JoLY — On the Reading of Meteorological Instruments. 159 



ment of contact will, by this means, be known to the observer ; 

 and if, further, the position of the movable wire, when started on 

 its downward course, was known to him, a record of the number of 

 contacts will give him the reading of the thermometer or other 

 instrument. 



In Plate III. a plan and sectional elevation of the necessary 

 mechanical arrangements are given. 



The vertical shaft or spindle, s, shown in section in the plan, is 

 pivoted at each extremity, so as to turn freely on its axes. It 

 carries for a portion of its length a screw thread, which, as shown, 

 has a pitch of ^o^^ of an inch. It also carries a small wheel, 

 notched with triangular teeth, twenty in number. A still smaller 

 wheel, IV (elevation), gears with the cogged wheel, tv, so that, as 

 will be readily understood from the figure, the spindle s, in 

 turning on its axes, either moves or is moved by the weight, P, 

 contained in the hollow pillar supporting the apparatus. 



A nut carrying a horizontal arm, /, bites on the screw. This 

 arm is held from rotating with the screw by the vertical guides o, o. 

 The rotation of the screw, therefore, imparts a vertical motion to 

 the limb /. The fine platinum wire, p (elevation), hangs loosely 

 from the extremity of /; it is, in fact, retained from falling 

 through an aperture provided for it by a small enlargement or 

 stop. A thermometer, ^, is so placed, that vertical motion im- 

 parted to /raises or lowers this wire in its tube, which, to that end, 

 is left open. 



Two pairs of electro-magnets are shown in plan. One of these, 

 e, e, serves to impart motion to the triangular-toothed wheel, and 

 hence [to the screw, which it turns in such a direction as to loioer 

 the platinum wire into the tube of the thermometer. The other 

 pair, e', e\ serves to release the wheel from the control of the 

 pallet, g, so that the descent of the weight P [raised at the same 

 time that / was being urged downwards) may restore the arm / 

 and wire p to their highest positions. The return wires from both 

 electro-magnets are put to earth through a small resistance. An 

 adjustable and insulated limb, for contact with the wire p when in 

 its highest position, is shown at n (elevation). A single wire from 

 the home station branches -to this and to the mercury in the ther- 

 mometer. Two more wires are needed — one to e, e, another to /, e^ 

 A reading is effected as follows : — 



