244 LECTURES TO SCIENCE TEACHERS. 



down motions of the body. All three recording levers 

 work in the same plane. Their points are directly one 

 above another, and they write on the same cylinder : here 

 is the record obtained. We can see at a glance that in 

 running, the body is entirely unsupported during a short 

 interval between each step and its successor, and we can, 

 if we like, measure the duration of that interval. We can 

 also determine at what moment during the period that the 

 foot rests on the ground the pressure is greatest, and what 

 is the effect of that pressure in raising the body. I regret 

 that there is not time to go further in describing M. 

 Marey's beautiful contrivances for carrying out his other 

 researches in the same field ; particularly those most in- 

 teresting ones which relate to the mechanism of the flight 

 of birds. I must content myself with stating that by 

 training pigeons to carry suitably contrived recording ap- 

 paratus he has obtained graphic records of their motions 

 of flight, which are quite as perfect as those of which I 

 have given you examples. 



I will now show you one or two contrivances for record- 

 ing the motions of the chest in breathing. Here is a very 

 simple one of M. Marey. It is an inelastic cincture, the 

 length of which can be varied so as to adapt it to the girth 

 of the chest. Each end of the band is connected with a 

 receiving tympanum, which is expanded when the chest 

 enlarges, and vice versa. In this way the variations of girth 

 of the chest can be recorded, with a certain degree of 

 accuracy. 



The other instrument (Fig. 7) is perhaps not quite so easily 

 applied, but gives results of much more value. Its purpose 

 is to measure not the variations of girth but those of dia- 

 meter of the chest ; and it affords a good illustration of the 

 possibility of obtaining results of great accuracy by this 

 method. In ordinary breathing the variations of the trans- 

 verse diameters of the chest, whether measured from side 

 to side, or from before backwards, are very inconsiderable, 

 that is to say, the increase of diameter which takes place in 

 inhaling air is very small indeed, as compared with the 

 total diameter of the expanding chest. Thus in tranquil 

 breathing, the chest of a full-grown man becomes wider 

 only by a line, whereas its total width may be from ten 

 inches to a foot. The measurement of so small an increment 



