RELATION OF AIR TO THE HOUSE WE LIVE IN. 215 



draughts is one of the few hygienic principles which have become 

 thoroughly popular. Perhaps this was not all profit, because with 

 many people ventilation and draught are synonymous; they are 

 afraid of a draught coming from an open window, an open door, and 

 find themselves in collision with ventilation. 



There is certainly and frequently danger in being exposed to a 

 draught a danger which has, perhaps, been over-estimated, because 

 men have an irresistible desire to fix a certain cause for a certain evil. 

 All collision is avoided if the proper meanings of ventilation and 

 draught are thoroughly understood. 



Ventilation is the necessary change of the air in a closed space, 

 at which the velocity of the air is still taken for a complete stillness, 

 and its motion takes place all round our body. It must not be more 

 than a little above nineteen inches per second. 



Draught is a one-sided cooling of the body, or some part of it, fre- 

 quently caused by a corresponding motion of cold air, but also in 

 other ways, as by increased one-sided radiation. The danger is, in 

 the first instance, the local perturbation in our heat-economy, which 

 has partly local consequences, but also and chiefly disorders the 

 nerves, acting on the calibre of our blood-vessels, our vaso-motor 

 nerves, which have to regulate the outflow of our heat. When we 

 are in the open, and the air is in more motion than the air of a 

 draught, we speak of wind, etc., but seldom of draught, because the 

 whole air-current flows equally all round us, just as in a well-venti- 

 lated room, only with greater velocity. 



The vaso-motor nerves, regulating the circulation in our skin, are 

 beyond our control, and we cannot bid them to defend us simply at 

 the place attacked by the draught. They know only how to serve 

 our heat-economy when the outflow of heat from our bodies is equal, 

 or nearly so, on all sides. They misunderstand the local irritation 

 for one spread over the whole surface, and act at once on this error. 

 If one perspires and goes to the window with bared neck or chest, 

 one feels a shiver not only there but all over the body, and the per- 

 spiration becomes suppressed accordingly. The blood which at the 

 time filled the blood-vessels of the glowing skin is displaced by the 

 contraction of its channels; but by the misunderstanding of the 

 vaso-motor nerves it is driven not only from the exposed parts but 

 from the whole surface toward the internal parts. If one or some of 

 them are in some state of weakness, danger or bad consequences 

 cannot fail. It is the same thing as with a large quantity of cold 

 water taken in too quickly when the body is heated. A draught, then, 

 is injurious only in so far as it causes perturbations in our heat-econ- 

 omy, and as these perturbations can be caused in different ways we 

 often accuse the draught wrongly. 



We hear often, "I don't like sitting near this window, close to 

 this wall," and so on ; there is always a slight draught coming from 



