178 



1'HILOSOPHICA.L TRANSACTIONS. 



[anno 1777. 



is really the case, viz. that he knew how well pitch was calculated for purposes 

 of this kind. Only it may be remarked, that as glass is much harder, polishes 

 much slower, and consequently does not wear away and alter its figure so soon as 

 the metal of which the speculum is made; and as at the same time (on account 

 of the very small apertures allowed to telest;opes of this sort) nothing more than 

 a spherical figure is proposed; he is thereibre obliged to use pitch in a hard, 

 friable, and stubborn state; whereas, considering the delicate substance of the 

 metal speculum, and the figure intended to be given to it, the soft pitch of the 

 common sort, by sutFering the putty to bed itself in its substance, produces the 

 most beautiful polish; and by its pliability is better calculated for that nuitual ac- 

 commodation between polisher and metal, so necessary to the figure proposed. 

 Explanation of the Figures — PI. 9, fig. 1 shows tlie grinder for working off the rough face of tlie 

 metal ; die black strokes represent deep grooves made ^^■ith a graver. Fig. 2 the bed of hones, to 

 complete the spherical figure of the speculum, and to render its surface fit fur the polisher. Fig. 3 an 

 apparatus for examining the parabolic figure of the speculum, a a The moutli of the telescope, or 

 edge of the great ti'be. bb A thin piece of wood fastened into, and flush with the end of the tube; 

 to which is permanently fixed tlie annular piece of paste-board cc, intended to cover, and to prevent 

 the action of the corresponding part of tlie speculum, d Another piece of paste-board, fixed by a 

 pin to the piece of wood bb, on which it turns as on a centre ; so tliat the great annular opening hh 

 may be shut up by the ring ff, or the aperture gg by the imperforate piece e in such manner that, 

 in the first instance, the reflexion may be from the centre, and in the latter from die circumference, 

 of the great speculum. 



XVIL Extract of a Register of the Barometer, Thermometer, and Rain, at 

 Lyndon, in Rutland, 1776. By Thomas Barker, Esq. p. 330. 



Barometer. Thermometer. Rain. 



Jan. 



Feb. 



Mar. 



Apr. 



May 



June 



July 



Aug. 



Sept 



Oct. 



Nov 



Dec. 



Highest. Lowest. , Mean. 



Morn. 



Aftem. 



Morn. 



Aftem. 



Morn. 



Aftern. 



Morn. 



Aftern. 



Morn. 



Aftern. 



Morn. 



Aftern. 



Morn. 



[Aftern. 



'Morn. 



[Aftem. 



Morn. 



Aftern. 



Morn. 



Afteni. 



Morn. 



Aftern. 



Morn 



Aftem. 



•29.50 

 29-9J 



29.99 

 29.87 

 29.86' 

 29.83 

 29.93 

 29. .S7 

 29.N5 

 29.9t 



28.88 



28.24. 



28.50 



29.00 



28.73 



29.04. 



29.03 



29.00 



28.75 



29.0(1 



28.6<) 



28.73 



29.27 



28.89 



29.46 



20.58 



29.57 



29.42 



29.^4. 



29.-11 



29.40 



29.56 



29.42 



29.4.3 



In the House. 

 High. Low. Mean. 



Mean of all 



29.40 



42^ 



43' 



44^ 



45 



53.^ 



55 



54i 



571 



62J 



66 



66 



6'9i 



65 



6s J 



70 



75 



62 



64 



oil 



58 



52 



52 



48 J 



4-9 



23 



26 



24 



25 



381 



40 



43 



44 



47 



48 



55j 



57 i 



57 



m 



57 



59 



52j 



54^ 



4.8 



48 



■il 



37 



3+ 



34 



.301 



39 



40 



44.1 



46' 



50 



5li 



311 



53' 



59 



61 



62 



()'4 



61 



62 i 



oil 



58j 



52^ 



53J 



45. i 



46 



43 



50^ 



Hlgl 



Abroad. 

 Low. Mean, 



42 



45 



44 



45.^ 



47 1 



62| 



52 



64| 



58 



76 



62 



75 



6'^ 



80 



63.^ 



82 i 



60 



69 



53 



62 



54 



54 



4-9 



10 



16 



II 



27 



27 



37 



31 



42 



38 



38 



48.2 



54 



53 



65 



46 



58^ 



37 



52^ 



36 



48 



26.! 



32.^ 



2 a 



26i 



26 



29 



-35 



4U 



38 



481 



421 



55 



47 



57 



54. 



64 i 



58 



69i 



54 



66 



49.^ 



61 



45i 



54' 



38^ 



44 



37 



40. J 



49 



2.511 

 3.195 

 1.51S 



0.887 

 1 .627 

 2.485 

 1.850 

 5.200 

 2.452 

 2.061 

 2.823 

 1.233 



27.842 



