202 



METEOROLOGY. 



[Sect. IX. 



the scale, which is often required. At the 20th second 

 prepare to observe; at the 25th begin to count beats, 

 0, 1, 2, .... 20; and at the 20th beat, ^. e. at 2^^ 16^ 

 SO^ read off, and enter the reading in column 3, A, as 

 the initial shade reading (45'''2). Then wait, as before, 

 till nearly a minute has elapsed, and at 2^ 17^ 20^again 



At 17^^ 25' begin to count beats; at 17^^ 30' 



read off, and enter this terminal shade reading (42 '8) hi 

 column 3, B, and, if needed, withdraw the zero. 



A.gair wait 20% in which interval there is time for the 



entry, &c. At 17"^ 50^ remove the screen, or expose the 

 instrument in the sun ; at 55^ begin to count beats ; and 

 at the complete minute, 18"^ 0% read off (M'^'S), and so 

 on for several alternations, tailing care to begin and end 

 each series with a sun ohsei'vation^ and to read off the 

 internal thermometer at the end of each set, or, if the 

 observations be continuous, at every fifth sun observation. 

 If the instrument be held in the hand, care should be 

 taken not to change the inclination of its axis to the 

 horizon between the readings, or the compressibility of 

 the liquid by its ovfn weight will produce a very per- 

 ceptible amount of error. 



In the annexed form column 1 contains the times, 

 initial and terminal, of each sun and shade observation. 

 Column 2 expresses by an appropriate mark, and x ? 

 the exposure, whether in sun or shade. Column 3 

 contains the readings, initial and terminal (A and B). 



Column 4 gives the values of B — A, with its alge- 

 braical sign expressing the rise and fall per minute. 



tfulness 



for£re 



And here it may be observed; that if by 



the exact minute be passed, the reading off may be made 



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s 



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1 



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