SURVEYING THE ROUTE. 21 



particular instruments he used. These are given in the Paper 

 above referred to, which should be consulted for a more detailed 

 study of the subject. 



Mr. Buchanan improved on this method by the simultaneous 

 use of a mercury piezometer. In this instrument the upper 

 bulb is filled with mercury rising a portion of the way in the 

 tube. The bent portion of the tube and the downward leg 

 are filled with water, and the column of water rests on the 

 short mercury column which forms part of that in the lower 

 bulb. 



The action of this instrument is precisely the same, the mass 

 of mercury being contracted by the increase of pressure and 

 decrease of temperature, and the maximum contraction at the 

 bottom of the sea being registered. The mercury instrument 

 is much more afi"ected by temperature than pressure, and the 

 water instrument more by pressure than temperature. Conse- 

 quently the reading which is liable to error in one instrument 

 is susceptible of great accuracy in the other. 



Both instruments are sent down together on the sounding 

 line. The depth indicated by the line out (considered as an 

 approximate measurement only) is then taken and used in 

 clearing the reading on the mercury piezometer for pressure, 

 leaving an exceedingly close first approximation to the tempera- 

 ture. This result is then used to clear the reading on the 

 water piezometer for temperature, which then gives the true 

 depth. And again, this result is used to clear the mercury 

 instrument for pressure, which then gives the true temperature. 



In addition to the barometric method of Buchanan, a simple 

 form of pressure gauge, in which the pressure acts through an 

 elastic diaphragm, has been introduced by Messrs. Bucknill 

 and Casella. The instrument is enclosed in a strong water- 

 tight case and lowered in the ordinary way on the sounding 

 line about six feet above the lead sinker. The index on the 

 dial records the maximum depth in fathoms. The advantage 

 of instruments of this class is that under-currents do not cause 

 errors in the measurement, and there is no necessity to keep 

 the sounding line absolutely vertical. 



We now come to the measurement of ocean temperatures. 

 Thermometers for use in the sea at great depths require pro- 

 tection against the enormous pressure to which they are 



