60 



REPORT 1861. 



neck, within which are placed a flat elastic ring and an elastic stopper. When the 

 water in the gauge is compressed, the stopper and ring are pressed down the stem 

 towards the cylinder ; and when it expands, the elastic stopper is pressed back, the 

 elastic ring remaining as an index of compression. 



Some few precautions are necessary before use, viz. the gauge should be well 

 rinsed with boiled water for the purpose of preventing the adhesion of air to its 

 inner sm-face. It should then be filled, to the top of the stem, with sea-water 

 boiled to free it from air. 



The elastic ring should now be inserted, and then the stopper, with a vent or 

 small grooved needle at the side, to allow superfluous water to escape, and it should 

 be pressed down the stem until its lower edge and the point or zero-line marked 

 20()() are coincident. The grooved needle should then be withdrawn, and the 

 stopper will tightly fit the stem. The stopper should be slightly lubricated to pre- 

 vent excessive fiiction. 



On descending into water of greater density the water in the gauge is compressed 

 until equally dense, and the elastic stopper and elastic ring are pressed down the 

 stem towards the cylinder. On ascending to water of less density the water in the 

 gauge expands, and the stopper is pressed upwards, leaving the elastic ring behind. 



Upon regaining the surface after the experiment, the water in the gauge should 

 press the stopper nearly back to its former position on the zero-line, a small dif- 

 ference being caused by friction. 



The elastic ring marks the extreme compression at the greatest depth attained. 

 This depth should be determined by the sounding line, to which the instrumeut 

 should in these experiments be for some time attached. 



The volume of the water in the cylinder and stem is considered to be divided 

 into 2000 parts, of which the stem contains one-tenth or 200 parts ; these are num- 

 bered fr-om 1800 to 2000. 



Each part on the stem may be easily read to a tenth, or a 20,000th part of the 

 whole quantity. 



A compression of sea-water of one part in 20,000 is caused by a pressure of 

 15-8 lbs. avoirdupois per square inch, or a depth of 35,456 feet, or nearly six fathoms. 



The experiments of Mr. Canton and Mr. Johnson confirm this estimate of pres- 

 sure, so that it appears to afford a basis for the compilation of tables for the com- 

 parison of pressure and depth. 



Table of Variation in the Volume of Sea- Water, boiled to free it from Air, 

 with Change of Temperature. 



* A gentle motion kept up to equalize the temperature of the sea-water has prevented 

 its freezing at 28° o. 



