ACCESSORY INSTRUMENTS. 139 



of German silver yielding but slightly, and the steel springs 

 being much stronger than the brass ones, the power of com- 

 pression is greatly increased. 



When a more powerful compressorium is required, the form 

 represented by fig. 92 is highly useful. It consists of a 



Fig. 92. 

 plate of brass, three or more inches long and one-and-a-half 

 broad, having in its middle a circular piece of plate-glass for 

 an object-holder ; this is slightly raised above the metal plate ; 

 at one end of the latter is a circular piece of brass, having 

 attached to it another piece of brass, carrying an arm capable 

 of being moved up and down by means of a screw at one end, 

 whilst at the other is a semicircle supporting by screws a 

 ring of metal, to the under side of which a piece of thin glass 

 is cemented ; the semicircle is made to turn upon the arm, and 

 the arm and aU that is attached to it is capable of being 

 turned upon the bottom plate. 



The use of this instrument is obvious ; if we wish to com- 

 press any substance, we must first, by means of the screw, 

 elevate the opposite end of the arm from the object-plate ; the 

 arm, with all its appurtenances, is then to be turned away 

 from the object-plate, and the object being placed on the plate 

 with a requisite quantity of fluid, the arm is then to be 

 brought into its proper place again, and by means of the 

 screw, the metal ring with the thin glass cover can be made 

 to exert as much pressure as the thin glass cover will stand 

 without breaking. Messrs. PoweU and Lealand have lately 

 constructed a much stronger instrument than that represented 

 by fig. 92, and have made their object-plate of a thick piece 

 of parallel glass, raised as much as the one-eighth of an inch 

 above the bottom plate, so that it can be cleaned without 

 much trouble ; the ring containing the glass cover is also made 



