1837.] 



Meteorological Observations. 



203 



well, and with common care in this mode of transport, its zero runs no 

 risk of change. 



The next correction, and *,he most important of all, is that due to the 

 temperature of the Mercury in the Barometer tube at the time of obser- 

 vation. To obtain this, every Barometer requires to have attached to, 

 or fixed very near it, a Thermometer, called the attached Thermometer, 

 which must be read and registered at each observation of the Barometer. 

 It is preferable in practice to read off this Thermometer first, to avoid 

 the error arising from breathing on, or standing long near it, whil e 

 reading the Barometer itself. The zero of this Thermometer should be 

 ascertained by comparison with a standard at the temperature of about 

 60° Fahr. 



The third correction applicable to barometric observations arises from 

 change of level of the mercurial surface in the cistern, owing to the 

 transfer of a portion of its contents to or from the tube. In Barometers 

 with small cisterns, and where the lower level cannot be adjusted at 

 each observation, its amount may be large, and its effect being always 

 to make the apparent fluctuation less than the real, in afixed proportion, 

 it ought, if possible, to be ascertained. The data necessary to be known 

 are — first, the internal and external diameters of the tube — secondly, 

 that of the cistern containing the mercury, at the surface, where the 

 tube plunges into it. These particulars, as they must be known to the 

 maker, ought to be inquired of him, and indeed ought to be engraved 

 conspicuously on some part of the instrument. 



Although all these corrections are necessary for the strict reduction 

 of registered observations, they ought not to be applied to individual 

 observations previous to registry. It is sufficient to know them. Their 

 effect is in all 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. — The 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) with- 

 in the range of the climate in which it is to be used. In fixing it, choose 

 a perfectly shaded but otherwise free exposure, and one where no re- 

 flected sunbeams from water, buildings, rocks, or dry soil, can reach it : 

 and easily accessible 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 stated times, notice should be taken of all sud- 

 den and remarkable changes of temperature, as indicated by the 

 external thermometer, whenever they occur. In the neighbourhood 



