7G EEPOKT OF COMMISSIONER OF FISH AND FISHERIES. [74] 



wholly filled with a mixture of creosote and watery the opposite limb 

 above the mercury is partially filled with the same mixture, the remain- 

 ing space therein being occupied by compressed air. In the mixture, 

 on each side, is a steel index having a horse-hair tied around it near the 

 upper extremity. The ends of the elastic horse-hair, being held in a 

 pendant position by the inner walls of the tube, exert enough pressure 

 to oppose a frictional resistance to a movement of the index in eleva- 

 tion or depression. As thus described, the instrument is a self-register- 

 ing maximum and minimum thermometer for ordinary use. The indica- 

 tions are given by the expansion and contraction of the creosote and 

 . water mixture in the large full bulb. 



" The instrument is set by bringing the lower end of the indices la 

 contact with the mercury by means of a magnet provided for the purpose. 

 Then, when the instrument is submitted to a higher temperature, the 

 expansion of the mixture in the large bulb depresses the column of mer- 

 cury on that side, and correspondingly elevates it on the other side. 

 A decrease of temperature contracts the mixture in the large bulb, and 

 by the elastic force of the compressed air in the smaller bulb, a trans- 

 ference of the column of mercury takes place in precisely the reverse 

 manner to that which occurs on a rising temperature. Thus the mer- 

 cury rises in the left limb for a lower, and in the right limb for a higher, 

 temperature. The greater the change of temperature the higher the 

 point reached in the respective limbs ; hence the scale on the left is 

 graduated from the top downwards, and that on the right from the 

 bottom upwards. The rising of the mercury in either limb carries with 

 it the index of that limb, and on the retreat of the mercury the index 

 remains at the highest point attained. The bottom of the index, being 

 the part which has been in contact with the mercury, gives the point 

 at which to take the reading." 



The large bulb of this thermometer is now i)rotected from pressure by 

 a glass shield which surrounds it; the space between the shield and bulb 

 is nearly filled with alcohol, which acts as a transmitting medium for 

 temperature performing the same function as the mercury in the shield 

 of the Negretti & Zambra thermometer. The shield above mentioned 

 has added much to the value of the instrument, as it has practically 

 eliminated errors arising from varying pressures. This thermometer 

 has been considered the standard for deep-sea work, and when several 

 were to be sent down to great depths on the same line it was unrivaled 

 until the jd resent imi)rovements in the methods of capsizing the ^>egretti 

 & Zambra thermometers were introduced. 



It is not as sensitive as the Negretti & Zambra, but under the above 

 conditions a delay of a few minutes is not of great importance. The 

 movable indices are a fruitful source of annoyance and vexatious delay. 

 An index may, without an apparent cause, absolutely refuse to move 

 in the tube; coaxing with the magnet is followed by lightly tapping the 



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