Mr. Galbraith on the Velocity qfSminJ. Ill 



nearly with his determination, which is 1107*O5 at 29*79 ba- 

 rometer and ^S^'GS Fahrenheit. 



If Mr, John Farey's number 1 1 262 representing the times 



mercury is heavier than air be taken, then v = 202*88 V p 

 = 1 107-32, agreeing almost exactly with experiment. 



Now, since in general terms v =■ n V p, the variation of v 

 for a change of pressure is as v/ jy — \/ p' (4) 



Supposing J) = 30 inches, and p' = 29 inches, then on that 



account Iv = n{V p — V p' ), or Su = w x 0'0921 x 8^ 

 nearly ; and for one inch, Iv = 0-0921 7i. But n = 202-88 ; 



therefore 5u = 0-0921 x 202-88 = 18*8 feet, (5) for one 



inch of the barometer, at about its mean state. 



Again, since according to the continental mathematicians 



V = c \/ I +0-00375^, t being the temperature b}' the centi- 

 grade thermometer above zero ; 



This becomes by Fahr. above 32=, v = c \/ I +0-002083/ 

 or V = c (1+0-001042/) = c + 0-001042 c/. 



But c = 333-44 metres, of 3-2809 feet each, or 1094 feet. 



Hence x> = 1094 + 1-14 /; and therefore the variation de- 

 pending upon the change of temperature is 1-14 / ... . (6)* 

 of Fahrenheit's scale. 



It is now only necessary to determine the variation for the 

 state of the atmosphere depending upon dryness and moisture, 

 as indicated by a hygrometer constructed upon proper prin- 

 ciples. This cannot easily and accurately be done, as we have 

 no experiments in which it is marked, except Mr. Golding- 

 ham's. 



Making the foi-egoing allowances for the barometer and 

 thermometer, which may be presumed to be nearly correct, 

 we may ascertain what his hygrometric variation is, accord- 

 ing to his own scale; though this is not comparable with others, 

 for want of a knowledge of the extent and fixed points of his 

 scale. 



• This is greater than Dr. Gregory's from experiment, as may be shown 

 thus, the barometric pressure being constant : 

 o inch. 



Then 27 Bar. 29-82 Vel. 1094-2 oi.n 



6G 29-82 1116-1 Therefore :l^= 0-5G1 foot. 



39 -00 21-9 



The Doctor, for want of a proper instrument, tlid not make any al- 

 lowance for the effect of moisture; therefore this may be somewhat er- 

 roneous on that account. We hope, however, that he will resume the 

 suiiject, properly [)rovided with such an instrument. 



Comparing 



