Jan. 20, 1876] 



NA TURE 



229 



during its detention, and before it reaches the cylinder, 

 unless, in fact, the heat abstracted in condensation be 

 restored to it, the loss of power is very great. This has 

 hitherto been the stumbling-block of compressed air 

 engines, and M. M^karski seems to have completely met 

 the difficulty. He adopts as a re-heater a cylinder hold- 

 ing about 100 litres of water, taken from the boiler of an 

 engine, at five atmospheres, and to obtain the maximum 

 of effect possible, the condensed air is delivered from the 

 reser\'oirs to the cyhnder only after traversing the entire 

 mass of water. 



B)- a clever contrivance M. Mekarski regulates at plea- 

 sure the action of the compressed air upon the piston. 

 Two hermetically- closed boxes are placed vertically upon 

 the re-heater ; their common face is formed by a caoutchouc 

 diaphragm, in direct connection with an obturator, which 

 opens or shuts more or less the opening which communi- 

 cates between the lower box and the chamber containing 

 a mixture of compressed air and vapour in the upper part 

 of the hot-water cylinder. It will be seen that this orifice 

 will be more or less uncovered according as the pressure 

 in the lower box wiU be above, or not, the pressure in the 

 lower box. This second box is itself filled with air, and 

 constitutes a small pump-barrel, in which a large plunger 

 piston works. The rod of the piston is a screw, and is 

 fitted outside with a small regulator, on which the driver 

 works. This may rapidly be made to vary the presence 

 of the air in the upper box, and consequently the pressure 

 be increased or diminished of the air which is delivered 

 from the lower box to the motory cylinder. 



THE GREAT TELESCOPE OF THE PARIS 

 OBSERVATORY 



Y^T'E have from time to time noted the progress of the 

 * * great telescope which for years has been in course 

 of construction for the Paris Observatory, and now that it 

 is completed and in its place we are glad to be able to 

 present a view of the instrument, for which, and for the 

 details which follow, we are indebted to La Nature. 



In 1855 M. Le Verrier purchased in England two large 

 discs, the one of flint and the other of crown glass, intended 

 to form the material for an object-glass. The late Leon 

 Foucault, the eminent physicist of the Observatory, was 

 charged with the investigation of the processes which 

 should be employed to cut these large glasses, whose 

 dimensions were much greater than those to which opti- 

 cians had been accustomed. It is known how Foucault 

 was led, by his series of investigations, to make mirrors 

 of silvered glass. Successive attempts enabled him to 

 present French observatories with reflectors of 40, of 50, 

 and finally of 80 centimetres in diameter, having a tube 

 in length only six times the diameter of the mirror. The 

 largest of the telescopes constructed by Foucault himself, 

 of 80 centimetres aperture, is at Marseilles, under the 

 care of M. Stephan ; by means of it this astronomer has 

 seen all that Herschel saw with his enormous metallic 

 reflector of r45 m. diameter, all that Lord Rosse has been 

 able to see with his leviathan of 1 70 m., and he has added 

 hundreds of new nebulse to the list given by his illustrious 

 predecessors. 



To crown his labours, L. Foucault wished to construct 

 the largest mirror which it would be possible to make 

 by his admirable method. This superior limit is vio m. 

 diameter. M. Le Verrier caused to be cast at St. Gobain 

 a block of glass weighing 700 kilogrammes, which was 

 rough-ground and shaped in the workshops of MM. Sauter 

 and Lemonnier. But to construct this telescope, with 

 its tube of 16 metres in length, required special funds, 

 the ordinary- budget of the Observator}- not being suffi- 

 cient. M. Le Verrier sought to obtain them from the 

 Corps Legislatif, which, in 1865, voted a sum of 400,000 

 francs. 



By the beginning of 1868, Foucault, notwithstanding 



his researches on regulators and the fatigue caused by 

 the active part he took in the Exposition of 1S67, had 

 prepared the plans for the large reflector, when death 

 snatched him from his work, and deprived France of one 

 of the most original and finest geniuses she has possessed. 

 This fatality, and the troubles which soon after and for 

 long disturbed the Observatory', seemed to have lost to the 

 country the work of years, and to have rendered useless the 

 liberality of Government. Happily, the Minister of Public 

 Instruction, M. Duruy, was willing to lend an attentive 

 ear to the suggestions of men of science, and place at 

 their service an intelligence eager for progress. At the 

 request of the friends of Foucault he ordered the work 

 which had been begim to be continued, and the authorities 

 of the Observatory eagerly complied with his orders. 

 An eminent mechanician, M. Eichens, indicated to M. 

 Le Verrier by the Grand Prize in Mechanics which he 

 obtained at the Exposition of 1867, and by his con- 

 struction of large instruments for the Observatory, re- 

 ceived the order for the construction of the telescope. M. 

 Adolphe Martin, whom Foucault had instructed in his 

 methods and associated with himself in his optical under- 

 takings, was charged with the polishing of the mirror. 

 Finally, M. Le Verrier entrusted to one of the astronomers 

 of the Observatory, M. Wolf, the general superintendence 

 of the work. 



The construction ought to have been completed in three 

 years. The war and the changes in the Observatory 

 caused the work to languish, and it was not resumed with 

 vigour until the return of M. Le Verrier as Director in 

 1873. At the commencenient of 1875 the mirror was 

 completed and tried upon terrestrial objects ; M. Wolf 

 had got a shelter constructed for the telescope and the 

 staircase for the observer ; finally, in the month of 

 October, M. Eichens sent in the instrument complete in 

 all its main details. 



The total cost of the instrument and of the observatory 

 amounts to 190,000 francs. 



The illustration which we give represents the telescope 

 in a position for obser\'ation. The wheeled hut under 

 which it usually stands, a sort of wagon seven metres high 

 by nine long aJ)d five broad, is pushed back towards the 

 north along double rails. The observing staircase has been 

 fitted to a second system of rails, which permit it to circu- 

 late all round the foot of the telescope, at the same time 

 that it can turn upon itself, for the purpose of placing the 

 observer, standing either on the steps or on the upper 

 balcony, within reach of the eye-piece. This eye-piece 

 itself may be turned round the end of the telescope into 

 whatever position is most easily accessible to the ob- 

 server. 



The tube of the telescope, 7"30 metres iji length, con- 

 sists of a central cylinder, to the extreiriities of which 

 are fastened two tubes of 3 metres long, consisting of four 

 rings of forged iron bound together by twelve longitudinal 

 bars also of iron. The whole is lined with small sheets 

 of steel plate. The total weight is about 2,400 kilogrammes. 

 At the lower extremity is fixed the barrel which holds 

 the mirror ; at the other end a circle, movable on the 

 open mouth of the telescope, carries at its centre a 

 plane mirror, which throws to the side the cone of rays 

 reflected by the great mirror. The telescope is thus on 

 the Newtonian system. That of Melbourne, so admirably 

 constructed in England, is a Cassegrain telescope ; the 

 metallic mirror is pierced at its centre by an aperture 

 which receives the eye-piece, a system so far advantageous 

 that the observer always remains at the lower part of his 

 instrument, and has to raise himself only a very- short 

 distance above ground, but less calculated perhaps to 

 produce a perfect image than the Newtonian , system 

 adopted by Foucault. 



The weight of the mirror in its barrel is about 800 

 kilogrammes ; the eye-piece and its accessories have the 

 same weight. Such is the load under which the tube of 



