690 



SCIENCE 



[N. S. Vol. XXX. No. 776 



conductors. Interesting results may be 

 brought out by comparing the effects of using 

 solid metal plates and similar plates cut into 

 sectors, insulated from one another. These 

 results suggest the use of different forms of 

 cores in a solenoid placed in series with the 

 secondary circuit, or with the lamp and coil 

 used to show induced currents. 



The screening effects of conducting plates 

 may be shown by placing them between the 

 secondary coil and another coil in circuit with 

 a lamp. For high frequencies, thin sheets of 

 copper or of iron may cut down the brilliancy 

 of the bulb very decidedly. 



If a short-circuited coil is used instead of 

 the plate, a similar screening effect may be 

 shown.'' 



To show "resonance," arrange a second os- 

 cillating system containing capacity, self-in- 

 duction, and a small lamp. If the coil of one 

 system is laid upon that of the other, and the 

 natural periods of the two systems are made 

 approximately the same, the bulb lights up 

 and the pitch of the note is changed. There 

 is a considerable range of response in the 

 second system, but with proper adjustment a 

 maximum of light for varying frequencies 

 may easily be observed. The use of two coils 

 in series in this second oscillating system is 

 convenient. One below the secondary coil and 

 the other sliding on top of it makes the ad- 

 justment for maximum effect easier. The 

 reaction of this new oscillating system on the 

 secondary and thus on the arc may be very 

 interestingly shown by making and breaking 

 the new oscillating circuit while changing the 

 capacity in the secondary circuit and noting 

 the resulting change of pitch of the arc-note. 

 The direction of this change depends on which 

 of the two free periods is the greater." 



In conclusion it should be said that the 

 above suggestions are made merely to call 

 further attention to a means of demonstration 

 which in some respects is simpler than the 

 spark-discharge, and which has certain ad- 

 vantages over models. 



W. B. Huff 



Bktn Mawr College 



* Cf. J. J. Thomson, " Recent Researches," § 427. 

 " J. J. Thomson, " Recent Researches," § 432. 



THE ROOK WALL OF ROCKWALL, TEXAS 



For many years reports of a more or less 

 definite nature have been circulated describing 

 the wonders of the ancient wall surrounding 

 the town of Eockwall, Texas. The writer was 

 able during the past winter to spend a few 

 days investigating this supposed historic struc- 

 ture. It proves to be not a wall, but a number 

 of disconnected sandstone dikes, strictly speak- 

 ing, not surrounding the town, but trending in 

 many directions. As exposures are few, they 

 have been discovered in such scattered locali- 

 ties in the town's environs as to suggest the 

 idea that they were fragments of a ruined wall. 



Eockwall is located in a rich farming dis- 

 trict about twenty-five miles east of Dallas. 

 Black waxy soil covers the rolling hills, and 

 only where erosion has been considerable can 

 the underlying rocks be seen. These, when 

 exposed, reveal blue limey strata of upper Cre- 

 taceous age in nearly horizontal attitude. A 

 white clay, the decomposed product of the lime 

 muds, generally occurs beneath the black soil. 

 These lime muds are remarkable in their fre- 

 dom from grit and in the peculiar property 

 which causes them to decrepitate when exposed 

 to the weather ; notable also in that, on drying, 

 cracks develop of various sizes. Within this 

 series of semi-consolidated beds a few sandy 

 layers occur. One is revealed by a drill record 

 1,800 ± feet below the surface ; another may 

 be seen near the town of Eockwall at the sur- 

 face and consists of thinly bedded flaggy sandy 

 limestone. 



Though good exposures are infrequent, 

 owing to the depth of soil, a peculiar condition 

 affords ample opportunity to observe the dikes 

 in place. These latter are natural courses for 

 undergroiTnd waters, and wells are often lo- 

 cated on them. Though these walls are filled 

 with water, the rock forming the dike, removed 

 during the sinking of the well, may be exam- 

 ined at leisure. 



The dikes are of various sizes, varying from 

 an inch in thickness to eighteen inches or two 

 feet. They stand vertically, or nearly so, and 

 have in cases been followed downward fifty 

 feet or more, always imbedded in the lime 

 muds. They are composed of exceedingly fine- 



^ Published with the permission of the Director, 

 U. S. Geological ourvey. 



