124 DISPLACEMENT INTERFEROMETRY APPLIED TO 



there is here a general similarity observable, it is not sufficient for such a 

 reduction, as tabulation showed. The plan finally adopted for want of a better 

 was somewhat cumbersome and as follows: Those observations in which 

 complete series of results were available between, say, p = 2 cm. and 76 cm. 

 were first constructed and the mean result taken; i. e., the mean graph drawn 

 through them, assuming Aj 2 = 3 for a plenum. By aid of this, the data taken 

 at all other pressures (incomplete series) were then reduced, assuming Aj 2 = 3 

 for the plenum, throughout. The table of corrected data is too bulky for 

 insertion here and the summary in figure 157 must suffice. The rates are 

 sometimes too low, probably from escape of air from the wood, sometimes too 

 high for the reversed reason of absorption ; but as to their testimony as a whole 

 and for this particular apparatus II there can be no question, I think. The 

 effective gravitational attraction when one of the bodies is in a partial vacuum 

 decreases at a mean rate of about A^ 2 = 0.03 or about i per cent per centi- 

 meter of mercury-pressure. This makes a drop of over Ay =2 for the com- 

 plete range from plenum to vacuum, about as originally obtained. 



It has been stated that this result is inconsistent with the efflux results of 

 figures 145 to 148, the interpretation of which is straightforward. Moreover, 

 these effects are relatively transient, whereas in the case of figure 157 the 

 results persist. If the graphs, figure 1 49 or 1 5 7 , are considered due to the influx 

 resulting from the leak with consequent rise of temperature, then these graphs 

 should slope downward from left to right and not upward. Under any cir- 

 cumstances, moreover, the thermal effect should vanish in a partial vacuum of 

 50 to 60 cm. ; i. e., the initial or plenum A;y 2 should be regained. There is no 

 suggestion of such an inversion in figures 149 or 157. The effect continues in 

 the same manner indefinitely, so far as observed. 



One can not but conclude, therefore, that so far as this evidence goes, there 

 is here a different phenomenon superimposed on gravitation; in other words, 

 that the effective gravitational attraction on a body in vacuum decreases 

 i per cent per centimeter of pressure, for reasons not understood. 



98. Apparatus No. Ill, brass and glass. It was necessary, therefore, to 

 endeavor to construct a new apparatus, which could both be freed from leak- 

 age and would be without the porous annoyances of wood. The new apparatus 

 /// was constructed much like No. II, except that a rectangular frame of 

 square brass tubing, the frame being 23 cm. long and 5 cm. high in the clear, 

 replaced the wood. Thick glass plates were cemented on in front and rear, the 

 distance apart of their outer faces being 3 cm. and 1.3 cm. between their inner 

 faces. The joints at the torsion-head, etc., were inclosed in cups, into which 

 melted soft sealing-wax was poured, obviating leakage. The quartz fiber 

 was somewhat finer than the above and, the center of the weight M = i .6 kg. 

 being able to approach the center of m to # = 4-5 cm., the deflections, other 

 things being equal, were larger. The needle moved more slowly. The 

 greater deflection here, however, is a doubtful advantage in view of the longer 

 period, for the readings are always very sharp. The needle when complete 



