Nov. 20, 1 8 73 J 



NA TURE 



47 



ON THE SCIENCE OF WEIGHING AND 

 MEASURING, AND THE STANDARDS OF 

 WEIGHT AND MEASURE* 



VIII. 



'X' HE ordinary method of commercial weighing by putting 

 ^ the weights in one scale and the commodity to be 

 weighed in the other, and then observing when a sufficient 

 equihbrium is produced, is inadmissible for scientific weigh- 

 ings, as it is subject to errors arising from defects in the 

 balance itself. To avoid any such errors, and obtain 

 scientific precision in the results, a check is required 

 which is found in a system of double weighing. There 

 are two methods of double weighing for the com- 

 parison of two standard weights. One method, known 

 as Borda's, and generally used in France, is that of 

 substitutioit, or weighing first one of the standard weights 

 to be compared, and then the other substituted for it, 

 against a counterpoise placed in the other pan. The dif- 

 ference between the mean resting points of the index 

 needle in these two weighings shows the difference of the 

 two weights in divisions of the scale. The second method, 

 known as Gauss's, but which was first invented by Le 

 Pere Amiot, and is now generally used in England and 

 Germany, e.xcept for hydrostatic weighings, is that of alter- 

 nai/on, or first weighing the two standards against each 

 other, and then repeating the weighings, after interchang- 

 ing the weights in the pans. By this second method no 

 counterpoise weight is required, and half the difference 

 between the mean resting points of the index needle shows 

 the difference of the two weights, in divisions of the scale. 



In all scientific weighings of standards with balances of 

 precision, it is necessary that the weights to be compared 

 should be so nearly equal that neither pan shall absolutely 

 weigh down the other. The balance must merely oscil- 

 late so that the pointer does not exceed the limits of the 

 index scale. In order to obtain an equipoise within this 

 limit, it is requisite to provide small balance weights, most 

 accurately verified, to be added to either pan, as may be 

 found necessary. 



The mode of reading adopted by the best authorities i:i 

 the process of weighing by Gauss's method is as follows : 

 — The comparing standard being in the left-hand pan, 

 and the compared standard in the right-hand pan, and 

 sufficient equipoise being obtained by adding small 

 balance weights, if requisite, the balance is put in action, 

 and the movement of the needle observed through a 

 telescope. The reading at the first turn of the pointer is 

 disregarded. The three next turns are noted, and the 

 reading at the third turn of the pointer, and half the sum 

 of the readings at the second and fourth turns are taken 

 as the highest and lowest readings. Their mean is the 

 resting point of the balance, or the reading of its position 

 of equilibrium. The balance is then stopped, and the 

 weights interchanged, when similar readings are taken 

 and dealt with in the same manner. These two observa- 

 tions constitute one comparison. In cases where great 

 accuracy is required, several successive comparisons are 

 taken, in order to obtain a mean result. Some additional 

 weighings are taken after adding a small balance weight 

 to either pan, in order to ascertain the value of a division 

 of the index scale. And if this balance-weight be added 

 successively to each pan the weighings may be used as 

 additional comparisons. 



In using Gauss's method of weighing, it is very desirable 

 to be able to transfer the pans and the weights contained 

 in them from one end of the beam to the other without 

 opening the balance case, and thus to avoid sudden 

 changes of temperature of air within the balance case and 

 consequent production of currents of air. For this pur- 



' Continued from p. 555. 



pose, the following plan is adopted. A grooved brass 

 rod is fixed inside the balance case over and a little 

 behind the beam. Upon this rod a brass slider is 

 made to traverse by being attached to a slender brass 

 rod drawn backwards or forwards from the outside of 

 the case. A descending wire with a hook at the end 

 is attached to the slider. For changing the weights, the 

 slider and hook are brought to the right-hand end of the 

 beam, when the pan and weight are lifted from the beam 

 and transferred to the hook by means of a brass rod curved 

 at the end and introduced through a small hole at the side 

 of the balance case. The pan and weight are then slid to 

 the other end of the beam, when the left-hand pan and 

 weight are lifted in a similar manner from the beam and 

 the right-hand pan and weight substituted. It only re- 

 mains then to transfer the left-hand pan and weight to 

 the right-hand end of the beam. 



This method possesses a further advantage. In making 

 a great number of comparisons between two standard 

 weights, they are exposed to some risk of being injured 



Fig. iS.— Mode of hydrostatic weighing (1 



by wear, if they are taken up in the ordinary way with a 

 piir of tongs. This risk is obviated by their being kept in 

 the pans when lifted. Two light pans are used of as nearly 

 as possible equal weight, each of which has a loop of wire 

 lorming an arch with the ends attached to the opposite 

 sides of the pan, so that it can be easi'y lifted with the 

 curved end of a brass rod. The pans are marked X and Y 

 respectively. By interchanging th'^ weights in the pans 

 after a series of comparisons, and making a second series 

 and taking the mean result, it gives the difference between 

 the two weights, unaffected by any possible difference in 

 the weight of the two pans. This contrivance is especially 

 useful, when either of the weights to be compared consists 

 of several separate weights. It was used by Prof. Miller 

 for all his more important weighings during the construc- 

 tion of the imperial standard pound. 



