JoLY — On the Reading of Meteorological Instruments. 161 



In the latter case, if failure of proper contact between the wire 

 and the mercury occurred, the observer might damage the ther- 

 mometer by urging the wire below the surface of the mercury. 

 As arranged, the wire, under such circumstances, would merely 

 rest on the surface of the mercury, however far / was urged 

 downwards. 



The weight P might perhaps with advantage be dispensed 

 with in favour of a spring coiled in a drum fixed on iv. With 

 the dimensions shown, this spring would be subject to five turns 

 during the complete descent of the wire. 



This arrangement, being of a purely mechanical type, admits 

 of many variations. One form has been made for me by Mr. 

 Teates, in which the elevation of the wire is effected by rotating 

 the toothed wheel in the opposite direction by the second electro- 

 magnet. The contrivance for effecting this is due to Mr. Teates. 

 In this case the screw itself advances downwards or upwards, 

 carrying the platinum wire attached to its extremity. The screw 

 bites in a nut borne by the toothed wheel, but is held from turning 

 with it, being slotted longitudinally, a fixed pin projecting into 

 the slot. Such a form, although found to work very satisfactorily, 

 has the great disadvantage of requiring as many makes and breaks 

 of circuit to lift the wire out of the thermometer as to take the 

 reading in the first instance. The quick return in the present 

 arrangement does away with this objection.^ 



It will be noticed that three wires are needed to work the 

 instrument — one to each magnet — that is, one to lower the movable 

 wire, and one to raise it, a third being necessary to notify the 

 position of the wire, whether in contact with the starting-point 

 contact or with the mercury contact. Now, it will be obvious that 

 this third wire might be common to any number of instruments 

 centained in an observatory, and the second wire might be dis- 

 pensed with by the addition to each instrument of an arrangement 



1 The instrument constructed for me reads to the ^th of a degree Fahrenheit. 

 This delicacy is unnecessary, and has the disadvantage of making reading a slow and 

 tedious operation. Its certainty of action is all that could he desired. In the form 

 proposed above a delicacy of ith of a degree Centigrade is intended, each degree 

 causing a rise of about ^th of an inch in the mercury. 



A little glycerine is placed over the column of mercury in the thermometer, to 

 exclude air bubbles. 



