86 



THE VOYAGE OF H.M.S. CHALLENGER. 



work, and generally known as the Miller- Casella thermometer. It is represented in fig. 27, 

 and the copper case in which it is enclosed when sent down is shown, on a smaller scale, 

 in fig. 28. The instrument is of small size (9 inches in length), to reduce, as far as 

 possible, the friction in passing through the water. The tube is mounted in ebonite, 

 and the scale (Fahrenheit's) is engraved on slips of glass which are fixed to the ebonite 

 alongside the capillary tube of the instrument. The primary bulb of the thermometer 

 is enclosed in a secondary one, and the space between them partially 

 filled with spirit. The thermometer is filled with a solution of creosote 

 in spirit. The capillary portion is bent in the form of a U, and 

 the bend is filled with mercury ; the limb furthest 

 from the bulb ends in a cylindrical reservoir, 

 partially filled with the thermometric liquid, but 

 with a large space empty, or rather containing 

 the vapour of the liquid and slightly compressed 

 air. A small piece of steel wire enclosed in a 

 very thin glass tube forms the index ; it retains 

 its place in any part of the tube by the spring 

 of a hair tied on one end of it. Each limb 

 carries an index of this kind. When the 

 thermometer is to be used, the indices are 

 drawn down in each limb of the tube by a 

 strong magnet till they rest on the surface of 

 the mercury on each side. When the ther- 

 mometer is brought up, the height at which the 

 lower end of the index stands in each tube indi- 

 cates 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. 



During the course of the voyage, it became evident that the ther- 

 mometers as supplied were wanting both in delicacy and in accuracy. 

 It is true that the great source of error had been removed by the application of the 

 secondary bulb, so that the indications were practically unaffected by pressure, but when 

 it had been found that the great bulk of the ocean water is at a low and nearly uniform 

 temperature at great depths, it became of importance to be able to distinguish accurately 

 fractions of a degree. With the thermometers supplied this was impossible, because 

 they were so short for the range of temperature they had to show, that the length 

 occupied by one degree could not easily have been subdivided beyond a quarter, even if 

 the scale had been engraved on the stem, and it was impossible to attain even that degree 

 of accuracy with certainty when the scale was on a slip of glass at the side of the stem, and 



Fig. 28. —Case for enclos- 

 ing Six's (Miller-Casella) 

 Thermometer. 



Fig. 27.— Six's Deep-Sea 

 Thermometer. 



