44 DENsiTii'.s oi' (i.\V(;kn and hvdkockn 



It will l)e observed th:it ourreuts of ;iii- pioduced outside of the closet of the 

 weighing,' apparatus have but little effect iu disturbing the balance. The openings 

 in the desiccatoi- are small, and are at the same level. It is therefore impossible 

 for a current of air to enter the desiccator near the top, and leave it near the 

 l)i>ttoni, striking a<rainst one of the globes during the pa.ssage. If the whole body 

 of air in the small looin snrroniuling the clo.set becomes less den.se, the whole body 

 of .lir within the closet will ooze out at .some opening near the bottom ; but this 

 movement will not take place in [>art through the desiccator. 



But if convection currents are produced within the closet itself, they are very 

 likely to enter the desiccator, unless this has a certain symmetrical exposure to 

 them ; ami if convection currents are produced within the desiccator itself, they 

 are still more likely to disturb the balance to which the globes are suspended. 



Suppose, for instance, that the distribution of temperature in the closet is such 

 as to produce currents; a downward current may well enter the <lesiccator ami 

 strike upon one of the globes. Cuiiciits in the same direction will also Ije pro- 

 duceil within the desiccator, since its inequalities in temperature will be in the 

 same direction as those of the walls of the closet. The globe on the left hand, let 

 us say, will ai)(iear too heavy. If now the positions of the globes be reversed, the 

 cui-rents which enter the desiccator from without are reversed in their effect on the 

 globes, but those which are pi'oiliiced within the desiccator itself will continue for 

 some time, making the right li.iiid globe appear too heavy. These currents, there- 

 fore, make the globe appear too heavy in both positions. 



As a matter of observation, the convection currents produced within the 

 desiccator are so much the more important that the ctlVct actually noticed in con- 

 ditions when weighing is possible is attribiitalile entirely to them, ^^'lu•n the 

 effect of the currents enteriuij the desiccator from without is noticed, the ecniilili- 

 rium of the balance is so inconstant that weighings have to Ite postponed. This 

 has occurred but seldom. 



Fortunately the effect of the currents .set up within the desiccator is easily 

 eliminated. If we wait for, say, half an hour, until we may assume that a new 

 constant condition of temperature has been attained, and again weigh by reversal, 

 the convection currents produced within the desiccator will produce an effect on 

 the a[>parent difference of weight of the two globes which is opposite to the effect 

 produced in the first weighing. If the conditions of temperature have not materi- 

 ally changed, and if the time used in the nianijiulation is about the same, the two 

 opposite effects will have about the same magnitmle, and will nearly disappear 

 from tile mean. The mean of two successive weighings by reversal with an inter- 

 val of half an hour or an hour ought, therefore, to give very nearly the tiiu' differ- 

 ence of weiiflit of the globe and its counterpoise. 



