PHYSICS. 459 



As the telephone is an indispensable convenience throughout these 

 papers, an interferometer investigation on the vibrations of the plate 

 of that remarkable instrument is inserted. What comes out definitely 

 in the research is the readiness of the plate to quiver in overtones, after 

 each electromagnetic shock. A small mirror at the center is not there- 

 fore displaced (as a rule) translationally, but rather rotationally, 

 giving rise to very complicated wave-forms, difficult to analyze. In 

 corroboration of this, it was found later that a telephone current may 

 often be commutated. 



The endeavor to place the Foucault mirror on the interferometer has, 

 for incidental reasons, thus far failed of achievement; but as a variety 

 of apparatus useful in experiments of the present kind were tried out 

 in the course of the work, a brief account of it is given. 



In deference to the wishes of Dr. R. S. Woodward, the author has 

 begun a search for methods of measuring the acceleration of gravity 

 other than those classically in use. Such an inquiry necessarily con- 

 sists in referring gravitational pull to forces generated in other mechan- 

 isms. An interferometer torsion balance is first tested ; but the results 

 are found to encounter relatively large and uncontrollable temperature 

 coefficients, both of rigidity and viscosity, even if the ordinary effects 

 of viscosity can be allowed for. The other (pneumatic) method 

 for g, in which gravitational force is referred to the pressure of a gas, 

 has at the outset much to recommend it; for it admits of rough treat- 

 ment, in spite of the otherwise surprising precision of results. The two 

 errors which offer a serious menace to the accurate hydraulic weighing 

 of the cartesian diver, viz, the diffusion and solution discrepancies, 

 though at first approach apparently insuperable, may not remain so 

 indefinitely. At least, in experiments on the diffusion and convection 

 of gases in narrow tubes made in the lapse of years, coefficients of a 

 negligibly small order of value were obtained. Though the work is very 

 laborious, it seems worth while to carry it further. 



The remainder of the volume is largely concerned with work bearing 

 on the constant of gravitation. The object of these experiments was 

 at the outset a mere endeavor to read the deflections of the gravitation 

 needle by displacement interferometry. The plan succeeded at once, 

 but on computing the Newtonian constant it came out actually several 

 times too large. It was obvious that this could be explainedonly by the 

 presence of relatively large radiant forces. In the course of the work, 

 the latter, in some mysterious way, were almost always found to act in 

 the same sense as gravitation. Hence the need of tracing the radiant 

 forces to their source and, if possible, to learn to control them, became 

 urgent. ]\Iuch of the succeeding paper is given to work of this kind. 

 Among other things, the attempt is made to refer the constant of 

 gravitation to the viscosity of the medium in which the needle moves. 

 The most curious results in relation to the radiant forces were obtained 



