503 REPOBT — 1867. 



§ 24. The guard-plate earrying, by the platinum-wire suspeusion, the alu- 

 minium balance, is attached to the bottom of a small glass Leyden jar (m m), 

 and is in permanent metallic communication with its inside coating of tinfoil. 

 The outside tinfoil- coating of this jar is in permanent metalHc communication 

 with the outside brass-protecting case. The upper open mouth of this case 

 is closed by a lid or roof, which bears on its underside a firm frame projecting 

 downwards. This frame has two V notches, in which a stout brass tube (o) 

 slides, kept in the Vs by a properly placed spring (p), giving it freedom to slide 

 up and down in one definite hue*. Firmly fixed m the upper end of this tube 

 is a nut («, fig. 8), which is caused to move up and down bj' a micrometer- 

 screw. The lower end of the shaft of this screw has attached to it a convex 

 piece of polished steel (b, fig. 8), which is pressed upon a horizontal agate- 

 plate rigidly attached to the framework above mentioned by a stiff' brass piece 

 projecting into the interior of the brass tube through a slot long cnoxigh to 

 allow the requisite range of motion. This arrangement will be readily under- 

 stood from the accompanying drawings. It has been designed upon obvious geo- 

 metrical principles, which have been hitherto neglected, so far as I know, in all 

 micrometer-screw mechanisms, whctlier for astronomical instruments or other 

 pui'poses. The screw-shaft is turned by a milled head, fixed to it at ics top 

 outside the roof of the instrument, and the angles through which it is turned 

 are read on a circle divided into 100 equal parts of the circumference (or 3°'(> 

 each) from a fixed mark on the roof of the instrument. The hole in the roof 

 through which the screw-shaft passes is wide enough to allow the shaft to 

 turn without touching it, and the lower edge of the graduated circle turning 

 with the screw is everywhere very near the upper side of the roof, but must 

 not touch it at any point. A second nut (c, fig. 8) above the eftcctive nut fits 

 easily, but somewhat accurately, in the hollow brass tube, but is prevented 

 from turning round in the tube by a proper projection and slot. Thus the 

 screw is rendered sufficiently steady, -with reference to the sliding-tube ; that 

 is to say, it is prevented from any biit excessively small rotations roimd axis 

 perpendicular to the length of the screw-shaft ; and when the nut is kept 

 from being turned round its proper axis, it forms along with the sliding-tube 

 virtually a rigid body. A carefully arranged spiral spring presses the two 

 nuts asunder, and so causes the vippcrsides of the thread of the screw-shaft 

 always to press against the underside of the thread of the eflfective nut, 

 thus doing away with what is teclmically called in mechanics " lost time."' 

 In turning the micrometer-screw, the operator presses its head gently down- 

 wards with his finger, to secure that its lower end bears firmly upon the agate- 

 plate. It would be the reverse of an improvememt to introduce a spring 

 attached to the roof of the instrument outside to press the screw head down- 

 wards, inasmuch as however smooth the top of the screw-shaft might be 

 made, and however smooth the spring pressing it down, there would still be 

 a very injm-ious friction impeding the proper settlement of the sHduig-tubc 

 into its Vs. A stiff fork (^) stretching over the graduated circle is firmly 

 attached to the roof outside, to prevent the screw from being lifted up by 

 more than a very small space ; perhaps not more than -J- of an inch at most. 

 In using the instriiment, the observer shoiild occasionally pull up the screw- 

 head and press it down again, and give it small horizontal motions, to make 



* In consequence of suggestions by Mr. Jenkin, it is probable tliat the spring may be 

 done away with, and the Vs replaced by rings approximately fitting round the tube, 'but 

 leaving it quite tree to fall down by its own weight. In consequence of the symmetrical 

 position of the convex end of the screw over the centre of the attracted disk, slight lateral 

 motions of the tube produce no sensible effect on the electric attraction. 



