Royal Astronomical Society. 537 



spindle of the third wheel carries a spike pointing downwards (the 

 distance of this spike from the centre of the spindle of the third 

 wheel being adjustable), and this spike lodges loosely in the centre 

 of the grinder and moves it upon the face of the mirror. Thus it 

 will be seen that the centre of the grinder is always moved in an 

 epitrochoid (including the circle as a possible case, if the adjustment 

 is made for that curve), in which the proportion of velocities of the 

 two circles is fixed, but the radii and their proportions are adjustable. 

 The grinder, as in Lord Rosse's construction, is allowed to take 

 freely the rotatory motion which it may receive from the friction on 

 the mirror ; but no counterpoise is used, the weight of the grinder 

 being comparatively small. 



The essential difference of these constructions, as regards the 

 movements of the grinder, is therefore this : that in Lord Rosse's 

 apparatus every stroke is very nearly straight, while in Mr. Lassell's 

 apparatus there is no resemblance to a straight movement at any 

 part of the stroke. 



IV. The process of grinding is nearly the same (with differences 

 corresponding to the difference of dimensions) in the operations of 

 both constructors. 



In Lord Rosse's grinding, the speculum having received an ap- 

 proximate figure from the form of the annealing furnace, the cast- 

 iron grinder is brought very exactly to form by turning upon the 

 lathe, with proper reference to a gauge, and (if not done before) its 

 surface is scored with cross-furrows about two inches apart and 

 nearly an inch deep, leaving the acting part of the surface in squares. 

 The grinder is then mounted in the apparatus ; and this is the most 

 dangerous part of the whole operation. The slightest jar of the iron 

 grinder upon the mirror would break the mirror ; and to avoid this 

 risk, a great number of thin wooden wedges is placed upon the edge 

 of the mirror, the grinder is slowly lowered upon them, and then by 

 degrees they are gently withdrawn. The grinder is then used to grind 

 the surface, with the intermediate powder of emery and water; 

 coarser and finer emery in succession. A heavy weight is allowed 

 to press the grinder upon the mirror ; and as the grinder itself suf- 

 fers much in form, it is repeatedly re-turned upon the lathe. This 

 operation sometimes lasts many days. 



Of the grinder used by Mr. Lassell the Astronomer Royal could 

 give no account, but believes that it is of wood, the same which is 

 used for polishing. The abrading powder used is the same as Lord 

 Rosse's (emery, coarser and finer in succession). 



V. The next point deserving attention is the important process of 

 polishing. 



When the figure of the speculum given by grinding is supposed 

 by Lord Rosse to be sufficiently accurate, the projecting squares of 

 the cast-iron grinder are covered with a coating of resin and tur- 

 pentine, of such a consistence that, at a temperature of about 50° 

 Fahrenheit, the nail can easily make an indentation in it. This is 

 then covered with another coating, of a substance formed by com- 

 bining the mixture last-mentioned with a certain quantity of wheat- 



