567 



observations of so short a duration cannot be uniform. The aim 

 must be to ascertain the precise number of respirations and the 

 quantity of air inspired then occurring. To control the respiration 

 by inspiring a predetermined number of times per minute, or by 

 breathing with an assumed uniformity of depth, is to render the act 

 and the results alike unnatural. It is doubtless possible to fix the 

 number of respirations, but it is impossible to regulate the quantity 

 of air inspired except by the aid of a spirometer. The effect of exer- 

 tion may be accurately determined by fixing the mask upon the face 

 with bands, and by carrying the spirometer in the arms, or fastened 

 upon the back with knapsack straps. The instrument must not 

 exceed a very few pounds in weight. The distance to be traversed 

 must be accurately measured, and subdivided into short distances also 

 accurately measured, so that, with the watch in hand, the rate may be 

 tested every half minute and over every small part of the course. 

 Each subdivision should be a known part of a mile, and at the rate 

 selected must be traversed in a given number of seconds. Thus, at 

 the rate of two miles per hour, a course of 58f yards would be tra- 

 versed every minute ; and if that be subdivided into six equal parts, 

 each one would be walked over in a very little less than ten seconds. 



THE CARBONIC ACID EXPIRED*. 



The remarks already made in reference to posture and rest are also 

 applicable to this subject. It is impossible so to regulate the respi- 

 ration that a fair average of the carbonic acid evolved may be made 

 from inquiries of one or two minutes' duration, both from the impossi- 

 bility of withdrawing the attention, and of obtaining an exact propor- 

 tion of the expiration in so short a time. We almost always found 

 that the rate of respiration was greater during the first than during 

 subsequent minutes ; and this was no doubt attended by a change 

 in the quantity of carbonic acid expired, and it was due to the action 

 of the mind. Five minutes is the shortest period during which such 

 an inquiry should be continuous, and ten minutes would be better if 

 the duration of each experiment should not interfere with the neces- 

 sary frequency of repetition ; but the latter might be an essential 

 character of the inquiry. An inquiry of five minutes duration may 

 be repeated every twelve minutes. The apparatus' required must be 

 * See Phil. Trans. 1859. 



