330 Instructions for Making and Reyistering 



detached from the fastening and lified up, so as to let the 

 indexes within the boxes slide down to tlie ends of the flnid 

 columns, a more convenient mode, when the steel index is free 

 enough to allow it, than the use of a magnet. 



Both the thermometers should be read off as early as possible 

 every morning, and the indexes re-adjusted. But as double 

 maxima frequently, and occasionally double minima occur, In 

 consequence of sudden changes of temperature, il is recom- 

 mended occasionally to inspect both oi them, with a view to 

 ascertain whether the motion of either the mercury or spirit has 

 been reversed in an unusual manner, and such double maxima 

 or minima, when remarkable, should be recorded as " super- 

 numerary," with their dates and leading features. 



The Self-registering Thermometer is extremely apt to get out 

 of order, by the indexes becoming entangled in the column of 

 fluid. In travelling they should not for a moment be carried 

 with the mercury bulb downwaids — if this should happen, they 

 are iwe to arrive in a state unfit for use. To correct them is 

 tedious, and always hazards fracture. With great care however 

 it maybe done, as follon-s: — 



Ist. The Spirit Thermometer. By many jerks force the in iex 

 down to the junction of the bulb and tube ; then by cautiously 

 heating and cooling alternately the bulb, tlie tube, or the air 

 vessel at the top, as the case may require, the disunited parts of 

 the spirit may be distilled from place to place, till the whole is 

 collected in one column in union with the spirit in the bulb. 



2d. The IMercurial Thermometer. When the steel index gets 

 immersed in the mercury it cannot be moved by a magnet, and 

 lets the mercury pass by its side. First cool the bulb (by 

 evaporation of ether, if necessary) till the mercury is either 

 fairly drawn down below the index, or a separation takes place 

 in the column, leaving the index with mercury above it. 

 Endeavour then by tapping, warming the tube, or by the 

 magnet, to loosen the index ever so little, then apply beat to 

 the bulb and drive up the index with its fcuperabundant 

 mercury quite into the air vessel. This requires many trials 

 and much patience. When there, hold the instrument bulb 

 downwards, and suspend the index by a ma^rnet at the top, 

 allowing any globule of mi>rcury to drop into the origin of the 

 tube below, then heat the bulb cautiously over a very small 

 clear flame of an oil lamp till the mercury rises to the very top 

 of the tube and fairly unites with the jrlobule there awaiting it. 

 Let the bulb cool and the mercury will sink in one united 

 column ; if not, heat it again. When this is accomplished, the 

 index may be set loose, by withdrawing the magnet, and restored 

 to its proper position in the tube. 



A Self registering Thermometer may be advantageously left 



