1S50.] 



THE CIVIL ENGINEER AND ARCHITECT'S JOURNAL. 



3S9 



tion has to be changed, so as either to follow the object or pass 



to another, is felt to be very great; 

 and even although many excellent 

 arrangements of mounting have 

 been proposed to overcome this 

 difficulty, yet so much personal in- 

 convenience, discomfort, and trou- 

 ble yet exists in all of them that 



the attention of the observer is in 



'^s'Si,45.s<iSSS?s---s<S'5SSS^^^^> no small degree occupied and di- 

 Fig.e. vided between regard to his own 



comfort and safety, and the actual object in view. Although such 

 discomforts might in some respect be reduced in the case of the 

 employment of the arrangement, fig. a, namely, that of the Gre- 

 gorian; or (ig. 3, that of the Cassegrain construction, in both of 

 which it will be seen that the observer views the object from 

 the lower end of the tube, thus, fig. 7 ; yet although the observer 

 may not in this case have so often to 

 mount up to the top of high ladders, 

 and be so far in bodily fear as is but too 

 much the case, as in fig. 6, yet the con- 

 tinual change in his position and the 

 awkwardness of having frequently to 

 crouch down in uncomfortable postures, 

 detracts so much from that ease of per- 

 son which is so requisite in the conduct- 

 ing refined observations, or for enjoying 

 with due tranquility views of the glo- 

 rious objects scattered throughout space, 

 that after considerable experience with telescopes of a large class, 

 Mr. Nasmyth bethought himself of such an arrangement as would 

 remove most of these objections. The optical department of this 

 arrangement is seen in section in fig. 4, where it will be observed 

 that, by the union of the Newtonian and Cassegrain construction, 

 in so far as respects the turning back of the cone of rays by the 

 small conve.x mirror, C, and receiving them at D, by a small 

 diagonal plane mirror, D, the rays which ultimately form the image 

 of the object are sent out sideways tlirough the ti-uiimon,G, in which 

 the eye-piece is placed, and through which, in fact, the observer 

 views the object. 



By having a corresponding trunnion at the opposite side, T, and 

 employing these trunnions as the supports of the telescope, and 

 using them as the axis on which it is moved in altitude, it will be 

 evident that, as the eye-piece, G, is thus in the centre of motion, 

 whatever be the sweep of elevation in moving the telescope verti- 

 cally from object to object, no change in the position of the 

 eye of the observer will be required; his eye, while opposite to the 

 trunnion, is conmion to all positions of the instrument in altitude; 

 his eye is virtually in the centre of motion. 



But as the telescope has to be moved round so as to follow the 

 motion of an object in azimuth, it is desirable that the observer 

 should not have to change his position even in this respect. There- 

 fore, in order that he may sit at his ease opposite to the eye-piece 

 while the telescope is moved either in altitude or in azimuth, all 

 that hasto be done toattain this object is to place the entireinstrunient 

 on a tarn-table^ and have a comfortable seat for the observer also 

 on the turn-table, and then, whatever be the elevation or direc- 

 tion in which the telescope is pointed, the observer need never 

 stir from his comfortable seat; and as we all now know with what 

 ease ponderous machines, such as railway wagons or locomotive 

 engines, can be swung round on properly constructed turn-tables, 

 and also the ease with which a well-balanced mass can be swung 

 when it centres, some idea may be formed of the perfect ease and 

 facility with which such an instrument as tliis of Mr. Nasmyth's 

 can be governed and directed by the observer, who has, by means 

 of suitable handles brought close to his chair, the most perfect 

 command of every requisite movement. The instrument in ques- 

 tion, which is represented in fig. 5, weighs upwards of two tons, 

 can be moved in every direction by the point of the finger, swung 

 round in an instant, or elevated to any object on a slow motion 

 given to it so as to enable the observer to keep the object in the 

 centre of the field for hours. Such is the perfect steadiness of the 

 motion, that not the slightest tremor is perceptible, even when 

 observing with a magnifying power of -toO times. Some objec- 

 tion may be urged against the optical arrangement by which iMr. 

 Nasmyth has brought his telescope to yield this central vision, in 

 so far that it is requisite to employ a third reflecting surface, 

 namely, the small plane diagonal mirror (U, fig. 4,) by means of 

 which we are enabled to view the object through the hollow 

 trunnion T, fig. 4, or B, fig. 5; no doubt some portion of light is 



sacrificed by the employment of this third reflector ; but when we 

 obtain in exchange so vast an amount of convenience and comfort 

 as result from the adoption of this arrangement of Mr. Nasmyth, 

 most observers will be happy to accept the exchange, and with the 

 advantage of the ease, comfort, and tranquility resulting from the 

 absence of all personal sources of interruption, Mr. Nasmyth pre- 

 sumes that by thus inducing more frequent and careful observa- 

 tion'^, scieice will be advanced. 



Mr. Nasmyth stated, that the main object he had in view in 

 constructing this large telescope was not so much to pursue observa- 

 tions of objects of the fainter class, as nebula?, &c., but rather for 

 following up a series of observations in reference to the structure 

 of the lunar surfiice, on which subject he has been occupied for 

 several years; and such has been the increased comfort and facility 

 which this truly manageable and powerful instrument has given 

 him, that, judging from the specimens of the "drawings from 

 nature," of the remarkable features of the lunar surface, which 

 he exhibited to the Section, the optical powers of his instrument are 

 equal to its convenience and comfort to the obsei-ver. 



Fig. 5. 



Fig. 5 is a perspective view of Mr. Nasmyth's "Comfortable 

 Telescope;" C is a cast-iron turn-table, which, on being moved 

 round, carries with it the entire instrument, and the oliserver, 

 who, seated in a comfortable chair, has complete control of the 

 elevation and round-about motion; the former by means of a tan- 

 gent screw and wheel, F, the latter by tangent screw and pinion- 

 shaft, E, which commands the roundabout or azimuth motion. An 

 eye-piece is placed convenient to the eye of the observer at C. 

 Some idea may be formed of the facility w'ith which the movements 

 can be controlled, wlien it is stated that within two minutes 

 iAIr. Nasmyth has frequently directed this large instrument to 

 nine diflferent objects situated in various parts of the heavens. 



Mr. Nasmyth, at the request of the president of the Section, 

 gave some description of his mode of securing perfectly sound 

 castings of specula for such large instruments, of wbich we hope to 

 furnish our readers some account in our next number. 



On a Patent Steam Plough. By James Usheb. 

 Mb. Ushek described his Patent Steam Plough, and stated that 

 many fruitless attempts had been made to cultivate the land by 

 steam-power, the reason of which had been that the parties had 

 proceeded on an entirely erroneous principle; as, from the method 

 they have pursued, they could never get the machine to proceed alonj;' 

 the land. Tliis can be simply explained by stating that all former 



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