HORIZONTAL SECTIONS. 65 



of the needle, the observer standing at the time on the 

 selected bench mark ; the instrument should have been 

 previously carried about the person for a few minutes, so 

 that the difference of temperature may take effect, if at 

 all, before and not after this adjustment. 



On arrival at each point where an observation should 

 be made, as the summit of a hill, the bottom of a valley, 

 the site of an exposed section, and so on, note is made of 

 the variation of the needle-point as a rise or fall of 

 pressure, if the instrument is graduated for pressure 

 only ; as so many feet higher or lower, if also for eleva- 

 tion. The indicator is not to be shifted at each station, 

 but the height of every one taken as so much above or 

 below the B. M. chosen for a starting-point as stated 

 before, these heights will not be correct if the atmo- 

 spheric pressure has changed in the meantime. Three 

 methods are given for the rectification of error arising 

 from this cause ; a and 6, being of the simpler kind, 

 will give fair results ; should greater accuracy be required, 

 c must be adopted. 



(a.) If the observer can return home along the route 

 by which he came out, and has the time at command for 

 making a second series of observations at each point, the 

 error arising from atmospheric change may be to some 

 extent eliminated. The indicator must remain in the 

 same position as at starting, and the heights on return- 

 ing be again taken in reference to the primary bench 

 mark. For example, suppose four points on a line, at 

 which the two series of observations are as given below 

 (the sign + indicating that the instrument registers so 

 many feet above the starting-point, when below the sign 

 being prefixed). 



5 



