Interferometer to the Horizontal Pendulum.. 511 



Thus, if AN = 10 -4 cm., a — 10" 3 seconds of arc, or the change 

 of a per vanishing interference ring (AiV r =10~ 6 X 30) is 

 •000310 seconds of arc. This limit could easily be decreased 

 to 10" 6 30" by increasing T a little over three times. 



Similarly, the forces at distance R from the axis of the 

 horizontal pendulum are 



F' = F[l + 



M h 



F R = 42-9 AN; F B ' = 53-7 A^ 



Thus if AN - 10- 4 cm., F' R = -0054 dynes or about -0016 

 dynes per vanishing interference ring, in case of the pendulum 

 loaded with the disc m. In case of the two discs used below, 

 m = 227 grams, m' = 1035 grams, placed all but in contact, 

 d = '5 cm., the gravitational attraction should exceed 



_8 1035 X 227 

 /= 6-7X10 -~- = -063 dyne 



or AN should exceed "00117 cm. or 2*3 drum parts on the 

 micrometer or over 5 drum parts (about 80 interference rings) 

 for the difference of deflection on the two sides of the sus- 

 pended disc. This should already have been measurable in 

 spite of the pendulum used, which was heavy and ill designed 

 for the purpose. 



In fig. 4 an example of the observations has been inscribed, 

 the ordinates being the inclination of the pier, a, in hundredths 

 of a second of arc, very nearly. It will be seen that the 

 inclination increases as a whole from the beginning to the end 

 of the month, the total range lying within something over two 

 seconds of arc. The rise is particularly marked and sustained 

 after the 14th, and the difference of inclination between the 

 first and second half of the month is about one second. 



As the observations were made in an unavoidably steam- 

 heated room it is probable that the flexure of the pier, etc., 

 due to thermal causes has been largely operative in modifying 

 the trend of the curve. For on comparing the curve as a 

 whole with the thermostat sheets (not shown) a retarded effect 

 is possibly suggested, such as one would suspect if variations 

 of surface temperature should penetrate massive masonry. It 

 would then be possible for the curve to have different heights at 

 the same temperature. Naturally, such comparisons are very 

 vague, and it is the range of values of admissible in the appar- 

 atus which is here of paramount interest. Furthermore, as 

 the hill on which the laboratory stands is, at present, being 



