ACOUSTICS AND GRAVITATION. 77 



Much larger weights than this are available without exceptional risk from the 

 viscosity of the wire. Moreover, if the mass weighed increases as the torsion 

 coefficient of the wire, the sensitiveness is not changed, again with a reservation 

 as to viscosity. 



The experiments with the phosphor-bronze wire were carried out both in 

 terms of AN at the mirror micrometer and of AS at the tangential micrometer. 

 There was no difficulty in finding or controlling the fringes, so far as the 

 apparatus was concerned, either at the zero position or at the two elonga- 

 tions. They appear whenever the guide-wire across the slit is sharply in 

 coincidence with the intersections of cross-wires in the telescope, so that 

 telescope and collimator are to be rigidly fastened, although only the 

 collimator cross-wire (fringes coinciding with its image) determines the 

 measurement. To this extent the damping seemed to be sufficient. Fringes 

 large and small were tested; but in a steam-heated room the fringes were 

 always in slow and deliberate motion over half the length of the slit-image, 

 irregularly to and fro. Possibly this might have been avoided by placing a 

 second case over the apparatus as a whole, to obviate air-currents and 

 unequal distributions of temperature. This however, would have been 

 inconvenient, particularly so as a modification apparatus with a more rigid 

 wire and larger weights conduces to further practical advantages. 



The next experiments were made with a steel wire (music wire) 0.02 2 4 cm. 

 in diameter and very hard drawn. The modification already referred to, of 

 an independent scaffolding for the wire, its end supports (torsion- head, etc.), 

 the balance beam and dash-pots, was now introduced. Again the operations 

 for measurement proceeded very satisfactorily ; but the fringes were still in 

 continual motion, though perhaps not so much so as in the preceding case. 

 The weight used was just short of 0.05 gram, the double deflection about 

 68.7. In some 12 alternations made between the stops on 5, the fringes, though 

 moving, always reappeared in place in the field. If at the scale 5, 89 = \/b and 

 A0 = 68.7Xo.oi75, 



dg_8d_ 6Xio~ 5 _ 6 



g " A0~ 10X68.7X0.0175" 



per fringe passing the cross-wire of the slit-image. It seems a pity, therefore, 

 that this sensitiveness could not be utilized. 



68. Thicker wire. As the observations with the above wire were useless, 

 because of convective temperature discrepancies, I inserted a thicker drawn- 

 steel wire 0.05 cm. in diameter. The forces being thus 20 times larger, the 

 apparatus shewed much greater freedom from the annoyances specified. The 

 behavior was far steadier, though the damping coefficient had proportionately 

 decreased. It was not, however, even now possible to rely on a single fringe 

 in a heated room; but in the summer time this would be the case. Tempera- 

 ture also modifies the elastic constants seriously. 



On the other hand, the effect of viscosity was now much more obvious, there 

 being a proportionately greater twist of the individual fibers of the wire. 



