42 Meteorological Observations, [Jan. 



electricity^ wind, moisture, and dryness of the air, rain, 

 evaporation. To detect, and to measure these, the follow- 

 ing instruments are employed : the thermometer, barometer, 

 CEthrioscope, photometer, electroscope, hygrometer, ombro- 

 meter or rain gage, atmometer. These I shall now de- 

 scribe; but must premise that all of them, with the excep- 

 tion of the oethrioscope, photometer, and ombrometer, ought 

 to be placed in a situation where they will be completely 

 shaded from direct, or reflected light, or heat, while, at the 

 same time, they are thoroughly exposed to the wind. The 

 best situation, therefore, would be the centre of a room hav- 

 ing a thatched roof, surrounded, on all sides, with Venetian 

 Minds. It is not uncommon to place a thermometer against 

 a wall, which, I need scarcely remark, is very objectionable, 

 for the temperature of the wall, rather than that of the air, 

 is thereby indicated. 



Thermometer.— The construction of this simple and well 

 known instrument requires no description, but as its scale 

 is not always graduaded in the same way, it will be neces- 

 sary to explain the divisions most generally in use. The 

 scale usually employed in England is that of Fahrenheit, 

 upon which, the space between the freezing and boiling 

 points of water consists of 180 degrees, and zero is placed 

 32 degrees below the former ; so that the freezing point of 

 water is 32° and the boiling point 21 2<^. The scale best 

 adapted for philosophical purposes is the centigrade, on 

 which the freezing point is marked 0, and the boiling 

 point 100; and all temperatures below the freezing point 

 have therefore the sign minus. This will be found most 

 convenient for meteorological observations, and I have 

 therefore adopted it in the accompanying register and ta- 

 bles. The degrees of the centigrade scale may be con- 

 verted into those of Fahrenheit, by the following simple 

 rule. Double the degrees of the centigrade, subtract one 

 fifth and add 32, Thus 20 of the centigrade is equivalent 

 to 68 of Fahrenheit, for twice 20=40, subtract one fifth 

 or 4=36-1-32=68. To reduce the degrees of Fahrenheit to 

 those of the centigrade, subtract 32, multiply by 5, and 

 divide by 9. Thus 68—32=36, which multiplied by 5 = 

 180, divided by 9=20. To facilitate these reductions 

 table No. 1 will be found useful. 



