August 24, 18S3.] 



SCIENCE. 



•J51 



made in a screen opaque to lisl't; for the developed 

 curves show distinctly tlie outline of tlie holes. If, 

 therefore, the slieet-irou in the form of a circular 

 plate, pierce i with a number of holes, be rapidly ro- 

 tated between the poles of a magnet and small 

 induction bobbins, the action of the magnet on the 

 core of the bobbins will be periodically modified, be- 

 cause of the passing holes : and hence induced cur- 

 rents will flow throxigh a circuit including the bolibin. 

 A disk of sheet iron was pierced with two circles of 

 quarter-inch holes concentric with the disk, the num- 

 ber of holes in the two circles being thirty-two and 

 sixty-four respectively. On one side of the disk was 

 placed a horseshoe magnet with its poles very near 

 the rows of holes ; on the other side were arranged 

 two corresponding induction l)ol>bins. The circuit 

 was completed through a telephone and either bobbin 

 at pleasure. Upon rotating the disk rapidly, a clear 

 musical sound was produced in the telephone, the 

 pitch rising with the rapidity of rotation. Moreover, 

 the bobbin opposite the circle of si.\ty-four holes 

 gave the octave above the other, and each gave a note 

 of the same pitch as was produced by blowing a 

 stream of air through the corresponding holes. 



Magnetic survey of Missoviri. 



BY F. K. XIPHEB OF ST. LOUIS, MO. 



Ix the spring of 187S a survey of Missouri was 

 begun, whicli was expected to determine all points in 

 regard to terrestrial magnetism: 100 points have been 

 covered. The work was undertaken under private 

 auspices, most of the money tendered unasked, and 

 the work has been carried on successfully until the 

 present time. The first Jtliree years were spent in 

 making a preliminary survey. In the early part of 

 the survey we labored under great difficulties, because 

 I supposed that the lines of e<|ual value, laid down 

 upon the observations given in the roast -survey 

 charts, were substantially correct; so that time was 

 frequently lost in repeating values at stations left 

 behind, in order to be certain that no error had been 

 committed. But when we settled down to the con- 

 clusion that we really knew nothing about the mat- 

 ter, we had very much Jess trouble. At first, intensity 

 determinations were made at each station ; but in later 

 years, since the magnets have proved so satisfactory, 

 the plan was adopted of making absolute determina- 

 tions only at regular hitervals during the summer. 

 The temperature corrections for the magnet were 

 made twice, — once in 1S7S, and once two years 

 ago. — and they agreed very closely with each other. 



The dip circle was a large one, such as was for- 

 merly much used, and which w.-is found to be an 

 excellent instrument, though rather clumsy to carry. 

 The charts which have been prepared show what the 

 results were. In a former euinniunication to the 

 association at Cincinnati, I suggested an explana- 

 tion of the peculiar flexures of the isogonic lines, 

 as being due to earth-currents which seemed to 

 be deflected by the moist river-valleys. The map 

 upon which that hypothesis was based represenleil 

 observations taken over the entire state, lly re-dcter- 



mination we have found that those observations were 

 all correct; but more detjtiled work shows that this 

 explanation is not admissible. There is no explana- 

 tion of the fact that contour has any thing to do with 

 the deviation of the needle from the normal values. 

 Similar flexures are also seen in the lines of ei|ual in- 

 clination and the lines of intensity. One and perhaps 

 two years will be required to accomplish the work 

 properly. There is nothing new in the subject, ex- 

 cept the rather unexpected flexures which we found 

 in these lines. It shows very clearly that the isogonic 

 lines which are published for the use of surveyors are 

 of no earthly use. Work ought to be done in a de- 

 tailed way over the whole country; and I hope we 

 shall some time be able to combine with these deter- 

 minations a series of magnetic values at ten or twelve 

 dillerent stations in the state of Missouri, and also 

 simultaneous determinations of earth-currents upon 

 lines making angles with-each other at the ditTerent 

 stations. Similar variations would probably be fouud 

 in the states of Illinois and Iowa. « 



In the discussion which followed. President l!ow- 

 laiul said, that with respect to the earth-currents, he 

 liimself never saw any experiments which gave steady- 

 earth-currents. Earth-currents are usually supposed 

 to vary very quickly. They do not pass in a stculy 

 direction anywhere; and therefore he would inquire 

 whether Trofessor XijOier has any reason to suppose 

 there are such earth-currents, and, further, whether 

 these local changes of these lines may not be due to 

 hidden mines of iron, or something or other, rather 

 than to earth-currents. 



The question was also asked, whether, in comparing 

 earlier observations with the later, there are varia- 

 tions from year to year which would soon invalidate 

 any survey that could be made, and render it com- 

 I)aratively of no value. 



I suppose, replied Professor Xiplier, that, over 

 rather large areas of country, the annual change 

 does not vary very rapidly in space. In the western 

 states, so far as I know at present, it is pretty nearly 

 constant, though I do not know as we have auy rea- 

 son to say that it is really constant, licplying to the 

 president's last question, I slioidd say tliat the deler- 

 niinalion to which I have referred, as regards earth- 

 currents, was not for the purpose of testing thetheury 

 which I formerly had, but simply for the purpose of 

 examining a cause which certainly has some effect. 

 I think it is well enough known that it is a fact, and 

 it is well to invest igiite it, since we found so many 

 unexpected things. I should supi)Ose th.it the ex- 

 planation, that it is due to magnelicioatler under the 

 surf.'ice of the earth, is the much more i)robalile onu, 

 as the case stands now. As to the disposition of that 

 magnetic matter, yo\i can make a great variety out of 

 that, and locate your mines in various parts of the 

 state. 



Prof. A. E. Dolbear inquired whether any inves- 

 tigations have bei'n in.ade as to the direction of 

 eartli-curri'iils; and wlictlicr Professor Niplier knew 

 of any device which would enable him to detect the 



