KIDD'S OWN JOURNAL. 



175 



of this building to prevent the slightest 

 vibration ; but, if any disappointment in this 

 respect should arise (which, however^ Mr. 

 Gravatt does not anticipate), additional 

 weight can be obtained by loading the 

 several floors, and the most perfect steadi- 

 ness will be thus insured. 



By the side of this sustaining tower hangs 

 the telescope. The length of the main tube, 

 which is shaped somewhat like a cigar, is 

 seventy-six feet, but with an eyepiece at the 

 narrow end, and a dewcap at the other, the 

 total length in use will be eighty-five feet. 

 The design of the dewcap is to prevent 

 obscuration by the condensation of moisture, 

 which takes place during the night, when 

 the instrument is most in use. Its exterior 

 is of bright metal, the interior is painted 

 black. The focal distance will vary from 

 seventy-six to eighty-five feet. The tube at 

 its greatest circumference measures thirteen 

 feet, and this part is about twenty-four feet 

 from the object glass. The determination 

 of this point was the result of repeated ex- 

 periments and minute and careful calcula- 

 tions. It was essential to the object in 

 view that there should not be the slightest 

 vibration in the instrument. Mr. Gravatt, 

 reasoning from analogy, applied the principle 

 of harmonic progression to the perfecting of 

 an instrument for extending the range of 

 vision, and thus aiding astronomic research. 

 By his improvements the vibration at one 

 end of the tube is neutralised by that at the 

 other, and the result is that the utmost 

 steadiness and precision are attained. 



The iron work of the tube was manufac- 

 tured by Messrs. Kennie, under the direction 

 of Mr. Gravatt. The object-glasses are 

 also of English construction, and throw 

 curious light upon the manner in which an 

 enlightened commercial policy has reacted 

 upon and promoted the advancement of 

 science. Up to a recent period, the flint 

 glass for achromatic telescopes was entirely 

 of foreign manufacture. Since the reduc- 

 tion in the duty, great improvements have 

 been made in this department. The 

 making of the large flint glass was intrusted 

 to Mr. Chance, of Birmingham, who at first 

 hesitated to manufacture one larger than nine 

 inches in diameter. Upon being urged, 

 however, by Mr. Craig, he has succeeded in 

 producing one twenty-four inches ; per- 

 fectly clear, and homogeneous in structure. 

 Besides this, there is a second of plate glass 

 of the same dimensions, cast by the Thames 

 Plate Glass Company, either of which the 

 observer may use at his option. 



The manner in which these object-glasses 

 are fitted into the tube is a marvel of 

 artistic invention. By means of twelve 

 screws, numbered according to the hours of 

 the day, they can be set in an instant to any 



angle the observer may require, by his 

 merely calling out the number of the screw 

 to be touched. The object-glasses also 

 move round in grooves to wherever it may be 

 considered that a more distinct view can be 

 gained. The tube rests upon a light wooden 

 framework, with iron wheels attached, and 

 is fitted to a circular iron railway at a dis- 

 tance of fifty-two feet from the centre of the 

 tower. The chain by which it is lowered is 

 capable of sustaining a weight of thirteen 

 tons, though the weight of the tube is only 

 three. 



Notwithstanding the immense size of the 

 instrument, the machinery is such that it can 

 move either in azimuth, or up to an altitude 

 of eighty degrees, with as much ease and 

 rapidity as an ordinary telescope, and, from 

 the nature of the mechanical arrangements, 

 with far greater certainty as to results. 

 The slightest force applied to the wheel on the 

 iron rail, causes the instrument to move 

 horizontally round the central tower ; while 

 a wheel at the right hand of the observer, by 

 a beautiful adaptation of mechanical powers, 

 enables him to elevate or depress the object- 

 glass with the greatest precision and 

 facility. So easy, in fact, is the control 

 over the instrument in this respect, that a 

 very slight touch on the wheel lifts ten 

 cwt. It may be observed, also, that there 

 cannot be the slightest flexure in the tube ; 

 no error or deflection arising from that 

 cause can occur, while the ease with which 

 it can be directed towards any point of the 

 heavens will enable the observer to make 

 profitable use of any patch of clear sky, 

 however transient it may be. The great 

 value of this need not be pointed out to 

 those accustomed to making astronomical 

 observations. With respect to the magni- 

 fying power of this novel instrument, it is 

 only necessary to state that, though the 

 focus is not so sharp as it will be shortly, 

 it has already separated the nebulae in the 

 same way as Lord Rosse's. It has also 

 separated some of the double stars in the 

 Great Bear, and shown distinctly a clear 

 distance of fifty or sixty degrees between 

 them, with several other stars occupying 

 the intervening space. Ordinary readers 

 will better understand the extraordinary 

 magnifying power of the telescope, when we 

 inform them that by it a quarter-inch letter 

 can be read at the distance of half a mile. 



The preparations for this really national 

 work have been progressing for the last two 

 years under the superintendence of Mr. 

 Gravatt as engineer and mathematician ; but 

 it is only about three months since the 

 superstructure at Wandsworth Common was 

 commenced, and it is already near comple- 

 tion. 



We understand that the Observatory is 



