45, CORNHILL, B.C., AND 122, EEGENT STREET, W., LONDON. 



49 



the needle-point in contact with the mercury, and then withdraw the magnet, 

 having previously ascertained that no particles of mercury are attached to 

 the index. 



It may sometimes, though rarely, happen that, from the time a minimum 

 temperature is registered by the index, and the time an observation is made, 

 .the mercury may have risen so high in the tube as to completely pass the 

 index, as shown (fig. 3). Should it so happen, the space which the index 

 occupies will readily be observed, as it will be pressed to one side of the tube, 

 causing a different appearance in that part, although the point of the needle 

 may not be seen. If such be the case, apply a magnet to the spot where you 

 see the index is fixed : this will hold the needle firmly. Then, by slightly 

 tilting the thermometer bulb uppermost, the mercury will flow into the top 

 bulb, leaving the index attached to the magnet, and quite uncovered. Having 

 taken the reading, draw the needle into the top bulb, and hold ifc there whilst 

 you adjust the thermometer by again bringing it to the upright position. 



So \ 70 I 31 $9 





NEC PETTI & 2 AM BRA-, LONDON. 



FIG. 59. 



68. Negretti and Zambra's Patent Mercurial Minimum Thermo- 

 meter. The Patent Mercurial Minimum Thermometer is 

 constructed as follows : A is the thermometrical or indicating 

 tube, and B is a small vertical tube connected to it at right angles, 

 about one inch from the bulb. In the tube B, at the point C, 

 is inserted a platinum plug, which does not entirely fill the 

 bore, as may be seen by elevating either end of the instru- 

 ment, as the mercury will then flow in the tube A, either to or from the bulb, 

 depending upon which end of the thermometer is elevated or depressed. 



To set for Observation, and use the Patent Mercurial Minimum Thermometer. 

 Hold the thermometer with the bulb downwards until the bulb and tube B 

 are quite full of mercury ; then raise the bulb end of the thermometer, and 

 the mercury will flow from the tube B into the tube A, until it reaches the 

 plug C, where it will be checked by the mercury adhering to the platinum plug 

 the affinity of platinum for mercury being sufficient to arrest the flow of 

 mercury, if not allowed to flow too rapidly. Should it overshoot the mark and 

 go to the end of the tube A repeat the operation more carefully. 



Suspend the thermometer horizontally, and on a decrease of the tem- 

 perature the mercury will fall in the tube A until it attains its minimum 



