50 



REPORT — 1869. 



As above, 



yn= /P ffW^ /P ff^ 29;92 

 V 2 w" \/ 2 w p 



28-55. 



In cases where it is necessary to give the velocity in feet of air at a pressure of 

 29'92 inches, 



?' _ h 



-s/' 



2y'92 



^29i><^^'''- 



The complete formula, embodying the forniulro of correction for variations of 

 temperature, and also of barometric pressure, would therefore bo 



v=,^, 



h 519 



29-92 ■ 459+< 



X 28-65, 



V being the velocity of air at a temperature of t degrees Fahr., under a pressure of 

 h inches of mercury ; but the velocity is measiu-ed in feet per second of air at the 

 normal temperature and pressure. 



When drawing a sample of air from a chimney or iire-flue in order to examine it, 

 that sample is measured, by the aspirator employed, under the existing barometric 

 pressure ; we want, therefore, the velocity to be given in feet of air imder the same 

 condition. The following is the formula then to be used : — 



/ 29-92 



519 



X 28-55. 



459+^ 



The number 28-55 thus obtained by calculation diifers somewhat from the 

 number obtained by the experiments which were made two years ago. These were 

 not carried out with the accuracy that might now be obtained' by help of the experi- 

 ence which has been gained in the use of the instrument since that time j they have 

 therefore been repeated. 



The same method was adopted as formerly. A regular current of air was esta- 

 blished in a long flue or air-channel, one end of which was in connexion with a 

 high chimney, and the other end open. The speed of this current was measured 

 by the anemometer, and at the same time measured by noting the time a pufl' of 

 smoke took in travelling from one end of the flue to the other. These experiments 

 were made in three separate flues, and many experiments were made in each. 



The value of c is found in each case from the formula 



