WARMING AND VENTILATION. .S3 3 



lated. If slioultl be kept in operation at least two minutes, if a watch in- 

 dicating seconds be used ; and four or five, if a watcb indicating minutes 

 only is employed. From the number of turns made in this interval of 

 time may be deduced that corresponding to a second, whence the ve- 

 locity, V, may be obtained. This velocity, multiplied by the sectional 

 area of the flue, will give the amount of air passing through in a second, 

 and from this the amount passing through in 3,600 seconds, or an hour, 

 can be obtained. 



If it be feared that the velocity in the flue is variable, on account of 

 its large size or other circumstances, it will be necessary to try the 

 instrument at different jjlaces, which will then give with sufficient exact- 

 ness the mean velocity of the air. 



When it is desired to determine the volume of air which is carried 

 out or drawn in through an opening covered with a grating, the ane- 

 mometer should not be placed above or in front of this opening, as is 

 done by many observers, and the velocity resulting from the number of 

 turns observed taken as the mean velocity of the air passing through. 

 Serious errors will result from this method. The proper way is to place 

 before the opening, and fitting to it as closely as possible, a pipe having 

 at one end the form of the opening, and joining at the other end a cylin- 

 drical pipe at least 2 feet long, in which the anemometer should be 

 placed, which would then indicate the velocity of passage in this part 

 of the auxiliary pipe. The velocity of the air introduced or withdrawn 

 in a second or an hour may easily be deduced afterward. 



147. Means of insuring the regularity of the ventilatio7i. — While the use 

 of portable anemometers serv^es for experimental investigations and for 

 the determination of the results obtained by the ventilation, it is not 

 sufficient to secure the necessary regularity of ventilation in large estab- 

 lishments. 



In such cases it is necessary to introduce much larger anemometers, 

 connected with an electrical recording-apparatus placed in the office of 

 the superintendent, or in a conspicuous place where it can be seen every 

 hour, or every morning and evening, whether the renewal of air is pro- 

 ceeding with regularity and with the prescribed energy. 



This is not the place to describe the apparatus.* I confine myself to 

 stating that an anemometer of the kind has been employed with suc- 

 cess lor several years at the Conservatory of Arts and Trades, to insure 

 regularity in the ventilation of the lecture-rooms ; and that every year 

 it works for about five consecutive months without derangement, and 

 without requiring any care but the renewal of the solutions in the bat- 

 tery two or three times a season. 



A similar anemometer has been in constant operation for several 

 mouths in the ventilating-chimney at Lariboisiere Hospital, and has 

 served to show, every morning, the amount of foul air removed from a 



* See the AnnaJca du Conservatoire, vol. 5, 18G4, p. 341. 



