48 THE WATER 



The reversing frame is actuated by a propeller or a 

 messenger, and should be made to carry two ther- 

 mometers and to fit anywhere on the wire, so that a 

 string of instruments can be used simultaneously. The 

 propeller is somewhat unreliable, and not infrequently 

 fails altogether. It is fixed on a finely threaded screw, 

 and is so arranged that on the way down it revolves 

 till it butts against a stop. On hauling up, the direc- 

 tion of rotation is reversed, and the screw releases a 

 trigger which permits the frame to fall over. The 

 distance through which the frame must be hauled up 

 before reversal takes place is uncertain ; this is of no 

 consequence in great depths where the temperature 

 changes slowly, but it vitiates the results in the 

 upper highly-heated layers. The messenger is much 

 more reliable, and its sole defect is the time required 

 for it to fall through great depths. In such a case 

 the shock of its striking the frame cannot be felt, 

 and a suitable interval must be allowed, which can be 

 determined in shallower depths. If a string of ther- 

 mometers is used, a fitting is provided by means of 

 which each frame, as it reverses, releases a messenger 

 hanging below it, which actuates the next frame. 



Reversing water-bottles are made so as to carry 

 thermometers and at the same time to collect a sample 

 of water from any depth. The only form of which 

 the writer has had practical experience* consists 

 essentially of a brass tube, tinned inside, and mounted 

 inside a rectangular brass frame on two pivots. A 

 hinged lid with rubber washers is fitted to each end, 

 and these lids are also connected to eccentrically 

 pivoted arms. On reversal the arms cause the lids to 

 close firmly. Made to fit anywhere on the line, it is 

 specially adapted for serial observations and great 

 depths. 



* Ekman : Publ. de Circonstance, No. 23. 



