416 HYDROGRAPHICAL SURVEYING [chap, xix 



it is necessary to make experiments when the actual speed of 

 the current is known, by immersing the meter just below the 

 surface and taking careful observations of the surface current 

 by means of a current log or weighted pole. From the number 

 of revolutions registered by the meter in a certain number of 

 minutes, and taking the mean of several observations, a very 

 fair value for a revolution can be deduced. On every occasion 

 of using the meter for under-current observations, the value of a 

 revolution should be redetermined, as it is apt to vary owing 

 to small differences in the friction caused by want of oil or the 

 presence of dust or grit ; the force of the current is probably 

 another important factor in determining the value of a 

 revolution. 

 Rough In smooth water, a very fair approximation of the strength 



Estima- Qf g^jj under-current may be arrived at by the aid of a steam- 



tion of J J 



Under- boat and wire sounding macliine. The lead lowered to different 



Current, (jgp-tj^g being acted on by the under-current, whilst the surface 



current has little or no effect on the wire, the rate of the boat 



steaming to keep the wire up and down, as compared with the 



rate at which the surface current is running, will enable the 



under-current to be estimated. 



Tempera- The observations are made more complete by ascertaining 



Density, the temperature and density of the water at the depths 



experimented on. 



EXPLORING A RIVER. 



Running Narrow rivers, navigable for boats, will generally be suffi- 

 ^' ciently laid down on a marine chart by a sketch survey, made 

 from the boat (a steam pinnace, if possible), while passing up 

 and down. Patent log and compass will be the instruments 

 mainly used for putting down the direction and length of each 

 reach ; though if we have objects that we can use for a sextant 

 fix, we shall of course use them in preference, at any rate 

 from time to time. We must endeavour in every case to get 

 a good fix at our furthest point, and the course of the river, 

 as mapped by patent log and compass, will then be squared 

 in on that, and the fixed points at the entrance and any other 

 fixes we may have got. Any elevated points near, which we 

 can ascend and fix, and from them get angles to bends and 



