466 



NA TURE 



{Sept. 13, 1 88; 



protects the interior of the apparatus from every rapid 

 variation of temperature. By the movement of the carriage 

 the observer brings successively under the microscopes, 

 the two metres the difference between which he wants to 

 ascertain ; he bisects the lines of both, and this opera- 

 tion made at the two extremities furnishes the equation 

 desired between the two rules. 



Another of the comparateurs represented in the accom- 

 panying diagram (Fig. i) is designed for the measurement 

 of expansions. As in the preceding comparateur we here 

 also find two microscopes with fixed micrometers and a 

 carriage moving on rails. Here, however, the carriage 

 contains two distinct boxes or troughs ; each at a distance 

 of about one metre from the other. The two rules to 

 be compared are placed one in each of these troughs. 

 They are thus to some extent independent of each other, 

 and may therefore be introduced at different temperatures. 

 To measure the expansion of one rule, you place it in 

 one of the troughs, and in the other trough the other 

 rule called " dc comparaison!' This latter, so long 

 as the process of determination lasts, is maintained 

 at an invariable temperature, while the other is alter- 

 nately cooled and heated throughout the series of con- 

 secutive experiments between sufficiently extended limits. 

 The latter rule then alternately contracts and expands, 

 and in the case of each particular experiment you 

 compare the length the rule has reached at the tem- 

 perature to which it is then subjected, with the constant 

 length of the " de comparatson" rule. One of the great 

 difficulties connected with these measurements is the 

 maintenance for a sufficiently long time of an exactly 

 uniform temperature, particularly when this temperature 

 is notably at variance with the surrounding temperature. 

 To secure this requirement the rules to be compared are 

 immersed in a liquid, and this liquid is heated by means of 

 a constant circulation of water within the double walls of 

 the trough. Indiarubber pipes, as may be seen in the 

 diagram (Fig. i), are used for this purpose. The water is 

 supplied from a large metal reservoir outside the chamber, 

 being heated by a regulating system that causes it to issue 

 at an invariable temperature. Thence by pipes it is con- 

 veyed to the comparateur, traverses the trough in a con- 

 tinuous stream, and is then carried off by waste pipes, 

 conveniently arranged, into a drain. By this means a 

 constant thermal state is maintained, within a few hun- 

 dredths of a degree, at any point up to forty degrees, for 

 whole hours at a time. 



The diagram (Fig. i) indicates the principal details of 

 the mechanism employed. In front is seen the handle 

 which by means of an endless cord draws away the 

 carriage and allows the rapid substitution of one trough 

 in place of the other under the microscopes. On the sides, 

 the long rods provided with buttons, which the observer 

 finds always under his hand whatever position he may 

 occupy round the instrument, have the power of acting 

 equally on the carriage by means of a cog-wheel placed 

 under it, and of moving it backwards and forwards by a 

 uniform slow movement. On the lids are perceived the 

 heads of the different keys which enable rectifications of 

 all adjustments to be made, as also the lenses by means 

 of which the thermometers are read. The fly-wheels 

 placed in front of the troughs serve by means of cords 

 and pulleys to convey a movement of rapid rotation to 

 the agitators which are placed in the trough, and thus 

 vigorously intermingle the strata of the liquid, and secure 

 uniformity of temperature in all parts of the bath. 



With these apparatuses the difference can be determined 

 between two metres at a given temperature with an exact- 

 ness reaching to some ten-thousandths of a millimetre. In 

 order to obtain such nicety, it is of course necessary that 

 the lines of the metres be traced with sufficient fineness 

 and distinctness to fit them for the magnifying power 

 employed. 



The two instruments just mentioned are fitted for the 



comparison of metres alone. The comparateur universel, 

 on the other hand, allows comparisons to be made of any 

 lengths whatever from below a metre up to two metres. 

 The aspect of this new comparateur is entirely different 

 from that of the two preceding. The microscopes which 

 in all cases constitute the essential parts, instead of being 

 fixed, are here mounted on carriages, which can be moved 

 over a kind of bridge placed horizontally between two 

 stone pillars. This bridge is formed by a large block of 

 brass furnished with steel surfaces on its upper edges, 

 which serve as a support and guide to the microscopes in 

 their movements. It is perfectly rectilineal and horizontal 

 When, by moving the carriages, the microscopes have been 

 brought into the position they require to occupy for a 

 given work, they are fixed by tightening a lever with the 

 aid of a knob which controls a screw. Below, as in the 

 preceding comparateurs, is a missive carriage likewise 

 bearing supports on which are arranged the rules needing 

 to be examined. These supports are equally furnished 

 with all the necessary means of adjustment. These latter, 

 again, are worked by a mechanism too complicated to 

 allow so much as an idea of it to be communicated 

 without the help of diagrams. The comparateur contains, 

 besides, a standard rule of two metres, divided along 

 its whole length into centimetres, two supplementary 

 microscopes mounted on a special carriage and designed 

 for marking the subdivision of a metre, various accessory 

 pieces capable of serving for comparison of measures a 

 bouts, either one with another or with measures d traits, 1 

 &c. It is entirely inclosed in a large mahogany box. 

 This box is furnished with windows necessary for lighting 

 the various parts, and with the orifices required for the 

 transmission of movements to the interior, &c, and has the 

 appearance of an imposing and elegant piece of furniture. 

 We have still to mention a comparateur for metres a 

 bouts by Steinheil's method; and to add that this beautiful 

 collection will in the course of a few months be completed 

 by the introduction of a geodetical comparateur for rules 

 of four metres, which is actually in process of construction, 

 and the object of which is indicated by its name. 



{To be continued.) 



THE VIENNA INTERNATIONAL ELECTRIC 

 EXHIBITION 



F'OR two weeks, the arrangement of the machinery 

 being nearly complete, the Exhibition has been open 

 in the evening from 7 till 11. The effect of the illumina- 

 tion of the immense interior of the Rotunda and its 

 annexes by the various incandescent and arc lamps, and 

 of the surrounding places which are lighted by large re- 

 flectors, is brilliant. The electric railway of Siemens and 

 Halske between the Rotunda and the Praterstern is 

 already in operation. The theatrical performances at the 

 "Asphaleia" Theatre, which is lighted by 1500 Swan 

 lamps fed by a large Zipernowsky alternating current 

 machine, have also begun this week. 



The series of lectures to be held at the theatre during 

 the Exhibition was inaugurated on August 27 by Sir C. 

 W. Siemens, with a lecture " On the Temperature, Light, 

 and Total Radiating Power of the Sun." After a short 

 introductory sketch of the nature of the terrestrial sources 

 of light, the lecturer gave an account of the ratio of the 

 three forms of radiant energy, viz. heat, light, and actinism, 

 as produced by the sun and terrestrial light sources. 

 Then referring to the difference between the statements 

 of various astronomers and physicists relating to solar 

 temperature, he expressed his opinion that the tempera- 

 ture of the sun could not exceed 3000° C, and explained 

 the experimental methods he used for measuring the solar 

 temperature. The second lecture was delivered on Sep- 



1 A metre or other measure ii baits is one whose ends exactly coincide 

 with the ends of the material of which it is made ; a measure 4 traits is 

 bounded by lines within the margins of the material on which it is traced. 



