Royal Astrotwmical Society. SS6 



tached, by felt and pitch, a small plate of cast iron ; and in a small 

 hole sunk in this cast iron, opposite to the centre of gravity, a pro- 

 jecting pin of a triangular plane lever takes its bearing. This trian- 

 gular plane lever is merely a triangle, having three points to sustain 

 pressure, and a small hole for the fulcrum at the centre of gravity of 

 the three points considered as equal weights ; it is, then, evident 

 that if that point is made really the fulcrum, the pressures at those 

 three points are necessarily equal. The same construction being 

 applied to each of the nine groups, then it is only necessary to sup- 

 port the nine fulcra in such a way that the pressures upon them 

 are necessarily equal ; and this is done by grouping three of their 

 fulcra as points of pressure upon the three points of another and 

 stronger triangle, whose fulcrum is at the centre of gravity of 

 these three points considered as equal weights ; and this fulcrum 

 is one of the points of the fixed frame. It will thus be evident 

 that every one of the first- mentioned pins sustains exactly one 

 twenty- seventh part of the whole weight of the mirror, or rather 

 of the whole pressure of the mirror perpendicular to its surface, 

 neither less nor more. [When the mirror is in the telescope, it ex- 

 erts also an edgewise pressure, which in Lord Rosse's construction 

 was at first sustained by fixed pillars of the fixed frame ; of these 

 more will be said hereafter.] The fixed frame has small wheels ; 

 while it is in the grinding-trough, it is so lifted off the wheels that 

 it takes a firm bearing upon the rotating frame ; when it is to be 

 carried to the telescope it is lowered to take bearing upon its wheels, 

 the side of the grinding-trough is taken off, the fixed frame carrying 

 the speculum is wheeled upon a proper carriage, the carriage con- 

 veys it to a place very near the telescope, where is a railway at 

 proper height for receiving the small wheels ; the telescope is placed 

 vertical, its lower end is opened, and continuation-rails are laid to 

 it, and the fixed frame is thus wheeled into the telescope, carrying 

 the mirror ; then by powerful screws the bearing of the fixed frame 

 is received upon three points, the wheels being entirely lifted off 

 their bearings. [Since giving this account, the Astronomer Royal 

 has learnt from Lord Rosse that a speculum which was raised for a 

 few minutes only from its lever-bearings received in that time a 

 permanent change of figure.] 



In Mr. Lassell's operations the speculum is supported on eighteen 

 points, the grouping being first made by two and two, with straight 

 levers, and then the fulcra of the straight levers being by means of 

 triangular levers supported upon three ultimate points. The Astro- 

 nomer Royal was not able to say whether the same cautions as to 

 retaining the speculum at all times upon the same bearings, which 

 Lord Rosse found necessary, were required for Mr. Lassell's mirrors ; 

 but it is evident that the difliculties of support of every kind are here 

 very much less. The edgewise pressure, when in the telescope, is 

 here supported by a semicircular iron hoop, of which more will be 

 said hereafter. 



III. The next point deserving special comparison is the apparatus 

 for grinding and polishing. 



The apparatus used by Lord Rosse imitates very closely, but with 



