420 , THIRD REPORT — 1833. 



lation ; and the power of pressure or impulsion may be in- 

 creased, if necessary, by increasing somewhat the thickness of 

 each component lamina. One extremity of the series must 

 obviously be firmly abutted against a solid fulcrum or bearing, 

 while the opposite extremity gives motion to a working-rod of 

 a suitable kind. 



The author of the communication then describes in detail 

 tlie various mechanical adjustments by which the apparatus 

 may be applied to maintain any determined rate of ventilation 

 through the casements of church windows ; to give alarm and 

 open valves in water-cisterns in case of fire ; to preserve a cer- 

 tain rate of combustion in furnaces, a uniform temperature in 

 baths and stills, and to act as a safety-valve for a steam- 

 engine. 



On a Reflecting Telescope. By Thojias Davison, of Low 

 Brunton, near Alnwick. 



The author of the invention described in this communication 

 is a weaver of linen, who has devoted himself with great per- 

 severance and ingenuity to the construction of telescopes and 

 other instruments. The modification of the ordinary con- 

 struction of a reflecting telescope which Thomas Davison has 

 executed is intended to improve the performance of the in- 

 strument by diminishing the false or aberrant fight which in- 

 terferes with the distinctness of the image. 



From the nature of its construction, the Gregorian telescope 

 is most exposed to this defect, and it is, therefore, to that form 

 of the instrument that the invention more particularly applies. 

 To each reflector tubes are adapted, having their axes coin- 

 cident with that of the mirrors, and their free ends directed 

 towards each other. The tube connected to the great speculum 

 enters the hole of that speculum, and is of a slightly conical 

 form, diminishing outwards, and prolonged to such a degree as, 

 without stopping many of the rays which should meet in the 

 image, to prevent nearly all the false light from entering the 

 eye-tube. The tube connected with the small mirror is pro- 

 longed so as to meet the extreme rays which converge from 

 the great speculum towards the smaller. Constructed in this 

 manner, Thomas Davison's telescope was found more effective 

 than one upon the ordinary plan. By simple contrivances the 

 instrument can be converted to the Cassegrainian or Newtonian 

 form. 



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