54 



SCIENCE 



[Vol. LV, No. 1411 



that checking the readings, again, becomes 

 almost impossible. In an attempt to overcome 

 some of these difficulties the writer constructed 

 the following apparatus which has proved so 

 useful and convenient that it is offered as a 

 suggestion to teachers of geology. 



This apparatus consists of two circular 

 pieces of wood about eight inches in diameter, 

 connected by a spindle two inches in diameter 

 and eight inches in length. It is fastened to 



the edge of a table by means of wooden clamps. 

 Ordinary iron clamps are not used because 

 they cause a deflection of the compass needle 

 amounting to two or three degrees. The 

 absence of straight edges in the outline of the 

 upper disk necessitates finding the line of 

 strike by locating a position of the clinometer 

 in which no inclination is registered and then 

 drawing a line along the edge of the compass 

 box upon a piece of paper fastened to the top 

 board by thumb tacks. The strike line can be 

 changed to all points of the compass by clamp- 

 ing the apparatus in different positions. The 

 dip is constant for each model but, since several 

 models are necessary for an average class, this 

 is taken care of by making each model with a 

 different dip. Models could easily be made with 

 the dip adjustable but such modifications would 

 mean more expense and more trouble in check- 

 ing students' readings. Furthermore, it is the 

 direction of dip and not the amount which 

 seems to offer difficulties to the student. It has 

 been found that ten models with dips ranging 

 from three degrees to eighty-eighty degrees 

 answer the purpose. 



The models are securely clamped in various 

 positions in the laboratory and their dip and 

 strike determined by the student and checked 

 by the instructor. The models are then turned 

 on their bases, clamped, read, and checked 

 again. The students are then required to make 

 corrections for magnetic declinations, assum- 

 ing that the models are situated in their own 



region. The readings are again eoiTected on 

 the assumption t' t they were taken in Alaska 

 or Ohio or any c er place that the instructor 

 suggests. After liie student has gained the 

 ability to make accurate deteiToinations on a 

 series of such models, assuming different 

 geographic locations for the purpose of cor- 

 recting for magnetic declinations, he is fairly 

 well equipped to use the compass and clino- 

 meter in the field. 



H. G. Turner 

 Lehigh Univeksitt 



ARTIFICIAL PRODUCTION OF TIPBURN 



Experiments conducted at the Iowa Experi- 

 ment Station have proved that Empoasca mali, 

 the potato leafhopper, is the factor in the pro- 

 duction of tipburn or hopperburn of potato. 

 Emulsions were made by crushing a large num- 

 ber of adults of both sexes in water. Small 

 amounts of this material were injected into the 

 leaves of the potato plants and in several days 

 an injury was produced similar to, if not iden- 

 tical with tipburn. Difficulty was experienced 

 in getting large amounts of the emulsion into 

 the leaf tissue, but enough was injected to 

 produce burning. When this emulsion was 

 placed on the leaf and then the tissue pricked 

 with a fine needle, negative results were noted. 

 Emulsions made from crushed nymphs failed to 

 produce injury in more than a few cases, and 

 in these it was not pronounced. 



That these insects contain some toxic sub- 

 stance was further demonstrated by placing the 

 residue left over from the insects after the 

 emulsion had been poui'ed off on leaf petioles 

 and then pricking this in by means of a fine 

 scalpel. In every case, a lesion was produced, 

 the tissue at these points first turning yellow 

 and then brown. Later the cells collapsed 

 leaving a rather large sear. 



Although Bordeaux mixture is toxic to the 

 nymphs, yet it acts comparatively slowly so 

 that by keeping a leaf sprayed with this com- 

 pound colonized with live nymphs tipburn was 

 produced. This would appear to show that 

 Bordeaux mixture does not prevent tipburn 

 by its action on the leaf but rather by its 

 action on the insect. 



F. A. Fenton and I. L. Rbsslee 

 Iowa State College 



