a 

 coil, decreasing in magnitude as the distance 

 from the commutated coil increases and nearly 

 disappearing before the interpolar gap is passed. 

 These ripples were found to be more marked 

 with narrow than with wide brushes, which is 

 explained by the damping effect of the adjacent 

 short-circuited coils acting as secondaries to 

 each other. The ripples are also more marked 

 for heavy than for light currents and for motors 

 than for dynamos. 



TELEC4RAPHY AND MAGNETIC INDUCTION. 



S. EVERSHED, in an article on ' Telegraphy by 

 Magnetic Induction' in the same journal, de- 

 duces a formula for the mechanical energy 

 available in a distant secondary circuit in which 

 no capacity is used, in terms of dimensions, re- 

 sistance, frequency, etc. , and from this calculates 

 that in the case of two circuits using together 

 1,000 kgm. of wire, each 1,000 meters square 

 and 10 kilometers apart, with a frequency of 100 

 and 100 watts in the primary, there would be 

 available in the secondary .34 ergs, per second. 

 Experiment shows that 2.'9 X 10"'' amp. gives 

 easily readable Morse signals in an ordinary 

 telephone, this being double tlie audible current 

 (this presumably for a frequency of 400). He 

 then finds that in the above case, but with fre- 

 quency equal to 400, there is 12 X 10"" amp., 

 and that hence the readable signals could be pro- 

 duced with 250 kgm. of copper. For satisfac- 

 tory audible signals the frequency must be at 

 least as high as 400, and here the undetermined 

 effect of absorption of these waves by the ma- 

 terial of the earth comes in. If this proves 

 serious it may be necessary to use lower fre- 

 quencies and other forms of receivers. A re- 

 ceiver is described consisting of a tuned rec- 

 tangle of wire, vibrating in a strong field, or, 

 better, two rectangles vibrating synchronously, 

 but in opposite directions. Such instruments 

 are being used at Lavernock and Flat Holm as 



