February 10, 1899.] 



SCIENCE. 



211 



is the same as its own inherent period. If 

 the length of C be now slightly changed, 

 the phenomenon of beats is readily apparent. 

 An electric arc will throw a sharp shadow 

 of this apparatus upon a screen and make 

 the experiment visible to a large audience. 

 The spring C may be replaced by an electri- 



casts or impressions. The materials com- 

 monly used for this purpose are beeswax 

 (either pure or mixed with some stiifening 

 substance, such as ozocerite or parafl&ne), 

 dentists' modeling composition (which must 

 first be softened in water heated nearly to 

 the boiling point), glue, gelatine, melted 



oally excited tuning fork or other mechan- 

 ical appliance. 



The armature of a small electro-magnet 

 may be attached to the ring at E and the 

 current interrupted by some mechanical 

 circuit breaker whose rate can be varied. 



A molecule like Fig. 2 would have several 

 inherent rates depending upon the relative 

 masses of A, B, C, D and upon their con- 

 necting springs. Ingenuity will suggest 

 many variations or improvements upon 

 these suggestions. 



"William Hallock. 



Physical Labokatoey, 

 Columbia XJniveestiy. 



PLASTILINE, A NEW MODELING COMPOUND. 

 Paleontologists have constantly to deal 

 with organic remains preserved in the 

 rock in the form of natural casts, molds 

 and impressions, for the proper study of 

 which it is indispensable to take reverse 



sulphur, and, of course, the common plaster 

 of paris. One writer* has suggested the 

 use of tinfoil for taking repousse impres- 

 sions, the foil being afterwards coated with 

 varnish to insure retention of its shape. 



Each of the above-named substances has 

 its own special advantages and applicability 

 in certain cases. But a comparatively new 

 plastic material which is especially well 

 adapted for modeling purposes, and hence 

 is of interest to the taxidermist, cartog- 

 rapher and others, is that known as plasti- 

 line. This is the invention of Professor 

 Luighi Giudice, of Genoa, Italy, by whom 

 it has recently been perfected, and is, 



* Goodcbild, H. G., How to take Impressions of Fos- 

 sils (Geol. Mag. [3], Vol. IX., p. 206), 1892. See 

 also, for various hints on modeling : Osborn, H. F., 

 Models of extinct Vertebrates (Science, Vol. VII., 

 p. 841;, 1897. Davis, W. M., and Curtis, G. C, The 

 Harvard Geographical Models (Proo. Boston Soc. Nat. 

 Hist., Vol. XXVIII., p. 85), 1897. 



