Meteorological Observations, Z29 



apparent fluctuation less than the real, in n Jixed proportion, it 

 ought, if pos«il)le, to be ascertained. The data necessary lo he 

 known are — first, ihe internal and external diameters of tlie 

 tube- secondly, that of tlie cistern containing the ineicury, at 

 the surface, where the tube pluiiiies into it. These particulars, 

 as they must be known to the maker, ontjht to be inquired of 

 him, and indeed ought to be engraved conspicuously on some 

 part of the ii:strnment. 



Although all these corrections are necessary for the strict 

 red^Ktion of registered observations, they ou^ht not to bo 

 applied to individual observations previous to registry. It is 

 sufficient to know them. Their effect is in most cases easily 

 and safely applicable to mean results, and to the conclusions 

 therefrom deduced, and a world of troublesome and often 

 mistaken calculations may be saved by so applying them. 



Of the External Thermometer. 



Tlie External Thermometer should have a scale on which 

 whole degrees are read off, and divisions large enough to admit 

 of estimating tenths, or at least quarters of degrees, by the eye. 

 It should be compared with a standard, and the difference 

 stated, at one or more temperatures (the wider asunder the 

 better) within the range of the climate in which it is to be usrd. 

 In fixing it, choose a perfectly shaded but otherwise frt? 

 exposure, and one where no rtjiicled sunbeams from water, 

 buildings, rocks, or dry soil, can reach it : and easily acce. slble 

 for reading. There fix it firmly and upright. In reading it, 

 avoid touching, breathing on, or in any way warming it, by near 

 approach of the person. The quicker the reading is done the 

 better. 



Although read off at slated times, notice should be taken of 

 all sudden and remarkable changes ot temperature, as indicated 

 by the external ihermotreter, whenever they occur. In tlie 

 neighbourhood of tlic Ca])e, and in many other parts of the 

 continent, hot winds frequently 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 interesting 

 meteoroh>gical pha?nomcna peculiar to the climate of South 

 Africa. 



Of the Maximum and Minimum, or Self-register irig 

 Thermometer. 



This should be placed liorizontaliy in some place out of door«, 

 shaded from direct radiation and rain, and otherwise freely 

 expo&ed to air^ and ao fastened an to allow of oue aud bciu^ 



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