aie grown in quantity. In fig. 19 is represented a convenient hygro- 

 meter from Messrs. Negretii & Zambra, Holborn, but there are 

 several other forms, some being constructed with a mechanical 

 arrangement and dial. One of these, which ia termed a self-recording 

 hygrometer, is shown in the woodcut, fig. 21. In this " the two 

 thermometers with the water cistern are mounted upon a frame in 

 connection with the clock. An index hand on the dial of the clock 

 enables the observer to arrange the release of a catch or detent at 

 any moment, and by the action of the clock cause the hygrometer to 

 revolve on its axis. By this movement the mercury contained in the 



Pig. 19. 



Fig. 20. 



left-hand tubes of the thermometer breaks off and passes over into 

 the right hand tubes, where it records the temperature." This is a 

 very elaborate instrument, but the simpler forms answer the same 

 purpose. A cheap useful thermometer for registering the lowest 

 temperatures is represented in fig. 20, and cansists of zinc scale 

 enclosing the glass tube, containing a black glass index floating in 

 the spirit. 



Deciduous terrestrial Orchids require a decided period of rest, 

 during which water is either entirely withheld or is given in very 

 small quantities. Evergreen pseudo-bulbous Orchids must never be 



