64 NEGRETTI AND ZAMBRA, HOLBORN VIADUCT, E.G., 



DESCENDING. ASCENDING. 



FIG. 72. 



FIG. 71. 



FIG. 73. 



ward, then the mercury takes the temperature just as an ordinary thermometer. When at 

 any time or at any place the temperature is required, all that has to be done is to turn the 

 thermometer bulb upward, and keep it in this position until read off. 



" The reading may be taken at any time after, for the quantity of mercury in the lower 

 part of the stem which gives the reading is too small to be sensibly influenced by a change 

 of temperature, unless it is very great, while that in the bulb will continue to contract with 

 greater cold and to expand with greater heat, and in the latter case some mercury will pass 

 the contraction A, and may fall down and lodge at B, but it cannot go further so long as the 

 bulb is upward, and thus the temperature to be read off will not be vitiated. Now, when- 

 ever the thermometer can be handled, it can readily be turned bulb upward for reading off 

 the existing temperature. At a depth in the sea, some contrivances must be provided for 

 turning the thermometer bulb upward. For this purpose the thermometer is fitted into 

 a hollow wooden frame, loaded with shot, free to move from end to end of it, and sufficient 

 to render the whole instrument just vertically buoyant in sea-water. 



" In using the thermometer a cord is rove through the hole in the frame nearest the 

 bulb, and the instrument is fastened by this cord to the sounding line. In descending the 

 thermometer will be pulled down with the bulb downwards ; but upon being pulled up, the 

 instrument, owing to the resistance through the water, and consequent displacement of its 

 centre of gravity, will turn over and come up bulb uppermost, the temperature of the spot 

 where it turned over will then be indicated, as shown in the illustrations. See figures 

 72 and 73. 



" As regards the thermometer itself it was necessary, in order to make it perfectly 

 satisfactory, to protect it against pressure, even if intended for shallow seas, as well as for 



