204 



Instructions for Making and Registering 



[Jan. 



of the Cape, and in many other parts of the continent, hot winds fre- 

 quently set in with great suddenness, often in the night, and singular 

 alternations of hot and cold temperature, occur, disturbing the regular 

 laws of the diurnal fluctuation, and connected, doubtless, with many in- 

 teresting meteorological phenomena peculiar to the climate of South 

 Africa. 



Of the Maximum and Minimum, or Self -registering Thermometer. — 

 This should be placed horizontally in someplace out of doors, shaded 

 from direct radiation and rain, and otherwise freely exposed to air, and 

 so fastened as to allow of one end being detached from the fastening 

 and lifted up, so as to let the indexes within the boxes slide down to the 

 ends of the fluid 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 frequent- 

 ly, and occasionally double minima occur, in consequence of sudden 

 changes of temperature, it is recommended occasionally to inspect both 

 of them, with a view to ascertain whether the motion of either the mer- 

 cury 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 apttoget out of order, 

 by the indexes becoming entangled in the column of fluid. In travel- 

 ling they should not for a moment be carried with the mercury bulb 

 downwards ; if this should happen, they are sure to arrive in a state 

 unfit for use. To correct them is tedious, and always hazards fracture. 

 With great care, however, it may be done, as follows : — 



The Spirit Thermometer. By many jerks, force the index down 

 to the junction of the bulb and tube ; then, by cautiously heating and 

 cooling alternately the bulb, the 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 Mercurial Thermometer. When the steel index gets im- 

 mersed 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 either, 

 if necessary) till the mercury is either fairly drawn down below the in- 

 dex, or a separation takes place in the column, leaving the index with 

 mercury above it. Endeavour then, by tapping, warming the tube, or 



* The spirit thermometer is apt to undergo a gradual change of zero by the transfer 

 (by distillation) of part of its spirit to the upper end of the tube. It should, there- 

 fore, often be compared with the mercurial one, and the difference of readings ap- 

 plied as a zero. In this only case is the application of a zero be/ore registering 

 permissible, and indeed, essential. , 



