122 On the Size best adapted for Achromatic Glasses. 



Fig. 1 . In this the inner cylinder represents the working cylinder 

 filled with oil, or any other fluid body, (perhaps water might be 

 found to answer) up to the piston, with a portion of the same 

 fluid over the piston. This cylinder having no bottom, the same 

 fluid stands between it and the next cylinder (vvhich Mr. Woolf 

 calls his receiver) at a height proportioned to the distance of the 

 piston from the top of its stroke. The outer cylinder in this 

 figure shows the steam-case. — This is his arrangement for working 

 by the pressure of the atmosphere. The steam being admitted 

 only into the receiver, causes the oil or water to descend therein 

 and rise into the working cylinder carrying the piston before it ; 

 when (the communication with the condenser being opened) a 

 vacuum being formed, the oil or other fluid ascends in the re- 

 ceiver, and the piston then descends, when a fresh charge of 

 steam is admitted, and the stroke of the engine repeated. 



Fig. 2 differs from fig. 1 or.ly in having the receiver placed 

 beside the working cylinder, instead of inclosing the latter. For 

 our part, we prefer the first arrangement. 



When the engine is to be worked by steam upon the piston, in- 

 stead of the atmosphere, then the working cylinder as well as the 

 receiver must be covered, and the working cylinder, like the re- 

 ceiver, must have a communication with the condenser. 



The advantage which Mr. Woolf's new improvement pro- 

 mises is, the impossibility of any steam being wasted by passing 

 the piston — a source of more waste than is generally imagined. 



*^* In the plate of Mr. Woolf's boiler given in our last, the 

 top of the boiler, fig. 3, was, by a mistake of the engraver, car- 

 ried over the top of the brick-work. It should have been carried 

 only to the inner edge of the brick-work. 



XXVI. On the Size lest adapted for Achromatic Glasses; with 

 Hints to Opticians and Amateurs of astronomical Studies , 

 on the Construction ajid Use of Telescopes in general. By 

 W. Kitchener, M.D.* 



Jn reflecting- telescopes. Dr. Herschel says, the maximum of 

 distinctness is much easier obtained in a speculum of six inches 

 and a quarter aperture than in larger ones ; and this was the size 

 of the telescope he made his astronomical catalogues with, and 

 in his hands it has worked wonders. Dr. H. observes, that the 



* The greater part of tliis paper was orirrinally published in " Practical 

 Observations on T.lrscopes," by the same Author, wliich was noticed in 

 the Philosophical Magazine for December 1814. The present paper is 

 intended to form part of an enlarged and improved edition of the above 

 work now preparing for the press by the learned author. 



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