210 



SCIENCE. 



[N. S. Vol. IX. No. 215. 



bers of these classes at graduation and at 

 their entrance into the College. 



The line a h indicates what might have 

 been expected had no such radical and un- 

 precedented increase of the demands at 

 entrance been made. The College would, 

 at its then rate of growth, have attained a 

 census of 1,000 students in 1898 or 1900, 

 possibly 1,200 in the latter year. Numbers 

 were then restricted by thus cutting the ex- 

 pectant entrance-class in half and its num- 

 bers fell as shown, and the dotted line, c d, 

 indicates where the figures will probably 

 reach, at those dates, as now thus reduced. 

 Similarly, the lines / h and g i show what 

 numbers were promised, between the speci- 

 fied dates, under the one, and what under 

 the later, arrangement. The lines j /c and Z m 

 show what should have been and what actu- 

 ally will probably be the magnitude of the 

 graduating classes, in 1898 to 1900, inclusive. 

 The line D indicates the number of students 

 taking the Master's degree. The peculiar 

 ' hump' at the date'89, on B, indicates the ef- 

 fect of the unsuccessful attempt to restrict 

 numbers at that date by limiting the num- 

 ber accepted. 



Just what is to be considered the real 

 balance between advantage and disadvan- 

 tage due the noted elevation of the entrance 

 requirements, in '94, is perhaps difficult to 

 decide. It has given a vastly better course ; 

 but the difference between the lines a h and 

 c d shows that the College has lost the op- 

 portunity to benefit many hundreds of stu- 

 dents who have, as it is, been compelled, in 

 most cases, probably, to go into business 

 without professional training and who are 

 thus placed almost hopelessly in the rear of 

 their more fortunate fellows in their strug- 

 gle for success through life.* 



R. H. Thurston. 



Sibley College, Cornell 



University, January 2, 1898. 



* Proceedings Society for Promotion ot Engineering 

 Education, 1898. 



MECHANICAL ILLUSTRATION OF KIBCHOFF'S 

 PRINCIPLE. 



In teaching the reversal of the metallic 

 lines in the Fraunhofer spectrum it is often 

 difficult for the student to get a concrete 

 idea of the principle that a molecule or 

 atom will absorb especially radiant energy 

 whose period is identical with the inherent 

 period of the molecule itself 



A customary method of illustrating this 

 point is with two tuning forks upon reso- 

 nance boxes, but this requires very careful 

 manipulation and is not altogether satisfac- 

 tory. The following method has proved 

 quite satisfactoiy : 



The suggestion of Lord Kelvin for a me- 

 chanical illustration of a molecule having 

 inherent periods of vibration is used, re- 

 placing his spherical shells by rings. Such 

 a molecule with one rate is shown in Fig. I. 



The ring A is about 20 cm. in diameter and 

 made of brass rod about 1 cm. in diameter ; 

 the ball B is preferably somewhere near the 

 same mass as the ring A. The three spiral 

 springs jS are wound about 2 cm. diameter 

 of about No. 22 hard brass wire. 



Such a molecule has a rate of vibration 

 of about 4 or 5 per second when suspended 

 on a long string as at D. A close spiral 

 spring C, similar to S, but about 50 cm. 

 long, is attached to the ring at E, the other 

 end being held between the thumb and finger 

 at TF. 



While holding this spring slightljr tense 

 it can be set into longitudinal stationary 

 waves bj^ compressing the part at P toward 

 TF and then letting go. The period of 

 these vibrations depends upon the length 

 TF to F. Commencing with this length 

 about 15 to 20 cm. , it will be observed that 

 the stationary waves in C do not effect the 

 molecule. Taking C longer and longer a 

 point is reached where the waves in C are 

 taken up and a decided vibration is set up 

 between A and B. That is, the molecule 

 absorbs the energy from C when its period 



