758 



SCIENCE. 



[N. S. Vol. II. No. 49. 



Selling, and chemistry with Wislicenus. 

 After three years work at Wiirzburg he 

 went for a year to Helmholtz at Berlin. 

 After his return to America in 1882, and 

 until 1890, he was chiefly occupied with 

 commercial applications, having been for 

 several years with a large firm manufactur- 

 ing electrical supplies in England, and sub- 

 sequently he was connected with a similar 

 firm in this country. In 1890 he was ap- 

 pointed assistant in physics at Columbia, 

 and quickly rose to the rank of instructor, 

 which he held at his death, on October 25, 

 1895. 



As a man and a friend he was all that 

 the words in their best sense imply. 



If one were to judge Mr. Cushman's ser- 

 vices to science by his published work, one 

 would obtain a most incomplete idea of 

 their extent. Although deeply interested 

 in the more theoretical questions which 

 have specially occupied physicists in the 

 past score of years, and always willing to 

 discuss them, still his heart was chiefly 

 set upon the practical applications, not in a 

 commercial sense, but referring to teaching 

 or research. Many pieces of very useful 

 apparatus and many excellent laboratory 

 methods are the results of his activity in 

 this field. His chief service to science, and 

 the one which will remain as a lasting monu- 

 ment to the strongest side of his activity, is 

 the admirable organization of the labora- 

 tory teaching of physics at Columbia. To 

 his untiring industry and thorough devo- 

 tion is due a system which handles 400 

 students annually in a limited space, giv- 

 ing to each a course adopted to his indi- 

 vidual needs. For each of the experiments, 

 some 200 in number, he has prepared full 

 explanations and instructions for mak- 

 ing the determination. It is proposed to 

 issue this collection with some little addi- 

 tional material as a laboratory manual for 

 the use of our students. It is also pro- 

 posed to contribute to Science several 



papers on methods and apparatus prepared 

 by him and left unpublished at his death. 



Collimatmg Magnetometer and Local Vari- 

 ometer. 

 Unpublished paper by Holbrook Cush- 



MAK. 



Edited by W. Hallock. 



magnetometer. 



In laboratory determinations of the in- 

 tensity of the Earth's magnetic field it is 

 often desirable to be able to obtain the 

 deflection observations with an accuracy 

 greater than is possible with the usual glass 

 pointer over a 15 or 20 cm. divided circle 

 and yet not incur the expense and elabo- 

 rateness of the telescope and scale, or the 

 lamp and scale. 



The method devised is intermediate in 

 accuracy between the two methods above 

 mentioned and gives the readings in tan- 

 gents of the angle directly. 



A in the accompanying Fig. 1 is the mag- 

 netometer with a plain mirror about 2 cm . 

 in diameter with a vertical black line across 

 the center of its face, B & B' are the scales 

 upon which the deflecting magnet M is 

 placed, S is a scale, a common meter stick 

 will do, mounted perpendicular to B at a 

 distance of one meter from the magnetome- 

 ter needle at A. Upon the scale slides the 

 sight, P, shown ^ natural size in Fig. 3. 

 It consist of a piece of sheet brass about 

 0.5 mm. thick, slit and bent so that two 

 side sti'ips, n n, slide upon one side of the 

 scale and the middle one, o, upon the other 

 side, toward the magnetometer ; h is a hole 

 about 1 mm. diameter, and the side toward 

 the mirror is covered with white paper 

 upon which is a cross X. By a series of 

 simple sightings the magnetometer A is 

 oriented in line with the scales B and B,' 

 and upon their common zero. Placing 

 the sight upon the point of the scale S 

 directly over the axial line of B, A and B,' 

 the whole apparatus is to be so oriented 



