August 29, 1913] 



SCIENCE 



303 



VARIATIONS IN THE EARTH's MAGNETIC FIELD 



Observations made in a tent on the lake 

 shore in Mackinac County, Mich., during the 

 last month have fully verified the results and 

 conclusions published in my paper entitled 

 " Local Magnetic Storms." 



Cloud shadows diminish the permeability of 

 the space within them in precisely the same 

 way that the earth's shadow does at night. 

 The molecules of air are ionized by solar 

 radiation. They are then little magnets, which 

 tend to set along the lines of force of the 

 earth's field, in such a way as to add their 

 magnetic effect to that of the earth's field. 

 When solar radiation is cut off, the air mole- 

 cules begin to return to their normal condi- 

 tion. Wind gusts and falling rain drops assist 

 in this operation. They decrease the permea- 

 bility. 



When an iron bar is placed within a coil of 

 wire carrying a current, its molecules are not 

 quite so free to set in positions such that their 

 magnetic effect is added to that of the coil. 

 A blow from a wooden mallet then assists 

 them. Its effect is directly the opposite of that 

 produced by a gust of wind in air. 



These results appear to furnish a rational 

 explanation of the conditions which bring 

 about local, daily and annual Variations in the 

 earth's magnetic field. Local variations are 

 due to local variations in the weather. Clouds 

 and sunshine, wind storms and rain, are the 

 agents which bring about a continual swaying 

 of the lines of force. 



They also indicate an explanation of what 

 causes the difference in permeability of solid 

 matter. 



In this work the needle was enclosed in an 

 airtight case, and mounted on a silk fiber about 

 40 cm. in length. Its motion was damped. It 

 was deflected at right angles to the magnetic 

 meridian by magnets whose axes were at an 

 angle of 4.5° with the meridian. The resultant 

 field was partly compensated by bar magnets 

 120 em. in length. All magnets were sealed 

 within heavy rubber tubing, mounted in 

 U-shaped supports, and enclosed in ice. The 

 supporting table was a frame made of 2 X 4 

 inch timber, bolted together with brass bolts, 

 and the legs of the structure were set two feet 



into solid clay and gravel soil. The frame 

 was securely braced. Francis E. Nipher 

 Hessel, Mich., 

 July 26, 1913 



EXCUSING CLASS ABSENCES IN COLLEGE 



There is no general uniformity in the 

 matter of handling class absences in college. 

 In some institutions the individual teachers 

 still excuse for all absences in the course for 

 which the teacher is responsible. In an in- 

 creasing number of institutions the excusing 

 power is centralized in some one office and 

 in a large number of cases some form of the 

 cut system is used. In some cases the student 

 is allowed as many absences a semester from 

 a course as the course recites times per week, 

 that is, three absences from a three-hour 

 course, four from a four-hour course, etc. In 

 general the number of class cuts allowed seems 

 to run as a minimum about 15 a semester — 

 the number of absences allowed a semester in 

 all courses approximating the number of reci- 

 tation periods per week. 



In eases where this minimum is allowed it 

 means that 5J per cent, approximately, of the 

 class-room periods may be omitted by any or 

 all students without any account being given 

 for the absences. 



A system such as this seems almost an in- 

 vitation to a student to avail himself of the 

 number of cuts allowed and in a large number 

 of cases is so regarded. 



In Oberlin College all class absences are 

 reported to the dean of men and the dean of 

 women, respectively. Each student must give 

 an account to the proper officer of all absences. 



The results during the semester ending in 

 February, 1913, were as follows in the case of 

 the college men : the • average number of ab- 

 sences for each freshman was 6.1, for each 

 sophomore Y.9, for each junior Y.5, for each 

 senior 7.3. This includes absences for all 

 reasons, sickness, absence on athletic teams, 

 glee clubs, etc., and counts absences from all 

 classes, including physical training. The 

 record of no student is included who left col- 

 lege for any reason before the end of the 

 semester. 



The total number of men and absences were 

 as follows during the semester just closed: 



