ON BALANCES. 75 



than the imperial standard Ib. It is actually 2*368 gr. in 

 excess. Its apparent weight in air is therefore about O8 gr. 

 (3-217 2-368) less than 1 Ib., whilst that of P-i is (K gr. 

 less. 



To produce an equilibrium in air a weight of about 0'4 

 gr. should then be added to Q ; and if, after the weigh- 

 ing in air, this added weight is left in the pan with Q, a 

 weight of a little less than 2 -8 gr. (2*368 + 0'4) should 

 be added to P-i, in order to produce an equilibrium in a 

 vacuum. 



You may now see these two standard Ibs. compared in 

 air in the Mendeleef balance, and it takes nearly four-tenths 

 of a grain, actually 0*35 gr. added to the quartz Ib. to produce 

 an equilibrium. 



We will next exhaust the air from the receiver in which 

 the balance is placed, so far as our means will readily allow, 

 to show you the effect of withdrawing the air upon the 

 difference of the two weights. If a perfect vacuum could be 

 produced, and the thermometer and barometer had previously 

 stood at the normal heights, it would be necessary, as I 

 have stated, to add about 2*8 gr. to the platinum-iridium 

 Ib. to produce an equilibrium. Eut of course any differ- 

 ence from the normal temperature and barometric pressure 

 must cause a slight variation in this amount of difference, 

 as you have seen to have been the case in the weighing 

 in air. 



Before exhausting the air, we will then add somewhat less 

 than 2 '8 gr. to the platinum-iridium Ib., in order that an equili- 

 brium may be produced without going so far as an actual 

 vacuum. There is a vacuum gauge in connection with the 

 air-pump which shows a range of 4 inches of atmospheric 

 pressure, and by adding a weight of 2*5 gr. we ought to 

 produce an equilibrium with a pressure of about 4 inches 

 of air. (This result was shown.) 



41. In conclusion, I may mention to you that our collection 

 of balances at the Standards Office, 7, Old Palace Yard, is 

 well worth a visit. In particular, there is the large balance 

 which was constructed for Captain Kater for verifying the 

 imperial bushel by weighing its contents of distilled water, 

 and which has been reconstructed by Mr. Oertling. The 

 beam is of mahogany, 70 in. long, 2 J in. thick, and 20 in. 

 deep in the middle. The central knife-edge is placed at half 



