206 



MEASURING APPARx\TUS OF THE 



ground, and 1st, the heads of these pickets may be left more or less above 

 the surface. 2nd. Upon the pickets a large triangular frame of three 

 inches thick deal is placed, which in cases of emergency may be dispensed 

 with or doubled. 3rd. The tressels may be of different heights, and do 

 actually vary from thirty to six inches ; again, each bar is supported upon 

 a brass fabric called a camel which rests upon the tressel, and is furnished 

 with three levelling screws, and one in the middle for elevating, therefore ; 



4th. The additional height given by 

 the screws is three inches. These 

 four means of adjustment may all be 

 brought into play in descending or 

 ascending a slope ; let us suppose the 

 former case : then we should begin 

 with the lowest tressel, with the pickets as near the surface of the ground 

 as possible, perhaps without any triangle over the heads of the pickets, 

 and with the elevating and levelling screws of the camel quite drawn down 

 whilst at the advanced end of the 6th bar, we should leave one inch or 

 more of the pickets exposed, the highest tressel, supported on perhaps 

 two triangles, would be used, and the elevating and levelling screws would 

 be drawn out to their full extent. 



The extreme difference of height which might thus be produced, 

 would be about thirty inches in a set (or sixty-three feet,) but in this as 

 in all other cases of human life, it is desirable to avoid having recurrence 

 to extremes, and the medio tutissimus is the best guide. 



\n I have said that each end of the frame of the bar rests on a brass 

 support called a camel. These have rollers at top so that the motion 

 of the w^hole longitudinally is easily effected with the hand, but that one 

 of each pair of camels which supports the rear end of the bar, has a brass 

 plate appended to it with a steel screw, whereby after the dot has, as nearly 

 as can be, been brought into its proper position under the microscope, any 



