46 Meteorological Observations, [Jan. 



renheit). — This might be very easily done with the assis- 

 tance of Table 2,* which shows the alongation of the 

 mercurial column caused by different degrees of heat from 

 32«. to 112^ of Fahrenheit.! In the first column of the 

 table is the temperature ; in the line along the top is the 

 height of the mercury in inches, and where these two in- 

 tersect will be found the amount of expansion to be sub- 

 tracted. For example, suppose the barometer to be 28.235 

 and the attached thermometer 70.5. — Look in the top of 

 the table for 2S, and in the first column for 70, and where 

 these two intersect will be found the number 091, which 

 subtracted from 28.235 leaves 28.144 the height at which 

 the mercury would have stood at the reduced temperature 

 of 32^ 



In addition to the sudden variations to which the baro- 

 meter is liable, it also undergoes regular periodical fluc- 

 tuations, in all countries, but especially within the tropics. 

 — It rises and fails twice in the 24 hours ; and it has been 

 ascertained, from a great many observations, that its two 

 highest points occur in India very nearly at the hours of 

 10 in the morning and evening and its two lowest at about 

 4 o'clock in the morning, and 5 o'Clock in the afternoon. 

 — As so early an hour as four o'Clock in the morning would 

 probably be inconvenient for most observers, I have substi- 

 tuted 5 o'clock in the accompanying form for a register, 

 which can hardly produce the slightest inaccuracy. The mean 

 of the four observations will be very nearly the true mean 

 of the whole day. 



CEthrioscope. — In clear and calm nights, the ground, if 

 freely exposed to the sky^ is always colder than the super- 

 incumbent air, owing to the radiation of heat constantly 

 going on from the surface of the earth, towards the heavens ; 

 which may be easily demonstrated by placing a thermome- 

 ter on the ground, and suspending another several feet 

 above it, when it will be found that the former indicates 

 a lower temperature than the latter. If we now screen the 

 ground by a thin awning of cloth, we check the radiation, 

 and its te mperature rises. It is to this te rrestrial radiation 



* I am indebted for this table to Mr. J. G. Taylor, Astronomer to Gk>^^ 

 meat. 



t As ail English barometers have the scale of Fahronhiet to their attached 

 thermometers, i have adopted it lu this instance, in preference to the centigrade 



