[25] CONSTRUCTION AND WORK OF FISH-HAWK. 27 



The following description is from Sigsbee's "Deep-sea Sounding and 

 Dredging:" 



"A glass tube bent in the form of U is fastened to a vulcanite frame, 

 and to the latter are screwed white glass slabs containing the graduated 

 scales. Each limb of the tube terminates in a bulb. A column of mer- 

 cury occupies the bend and a part of the capillary tube of each limb. 



"The large bulb and its corresponding limb, above the mercury, are 

 wholly filled with a mixture of creosote and water; the opposite liml). 

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

 ing space therein being occupied by comjiressed 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 

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

 to oppose a frictional resistance to a movement of the index in elevation 

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

 maximum and miniinum thermometer for ordinary use. The indications 

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

 mixture in tlie large, full bulb. The instrument is set by bringing the 

 lower ends of the indices in contact with the mercury by means of a 

 magnet provided for the i)urpose. Then, when.the instrument is sub- 

 mitted to a higher temperature, the expansion of the mixture in the 

 large bulb depresses the column of mercury on that side, and corre- 

 spondingly 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 transference of the column of mer- 

 cury 1 akes place in precisely the reverse manner to that which occurs 

 on a rising temperature. Thus the mercury 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 resijective limbs; hence, the scale on the left is graduated from 

 the top downwards, and that on the right from the bottom uj^wards. 

 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 these instruments is now iirotected from pressure 

 by a glass shield, with which it is covered; the space between shield 

 and bulb is nearly filled with alcohol, which acts as a transmitting me- 

 dium for temperature, performing the same iunction as the mercury in 

 the shield of the Xegretti & 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 where several are to be sent down on the same line, particularly 

 to great depths, it is unrivaled. It is not as sensitive as the Xegretti 



