REPORT ON THE PRESSURE ERRORS OF THE THERMOMETERS. 7 



retain a record of the observation. They have also a little piece of needle inside, and can 

 thus be moved from the exterior by means of a horse-shoe magnet, so that the adjustment 

 can be made at pleasure, and without any alteration of the temperature. The thermo- 

 meters are plunged for some hours in the water in the press, and the indices are set in 

 an instant while the instrument is partially lifted out for the purpose. With the other 

 instruments one might spend days before he could get them introduced, except after 

 special cooling, into the press with the index suitably adjusted to the temperature of the 

 water. The whole difficulty might have been avoided by putting an exceedingly small 

 piece of iron or steel wire above the index, to be acted on by a sufficiently powerful 

 magnet. 



Thus, although these instruments are absolutely perfect so far as regards immunity 

 from pressure (and in other essential respects which will be mentioned later), it is not 

 easy to work with them under the circumstances of this investigation. 



Individual Peculiarities of some of the Challenger Thei-mometers. 



The Challenger thermometers are not all exactly similar to one another. Some of 

 them have their degrees very much longer than others ; others have the extraordinary 

 peculiarity that the degrees upon the maximum side are nearly half as long again as 

 those on the minimum side, and sometimes it is the reverse. In one of the instruments 

 which was occasionally used in the deep sea, the length of a single degree on the 

 maximum side is only about three-fourths of a millimetre, and thus a reading to a tenth 

 of a degree is not to be looked for. But on account of this unexpected peculiarity, this 

 particular instrument was of use, as will be seen later, in demonstrating that the effects 

 produced in the press were due partly to heating, partly to compression. Several 

 instances of useful peculiarities of a similar character were detected, and utilised. 



In fact, the instruments cannot be said to do more than furnish rough and ready 

 means of approximating to temperatures within about a quarter of a degree, or in the 

 most favourable circumstances a tenth of a degree Fahrenheit. Had they been more 

 nearly what would be called " scientific " instruments, they might have altogether failed 

 on account of the rough treatment to which they were necessarily subjected during use. 

 Letting them down into the sea presents in general no great difficulties, but when they 

 have to be hauled on board again they are subject to jerks and shocks, and sometimes 

 swing through large arcs at the end of the lead line. Such misadventures are unavoid- 

 able at sea, and are excessively unfavourable to accurate results, because the index is 

 necessarily not fitted so tightly in the stem that it may not in a few oscillations be 

 sensibly displaced. And there is a defect inseparable from the use of movable indices : 

 viz., that the reading of the mercury column is sensibly different according as the 



