110 



well-lighted room free from sudden jars, direct sunshine or artificial light is desir- 

 able, and the apparatus should be protected by a suitable case of wood and glass from 

 air currents and the heat of the body of the observer. 



This case must be so arranged as to allow ready access to the micrometer screw 

 and ready removal if the different instruments, telescopes, levels, etc., are to be 

 put into position entire. Ordinarily, if the shelf is built into the wall, or if the 

 apparatus is mounted on a "pier," the case may have three sides of wood ;md the top 

 of glass and the left-hand end open for the introduction of the observer's hand when 

 using the micrometer screw, or at a suitable height above the apparatus a plate of ordi- 

 nary thick plate glass may be supported on brackets when in use, and when not in use 

 it may be buttoned up against the wall. Another plate can be arranged to be rai- 

 lowered to protect the apparatus from front and side currents whenever necessary. 

 The screw end must be left open for the operator's hand. If the apparatus is mounted 

 on a window sill the case had better be made wholly of wood, with open ghiss top. 



Some such protection as is above suggested is indispensable when sensitive levels 

 are to be tested. Of course, for the test of levels of low degrees of sensitiveness, such 

 care may not, in general, be necessary. 



Having secured a desirable place for the Level Trier, the next thing is to properly 

 mount it. The heavy cast iron base plate is provided, on the bottom, with three pro- 

 jecting studs which should rest securely on the stone pier when the base plate occupies 

 as near a level position as can be assured by the use of a common carpenter's level. 

 Now carefully place the bar upon the base plate so that the pivots shall rest in their 

 receptacles in the hubs on the right-hand end of the base plate, and the micrometer 

 screw upon its proper bearing at the left-hand end, and the apparatus is ready for use. 



The points of contact of the micrometer screw and the pivots should be kept free 

 from dirt, grit or rust. The resting place for the micrometer screw will sometimes be 

 made so that it can be raised or lowered by a screw arrangement so as to bring differ- 

 ent parts into use and prevent local wearing. This resting place can also be moved 

 excentrically and bring new points of bearing under the micrometer. 



.Mathematical Part. It is clear that the rotation of the micrometer screw 

 raises or lowers the bar of the apparatus. Any point on this bar, rotating about the two 

 pivots at the right-hand end, moves in the arc of circle whose radius is the distance 

 from the pivots to the screw point. Knowing the pitch of the micrometer screw and 

 its distance from the pivots, it is a simple matter to find how much arc is travelled by 

 moving the divided disc through one or more divisions : in other words, to find the 

 value of the angle of inclination subtended by this motion of the screw. 



Example. Suppose the length of arm 17.9 in., the pitch of the screw one-sixtieth 

 of an inch and the disc to be divided into 100 parts. Through how many seconds of arc 

 will the bar move when the micrometer screw is changed 30 divisions? 



There are 206,265 seconds in the arc whose length is equal the radius. 



Let x = the number of seconds required : 



* - 05 ,= 57.6" 



206,205 17.9' 



If we wish to know the radius of curvature of a level, it is necessary to measure the 

 run of the bubble when the level has been changed a certain amount in arc, and then 

 solve as follows : 



Example. Observed run of bubble for 30 seconds change in arc is found to be one 

 inch. Find radius of curvature of level. 



Let r = radius required : 



r 206,265 seconds 



. r- = ~ -5 == 6875.5 inches, 



1 inch 30 seconds 



or 572.96 ft. 



Method of making Tests. The level to be tested is placed in the wyes on 

 the bar, which is raised or lowered till the bubble is at one end of the scale, or, if 

 the level has no scale, up to the point which will be the limit of the run of the bubble 

 in practice. The micrometer disc is then turned over equal spaces and careful notes of 

 the run of the bubble are then taken. Having moved the bubble over its course it 

 should be moved in the opposite direction in the same manner and the whole operation 

 be repeated several times, and, with very sensitive levels, at differing temperatures, to 

 ensure accuracy in the results. 



