iv PREFACE. 



In Chapter V, on the direct interferometry measurements of the compres- 

 sion of a sound-wave, much of my work has been superfluous, as it was an- 

 ticipated in an admirable paper by Raps, using the Jamin interferometer. I 

 have therefore given only as much as is necessary for the coordination of the 

 other chapters. My method, however, is, I think, superior, owing to its much 

 greater flexibility and the ease with which fringes in any orientation may be 

 produced and shortened to a string of silvery beads. The simple organ-pipe 

 blower or adjustable embouchure much used in the chapter will, I think, be 

 found serviceable for many purposes, both of research and instruction. 



As the telephone is an indispensable convenience throughout these chapters, 

 it was thought necessary to begin an interferometer investigation on the 

 vibrations of the plate of that remarkable instrument. What comes out 

 definitely in the research, Chapter V, is the readiness of the plate to quiver 

 in overtones. A small mirror at the center is not therefore 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 (in 

 Chapters IV and V, for instance) that a telephone current may often be com- 

 mutated. 



In the endeavor to place the Foucault mirror on the interferometer I have 

 thus far, for incidental reasons, failed of achievement; but as different appara- 

 tus useful in experiments of the present kind were tried out in the course of 

 the work, I have given a brief account of it in Chapter VII. In Chapters VIII 

 and IX, in deference to the wishes of Dr. R. S. Woodward, I have begun a 

 search for methods of measuring the acceleration of gravity other than those 

 classically in use. Such an inquiry necessarily consists in referring gravita- 

 tional forces to forces generated in other mechanisms. An interferometer 

 torsion-balance is first tested, but the results are found to contain relatively 

 large and uncontrollable temperature coefficients, both of rigidity and vis- 

 cosity, even if the ordinary effects of viscosity can be allowed for. The other 

 (pneumatic) method for g, in which gravitational pull is referred to the pres- 

 sure of a gas, has at the outset much to recommend it, for it admits of rough 

 handling 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, I think it will be worth 

 while to carry it further. 



The remainder of the volume is largely concerned with work (Chapters XI 

 and XII) bearing on the constant of gravitation. The object of these experi- 

 ments was at the outset a mere endeavor to read the deflections of the gravi- 

 tation needle by displacement interferometry. The plan succeeded at once, 

 almost beyond my expectations; but on computing the Newtonian constant 

 it came out actually several times too large. It was obvious that this could 



