200 On Constructing Geological Models. 



first of these corrections is easily made, the level being furnished 

 with screws at each end, by which it can be gradually raised 

 or lowered, so that however turned the bubble may rest steadily 

 in the middle of the tube ; but the second and not less im- 

 portant adjustment is a little more troublesome. It will be 

 easily understood that though the glass tube itself may be 

 quite correct, the horizontal hair in the telescope may be 

 either higher or lower than the true level, so that all the 

 readings taken on the levelling staff would be in error ; and 

 yet, however much this line may be out of adjustment, it is 

 possible, by taking the elevation of two points equally distant 

 from the level, to ascertain two exactly horizontal positions by 

 which to correct the position of this hair. Accordingly, upon 

 any tolerably even piece of ground, measure off about four 

 chains, and set up the level in the centre of the line thus 

 formed, and adjust it ; then make an observation of the level- 

 ing staff (a piece of wood fourteen feet long, divided into feet, 

 tenths, and hundredths of feet), placed at one end of the line, 

 and again an observation of it at the other ; one of these read- 

 ings subtracted from the other will give the true difference of 

 level of the two spots upon which the staff has been placed ; 

 now remove the level and erect it over one of these spots, and 

 measure upon the staff the exact height of the eye-piece, 

 which is of course the exact height of the cross hair inside, 

 then by either adding or subtracting the difference previously 

 procured, according as the further point is lower or higher 

 than that at which we now are, we get what the telescope 

 should read if the cross hair were in its proper place ; if it is 

 not, it can readily be corrected by the screws which will be 

 found on the telescope tube near the eye-piece. 



Our instrument being now ready for use, we proceed to 

 take with it what are called contour lines, or lines of equal 

 altitude, all over the country of which the model is to be made. 

 To do this, a map of the district is first procured : a tracing on 

 prepared linen of the Tithe Commutation maps, which are drawn 

 to the scale of six chains to the inch, answers extremely well. 

 If such maps cannot be obtained, we must either be content 

 with the Ordnance maps on a scale of one inch to a mile, or we 

 must measure with a chain from each position to the next, 

 where we take observations of the levelling staff, and also take 

 the angles from one point to another by a compass or sextant. 

 Not having myself been driven to these latter resources, I 

 shall not dwell upon them now. Our field apparatus will now 

 be complete with a levelling book, which is ruled in the follow- 

 ing way : — 



