DEEF SEA THERMOMETERS. 7-27 



contracts in the bulb, the mercury rises in this arm of the tube, 

 which is graduated from above downward, but falls in the 

 other arm. When* the thermometer is going to be used, the 

 steel indices are drawn down in each limb of the tube, by a 

 strong magnet, till they rest, in each arm, upon the surface of 

 the mercury. When the thermometer is drawn up from deep 

 water, the height at which the lower end of the index stands 

 in each tube indicates the limit to which the index has been 

 driven by the mercury, the extreme of heat or cold to which 

 the instrument has been exposed. Unfortunately, the accuracy 

 of the ordinary Six's thermometer cannot be depended upon 

 beyond a very limited depth, for the glass bulb which contains 

 the expanding fluid yields to the pressure of the water, and 

 compressing the contained fluid, gives an indication higher 

 than is due to temperature alone. This cause of error is not 

 constant, since the amount to which the bulb is compressed 

 depends upon the thickness and quality of the glass. Yet, as 

 in thoroughly well-made thermometers, the error from pressure 

 is pretty constant, it has been proposed to make a scale, from 

 an extended series of observations, which might be used to 

 correct the observations, and thus closely approximate the 

 truth. 



A better plan has been proposed, and being practically ap- 

 plied, has been found to work very well. This consists in 

 incasing the full bulb in an outer covering of glass, so that 

 there shall be a coating of air between the bulb and the outside 

 coating, and that this air being compressed by the pressure of 

 the water outside, shall thus protect the inside bulb. Observa- 

 tions taken in 1869 with thermometers constructed in this 

 way, as deep as 2,435 fathoms, in no instance gave the least 

 reason to doubt their accuracy. A modification of the metallic 

 thermometer, invented by Mr. Joseph Saxton, of the United 

 States office of weights and measures, for the use of the coas 

 survey, may be thus described. A ribbon of platinum and one 



