592 



NA TURE 



{Oct. 1 8, 1883 



and this has received excellent confirmation from Prof. 

 Hall's observations in JS77, which indicated a period of 

 ioh. 14m. 2SSs. Schrreter also glimpsed several spots 

 indicating periods of about twelve hours, but these obser- 

 vations are probably erroneous, as they differ so widely 

 from the corroborative results of Herschel and Hall. 

 But it is possible tha'. these distinctive markings on 

 Saturn give anomalous periods similarly to the spots on 

 Jupiter, though hardly to the extent of the differences 

 between the existing observations. Were this planet 

 more sedulously observed, it is certain that we should 

 obtain some new and interesting facts with reference to 

 his globe and rings. As to the belts, they are occasion- 

 ally very plain : Grover has seen them with only 2 inches 

 of aperture, and the writer distinguished them well in 

 18S1 with a 2i-inch O.G. What, therefore, can be so 

 readily seen in small telescopes ought to come out with 

 considerable detail in the large instruments of the present 

 day. As to the system of rings, it forms a complicated 

 object for study. The marked differences in their tints 

 and brightness should be recorded on all occasions. 

 Cassini's division is always plain, but the outer and more 

 minute division of Encke has sometimes defied the power 

 of our best telescopes. It apparently varies both in 

 position and intensity. Other faint subdivisions are 

 sometimes traced, but they are very difficult objects, and 

 seldom seen with certainty. The crape or gauze ring, 

 together with other details, such as the anomalous shadow 

 of the ball upon the rings, supply an endless store of 

 curious appearances requiring further elucidation. 



There is no doubt that, notwithstanding the mass of 

 interesting facts gleaned in past years respecting the 

 physical aspects of Mars, Jupiter, and Saturn, there re- 

 main many novel features to be distinguished, and many 

 new facts to be descried. Observers, therefore, who 

 make these observations a specialty should endeavour 

 not only to confirm former results, but to make some 

 advance upon our existing knowledge. 



We have not space in the present paper to refer to the 

 satellites of these several planets, but may possibly be 

 able to do so on a future occasion. W. F. Denning 



THE INTERNA TIONAL BUREA UOF WEIGHTS 



AND MEASURES^ 



II. 



HAVING now given a description of the instruments 

 used in the section of linear standards, we come to 

 the apparatus belonging to the section of standards of 

 u eight. 



As regards the essential instrument connected with the 

 process of weighing, the International Bureau possesses 

 one of the most remarkable collections of balances of 

 precision existing in the world. Of these the principal 

 ones have been constructed by the house of Riiprecht of 

 Vienna The large engraving accompanying this (Fig. 3) 

 represents in its entirety the spacious chamber in which 

 they are set up. In addition we gave a special sketch (p. 

 463, Fig. -) of the balances principally designed for com- 

 parison of standard kilogrammes. This balance is con- 

 structed in such a manner as to adapt it for being worked 

 at a distance, whereby it is kept free from the disturbing 

 influence always occasioned in the process of weighing by 

 the proximity of the observer, in consequence of the 

 variations of temperature which his presence close to the 

 balance gives rise to. In the case of the instrument now 

 in question, the observer, having prepared his weighing 

 apparatus the preceding day, — that is, placed the weights 

 he will have occasion to use in their right positions in the 

 pans of the balance, — avoids any longer going near the 

 b dance. Standing in front of his telescope he performs 

 all the operations involved in weighing that is, he puts 

 the weights on the pans, releases the pans, and then the 



1 Continued from p. 466. 



beam, and measures the oscillations of the beam ; then 

 changes the weights from one side to the other, placing 

 that to the right which was at the left, and conversely, &c, 

 the whole at a distance of four metres. For that purpose 

 the balance is provided with a mechanism very ingenious 

 and of perfect precision which works automatically by 

 means of handles fixed to the extremity of long rods. The 

 oscillations of the beam are read by the reflection of a 

 graduated scale on a mirror borne by the beam ; it is 

 the image of that scale the observer sees displacing it-elf 

 slowly in his telescope while the balance oscillates. He 

 notes a certain number of successive oscillations, and 

 thence calculates the position of equilibrium. 



Three other balances, of the same model but smaller, are 

 intended for comparisons and adjustments of lighter 

 weights. They have the same mechanism of transposi- 

 tion, only in the case of the two smallest, a little more 

 simplified and less complete. In the centre of the large 

 engraving (Fig. 3) are seen the large arms of the lever 

 which allow the weighings to be made at a distance. 

 These arms rest on three stone pillars, above which are 

 placed the telescopes for reading the oscillations of the 

 beam. 



The following are some details of the mechanism of 

 transposition employed in this balance. The pans of the 

 balance have a shape altogether peculiar. Each is formed 

 by a circular piece open at one point and extending itself 

 inwards by four triangular plites or teeth directed towards 

 the centre of the balance. A cross piece placed under- 

 neath can be passed between these plates. Suppose now 

 that weights, say of one kilogramme each, are placed above 

 the pans on each side ; and to make the idea the more 

 definite let the kilogramme A be on the left scale and the 

 kilogramme B on the right. Taking hold of one of the four 

 handles under his hand, the observer sets the mechanism 

 in motion. This is what happens : — the cross piece placed 

 under the pan mounts at first, ascends beyond the pi ine 

 of the pan and consequently lifts the kilogramme resting 

 above it. Arrived at a suitable height the cross piece 

 shifts its place laterally, and disengaging itself from the 

 pan it gradually gets deposited above one of the plates, 

 attached right and left to the pillar of the balance. 

 These plates follow an arrangement analogous to that 

 of the pans. By continuing the movement the cross 

 piece then commences to descend, and traverses the 

 plane of the plate, depositing on it the kilogramme which 

 it has raised from the scale. While these movements are 

 effected to the left in the case of the kilogramme A, they are 

 in simultaneous accomplishment to the right in the case 

 of the kilogramme B. The two weights to be compared are 

 thus transferred at the same time to the central plates. 

 Then taking hold of a second handle the observer turns 

 the two plates 180 round the axis of the pillar of the 

 balance, a movement which shifts the plate \( hich was 

 at the left to the right, and conversely. All that is needed 

 then is to cause the same evolution to be gone over 

 with the crosses which they have already done, but 

 inversely, in order to bring back the kilogrammes to the 

 scales of the balance; the kilogramme A is then at the 

 right, the kilogramme B at the left, and the observer can 

 proceed to the second part of the weighing. 



The two other handles control — one the movement 

 serving to release the pans, the other the movement 

 low ering the fork and setting free the beam. 



For a long time it has been known that the balance is 

 an instrument of precision par excellence. By means of 

 those here in question the minimum amount of error in 

 the process of weighing has been reduced to an almost in- 

 finitesimal degree. The difference of two kilogrammes, for 

 example, can by them be determined to a nicety reaching 

 to the hundredth of a milligramme, that is, the weight of a 

 kilogramme is ascertained do«n to within a hundred 

 millionth of its absolute value. 



In another part of the hall is shown the hydrostatic 



