44 Meteorological Ohservatlons. [Jaist, 



In addition to these, the maximum and minimum ought 

 always to be noted ; which are most easily obtained by 

 means of the self registering thennometer. lliis consistis 

 of two thermometers, one filled with mercury, the other 

 with spirits of wine. The former has a small index of steel, 

 which is pushed forward by the mercury, and is left at the 

 highest point — the latter contains a small index of glass, 

 which floats in the spirits, is dragged back by them, when 

 they contract, and is left at the lowest point. The Instru- 

 ment is set, merely by inverting it slightly. It may be 

 observed and set every morning at 10 o' Clock, and the 

 mean of the extremes noted, for the purpose of being com- 

 pared with the mean of 10 a. m. and 10 p. m. 



The Barometer measures the weight or pressure of the. 

 superincumbent atmosphere. 



If a glass tube a. h. (Fig. 1.) closed at one end, and 

 about 40 inches in length, be filled with mercury, and then 

 inverted perpendicularly in a basin of mercury c, so that 

 the open end may be under the surface, the mercury will 

 sink down from the upper extremity and stand at a 

 certain point d, which will vary with every alteration in the 

 pressure of the atmosphere. This is a simple form of th« 

 barometer. The mercury in the tube being supported by 

 the weight of the atmosphere pressing upon the mercury 

 m the basin, must evidently rise or fall, according as the 

 pressure of the air increases or diminishes. The weight 

 of the atmosphere, at the level of the sea, is generally equal 

 to that of a column of mercury of the same base, and of 

 the height of about 30 inches. It constantly varies, how- 

 ever, and to a much greater extent in cold and temperate 

 regions, than within the tropics. Variations in the baro- 

 meter are often connected with changes of weather ; and 

 persons who have been accustomed to observe it carefully, 

 in certain situations, are often enabled, by means of its in- 

 dications, to predict an approaching stomi. But this is 

 seldom the casein intertropical countries, where the rise 

 or fall of the mercury scarcely ever amounts to more than 

 a few hundredths of an inch, in the course of a day. 



To measure the variations of the barometer a scale of 

 inches and tenths, with a vernier, is placed parallel to the 

 tube, and is reckoned from the surface of the mercury in 



