>N A HI AKI/ TIII:I-:.\I I;I;AVITY BALANCE. 



221 



solution is iii'-Miisistent with the necessity of having the lever strong enough to bear 

 arresting without ehangr of form, and the latter has the drawback that it tends to 

 magnify the effect of such variations as do actually occur in the shajie of the lever. 

 In short, the design of a gravity balance l)ecoines a matter of compromise just like 

 any other engineering undertaking. In calculating the moment of the gravitational 

 forces brought to bear upon the thread by the weight of the lever, it must not be 

 forgotten that the effective weight is reduced in consequence of the flotation of the 

 lever by the air surrounding it. At an accuracy of one part in a hundred thousand 

 in the estimation of g the effect of variations of barometric pressure or humidity 

 would become sensible, and it is therefore necessary to protect the lever against 

 variations of air density. Thin is done by enclosing the whole apparatus in an air- 

 tight casing within which the air is kept at a constant density. In practice the 

 density selected corresponds to a pressure slightly below the minimum external pressure 

 of the air. The apparatus must also clearly be filled with dry air to avoid the deposit 

 of dew. As a consequence, the twisting of the thread necessary to afford a reading of 

 the instrument, has to be carried out by means of a shaft working through a stuffing 

 IMIX ; and the iv<|iiisito accuracy in the estimation of the twist given to the thread 

 makes it necessary that the rcnl shall work almost without friction in the stuffing lx>.\. 



\\V have devised a sort of mercury and tallow joint which has got over the 

 difficulty fairly well ; and which is also applied to the stuffing box through which the 

 rod of the arrester works. 



An aneroid in connection with the internal space enables any leakage to be detected. 



The constructional difficulties above indicated were further increased by our deter- 

 mination to have no iron or steel work about the machine : this was due to fear of 

 magnetic interference ; with which, however, we have never been troubled. 



The degree of portability attained will be understood from the following statement 

 of the sizes and weights of the various appliances requisite for an observation of 

 gravity, as, for instance, during a survey when everything must be provided. 



The above weights and dimensions refer to a balance of gun-metal and copper, and 

 accessories taken just as they came to hand from the laboratory. Both weights and 

 dimensions might U> gn-atlv iv<lu<vd if so desired. The weights certainly might be 

 easily halved. 



