88 



NATURE 



YDec. 4, 1873 



Some improvements on Deleuil's vacuum balance have 

 been designed by Prof. W. H. Miller, and have been practi- 

 cally carried out in a vacuum balance constructed by Mr. 

 Oertling for the Standards Department. The balance 

 case consists of a strong brass frame cast in one piece, 

 with a rectangular base, two sides, and an arched top. 

 Two solid glass plates, each i^ in. thick, form the front 

 and back of the case, being clamped to plane surfaces of 

 the brass frame, and made air-tight by interposing thin 

 india-rubber. They are thus removable when required, 

 for instance, when any alteration is needed in the balance. 

 There is a circular opening 4| in. in diameter, on each 

 side of the brass frame, similar to those on Deleuil's 

 balance, to which glass covers are fitted. There is no 

 stufftng-box, but when the Standard weights to be com- 

 pared are placed in the pans, and the balance case 

 exhausted, contrivances are arranged for putting the 

 balance in action and arresting it, for adding any balance 

 weights to either pan and removing them, and for inter- 

 changing the pans and weights by transferring them to 

 the other end of the beam, without any disturbance of 

 the vacuum, or necessity of opening the case. 



These arrangements enable the weighings to be made 

 by Gauss's method of alternation. The balance case is 

 firmly placed upon a strong mahogany stand. Two iron 

 tubes are fixed underneath and opening into the balance 

 case. They rest in iron cups containing a sufficient 

 quantity of mercury. Within each tube is a steel rod 

 rising to the required height inside the balance case, and 

 having at the top an arm ot convenient form. By means 

 of a simple lever handle outside the tube, either rod can 

 be lifted about an inch, and it can also be turned round. 

 By this rotary motion, when the left-hand rod is in its 

 normal position, it acts upon two bevelled wheels, and 

 thus lowers the supporting frame of the beam and puts 

 the balance in action ; and by reversing the motion, the 

 action is stopped. By raising cither rod to nearly its fuD 

 height, it can be made to take up one of several small 

 balance weights riding on a little rail fixed to the pillar of 

 the balance, and transfer it to a similar rail at the top of 

 the pan, or to transfer it back again. Again by raising 

 either steel rod to an intermediate height, and turning its 

 arm under the arched rods of one of the pans, and then 

 raising it a little, the pan and weight can be lifted off the 

 hook of the beam and transposed to a small carriage 

 standing upon a railroad near and parallel to the front of 

 the balance-case. In a similar way the other pan and 

 weight can be transferred to a second carriage at the 

 back of the case. By means of a cord and pulleys, one 

 of which is upon the right-hand steel rod and can thus be 

 turned round with the hand, the two carriages can be 

 moved to the other ends of the case, and then each pan 

 with its weight can be attached to the hook at the other 

 end of the beam. The desired results are all thus 

 attained, and the whole action of the balance is open to 

 view. 



The construction of this new vacuum balance may be 

 seen from Fig. 19. 



The balance itself is similar in construction to the other 

 Standard balances made by Mr. Oertling. It is con- 

 structed to weigh a kilogram in each pan. There are 

 two Standard thermometers inside the case, one fixed to 

 each pillar, and adjustible as to height, so that its bulb 

 may be on the same level as the centre of gravity of the 

 weight. A mercurial gauge is fixed between the pillars, 

 and there is the same arrangement of three tubes and 

 stopcocks communicating whh air-pumps and with a 

 mano- barometer, as in Deleuil's vacuum balance. Two 

 glass vessels containing chloride of calcium, are also intro- 

 duced for absorbing any moisture in the balance case. 



There are two ways of comparing and verifying standard 

 measures of capacity. The first and most accurate, as 

 well as scientific method, is by weighing their contents of 

 distilled water ; the second method, which is simpler and 



more ordinarily used, consists in comparing the measure of 

 water contained in them, with the contents of a verified 

 standard measure. 



In weighing the contents of distilled water contained in 

 a standard measure, when quite full to the brim, and with 

 the surface of the water made accurately level by a disc 

 of plate glass slid over it, Borda's method of weighing is 

 employed. The measure with its disc is placed empty in 

 one of the pans of the balance, and is accurately counter- 

 poised. A verified standard weight equal to the legal 

 weight of water contained in the measure is then added 

 to the pan containing the measure and disc, and is accu- 

 rately counterpoised, and a sufficient number of weighings 

 is taken until the mean resting-point of the balance is 

 determined and noted. The standard weight is then 

 removed. The measure is exactly filled with distilled 

 water, and its temperature, together with the reading 

 of the barometer noted. Any difference in the actual 

 temperature and barometric pressure from the normal 

 temperature and pressure is to be compensated by equi- 

 valent weights placed either in the measure pan or weight 

 pan as required. If an equipoise is not now obtained, addi- 

 tional weights are placed in the pan until an equilibrium 

 is produced, and any difference from the normal cor- 

 recting weight for temperature and barometric pressure 



Field of Micrometer of Microscope. 



either plus or minus, shows the error of the measure in 

 relation to its legal weight of water at the standard 

 temperature and barometric pressure. 



For ascertaining the exact amount of the proper cor- 

 rections for temperature and barometric pressure, authori- 

 tative tables are computed both for Imperial and for 

 Metric Measures. Such tables will be found in the 

 Papers appended to the Fifth Report of the Standards 

 Commission, published in 1871 (pp. Si, 193, and 196), 

 and to the Sixth Annual Report of the Warden of the 

 Standards, published in 1S72 (pp. 49 and 51). 



With regard to comparing instruments for standard 

 measures of length, their construction has necessarily 

 varied according to the form of the standard measure. 

 As has been already stated, the earlier scientific standards 

 of length were defined by two points, and all compari- 

 sons were made by means of a beam compass. 



The introduction by Mr. Troughton of the use of 

 the micrometer microscope was a great step in ad- 

 vance towards the attainment of scientific accuracy 

 in the comparison of our standard measures of length. 

 It enabled optical observations to be made without 

 injurious contact to the defining points or lines, and 

 thus without interference with the permanence of the 



