LECTURES. 



151 



of ventilation, illnstratinj]? the tendency of the air to assume rotatory 

 movements, and thus induce the removal of vitiated and the supply of 

 fresh air whenever expansion or any other cause produces a disturh- 

 ance in the atmospheric halance. The effect of the human frame in 

 inducing such currents was then pointed out. The hody always ven- 

 tilates itself if the natural currents it determines are not impeded hy 

 the architecture which surrounds it. 



A special ventilating shatt has been constructed in this Institution 

 for the illustrations, and a connexion is established between it and a 

 tube and chamber in the experimental table, by which a ventilating 

 power is br^'Ught to bear on any visible vapors used in explaining the 

 principles and practice of ventilation. 



SECOND LECTURE. 



Dr. Reid commenced this lecture with different illustrations of the 

 movement of air. Mechanical means — as pumps, fanners and bellows, 

 or a current of air or water, the action of heat, the impulse of steam, 

 and the repelling power of electricity — had all been employed with 

 the view of moving air ; and all these forces had been practically 

 applied in sustaining ventilating operations, with the exception of 

 electricity. This agent, hitherto, had only been used experimentally. 



For all ordinary purposes, no power was so generally useful and 

 available for ventilation as that arising from the action of heat on air 

 or other gases. Referring to the ventilating shaft connected with 

 the experimental table at which he lectured, it was shown that a 

 column of heated air in the interior could not balance or resist the 

 pressure of the colder air in the apartment from which it was supplied, 

 air being admitted freely into it from the external atmosphere. It 

 was not strictly accurate to say that heated air ascended, in describing 

 this movement in a technical manner. It was more correct to state 

 that air, when warmed, became expanded, and lost its power of bal- 

 ancing the contiguous air, which then pressed in upon it on every 

 side and forced it upwards. The right understanding of this point 

 was essential in the study of all the more familiar phenomena of ven- 

 tilation. It was then shown, that on establishing a free communica- 

 tion with the lower portion of the heated shaft, a flexible tube could 

 be made to carry a ventilating power in any direction, and, at the 

 fixtures connected with the table, flame, smoke and various colored 

 vapors were made to move upwards, downwards, laterally, and in other 

 directions, according to the position in which the apparatus used at 

 each was placed, and the amount of power brought to bear upon the 

 materials employed. 



The tendency of air, when falling in temperature, to descend to a 

 lower level, was then pointed out. This was illustrated practically 

 by the exhibition of a heavy, cloud-looking vapor, that was poured 

 with facility from vessel to vessel and rolled along the table in a 

 continuous stream, as if it had been an ordinary li(|uid. It was formed 



