CHAPTER IX. 



A PNEUMATIC METHOD OF MEASURING VARIATIONS OF ACCELERATION 



OF GRAVITY. 



74. Introductory. Some years ago I made an extended series of experi- 

 ments* on the diffusion of gases through water, the gas having been im- 

 prisoned in a Cartesian diver ; the very sensitive conditions under which the 

 diver floats at a given level were made the criterion of measurement. 



Inasmuch as such experiments consist virtually of a comparison of weights 

 with the forces derived from air-pressures, it must therefore be possible to 

 obtain the acceleration of gravity in terms of these pressures, just as in the 

 preceding chapter of this report the endeavor is made to evaluate the changes 

 of g in terms of torsion. It will not, of course, be possible to determine g 

 absolutely in this way, because so little air is used; but the question as to 

 what degree the changes of g should, in a proper environment, be determinable 

 with some precision is worth investigating. 



75. Apparatus. The chief difficulty with the experiment at the outset seems 

 to be the elimination of loss of gas by diffusion. This, in the case of a cylin- 

 drical diver, open below, amounted in the experiments cited to 0.41 per cent 

 per day. It would not, therefore, be long before the whole of the gas would be 

 lost. To avoid this, two resources apparently suggest themselves : (i) to place 

 the diver about midway between two layers of air, one below and the other 

 above it; and (2) to provide the diver with a long slender neck below, precisely 

 what was to be avoided in cases of diffusion. 



I shall describe two forms of apparatus in which these precautions are taken, 

 the first intended for laboratory experiments or information, merely; the other 

 suggesting a possibly definite form of apparatus. Figure 1 10 is the experimen- 

 tal type. Here AA is a stand-glass containing water ww, the diver ab, and the 

 lower air-chamber cd. The vessel is closed above by a rigid cork, carrying the 

 thermometer T in a central perforation, and a lateral tube / for exhaustion. 

 The water-level at e is at a distance h above the water-level a in the diver 

 when the latter is floating. The air at c below is contained in a porous cup, 

 wedged in place, and the difference of heights of water-levels at a and cd 

 (the top of the porous cup) is also to be about h. The bottom of the diver ends 

 in a narrow tube, only just wide enough to admit of easy egress or influx of 

 water, subject to changes of pressure. Under these circumstances the loss of 

 air by diffusion from a to e will be practically balanced by the opposed diffusion 

 from do a. The narrow neck also obviates danger from ingress of any minute 

 air-bubbles. When not floating, the diver should repose with its mouth at 

 some distance above c. The bottom of the thermometer bulb determines the 



* Carnegie Init. Wash. Pub. No. 186, 1913. 



87 



