2 THE VOYAGE OF H.M.S. CHALLENGER. 



structed and placed against the ship's side, under the shade of the pilotage bridge, 

 as represented in Fig. 1 supplied by Staff - Commander Tizard, R.N. The change 

 was effected on April 17th, 1873, the thermometers being separated from the ship's 

 side by wooden battens about three inches in thickness, and the maximum and 

 minimum thermometers were hung from the same battens. The thermometers were 

 occasionally tested for error of zero points, in melting ice, and further with a 

 standard thermometer by Geisler. 



The relative humidity was deduced from the dry and wet bulb readings by 

 Glaisher's Hygrometrical Tables, sixth edition. 



The temperature of the surface of the sea was obtained by drawing a bucket of 

 water from over the side of the ship, and immersing a thermometer in it, care being 

 taken that the water was obtained from a position sufficiently far forward to be clear 

 of the discharge-pipes from the engines, etc. When in the region of ocean stream cur- 

 rents, the temperature of the surface water was taken at hourly or half-hourly intervals. 



Atmospheric Pressure.- — The barometer was suspended on the main deck, just 

 outside the captain's cabin, the cistern being 9^ feet above the level of the sea. The 

 same barometer, which was used throughout, was compared with the standard at Kew 

 before leaving and again on the return to England, and also with the barometers in the 

 Observatories at the Cape of Good Hope, and at Sydney, N.S.W., its error remaining 

 constant throughout the voyage. The readings have been corrected for all instrumental 

 errors, and reduced to 32° and sea level. No correction for errors of observation were 

 made in the observations printed in the " Narrative," but in this discussion of the 

 results all evident errors of observation have been rectified. 



Wind and Weather. — In accordance with the practice on board ship, the compass 

 direction of the wind was recorded at the time of observation, but in the printed table 

 its direction has been referred to the true meridian. In describing the force of the 

 wind and state of the weather, the notation proposed by Admiral Beaufort was employed. 

 This notation is explained in all elementary works on meteorology. 



Clouds. — The amount recorded was in all cases estimated, 10 indicating a sky 

 completely overcast and a clear sky. Both the upper and lower clouds were 

 recorded, and the notation of the species was that generally in use, being essentially 

 Howard's classification. 



Currents. — The direction and rate of the current was derived from the differences 

 in the position of the ship at noon determined by astronomical observation and that 

 determined by courses steered and distances run by patent log from the astronomical 

 position of the preceding noon. Specific observations on the direction and rate of the 

 surface currents were frequently made during the voyage, either from a boat fast to the 

 trawl rope, or when the ship was kept stationary for sounding purposes, occasionally by 

 comparing the distances shown by patent log with the distances made good over the 



