ASTRONOMICAL INSTRUMENTS. 125 



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entirely at the object end, although it be changed from the 

 horizon to the zenith. 



In the other constructions, where the centre of motion is 

 nearly in the middle of the tube, it is at times inconvenient 

 to stoop to the eye-end of the telescope when the altitude 

 of the object is considerable ; this new and improved con- 

 struction of a stand remedies this inconvenience, which, 

 together with its unusual steadiness, recommends it highly 

 to astronomical observers. The frame-work is composed of 

 bars of hard wood, firmly braced, and screwed together in 

 a very durable manner, and is mounted on three castors ; 

 these castors may, by the motion of a lever, be so arranged 

 that the stand may rest on the floor more steadily without, 

 the rollers, when the proper position of the telescope has 

 been obtained. For the horizontal and vertical motion of 

 the telescope, the arrangements are very complete ; the 

 former being a toothed wheel and arch ; the latter having 

 a small wheel moving a larger wheel, at the ends of the axis 

 of which there are cog wheels working in links, forming an 

 endless chain, or band, for drawing up one part of the 

 frame, and elevating the telescope. The tubes are of brass, 

 five and a half feet in length, the aperture of the object 

 glass four inches, having two terrestrial and two celestial 

 eye-pieces. Price, $275.00 to $350.00. 



The Transit Instrument. (Fig. 134, next page.) The 

 transit is a meridional instrument, employed, in conjunction 

 with a clock or chronometer, for observing the passage of 

 celestial objects across the meridian, either for obtaining 

 correct time, or determining their difference of right ascen- 

 sion ; the latter of which, in the case of the moon and cer- 

 tain stars near her path, that differ but little from her in 

 right ascension, affords the best means of determining the 

 difference of longitude between any two places where cor- 

 responding observations may have been made. Such being 

 more especially the use of the portable transit instrument, it 

 forms a valuable accession to the apparatus of the scientific 

 traveller, who, remaining a short time at any station, is 

 enabled thereby to adjust his time-keepers both with ease 

 and accuracy, and to obtain the best data for finding his 

 longitude. It also may be employed very successfully in 

 determining the latitude. 



The following figure represents this instrument as con- 

 11* 



