14 REPORT OF THE SECRETARY. 



Marey, of the French Institute, in aid of his experiments on 

 air currents. This research has been materially furthered by 

 the successful application of chronophotography, a field in 

 which Dr. Marey's experiments have heretofore been note- 

 worthy. By this means it has not only been possible to ana- 

 lyze the movements of waves and currents of liquids which 

 arc invisible to the naked eye, but even the displacements of 

 molecules. 



From reports so far submitted, but as yet necessarily incom- 

 plete, it is believed that this research will aid materially in 

 the solution of various problems connected with the mechanics 

 of propulsion in fluids, at the same time rendering- service 

 in solving practical questions of ventilation, etc. The illus- 

 tration indicating' the method of making visible the course 

 of these otherwise invisible currents round an obstacle is 

 appended. 



The reader, if he has not noticed the rare experiment of 

 successful machine flight of heavy bodies through the air, has 

 probably had his attention called at times to the extraordinary 

 difference between the performance of small steam vessels like 

 yachts or tugs, where with equal power one glides through the 

 water almost as though it offered no resistance, while another 

 labors in rolling a formidable wave before it. The same dif- 

 ferences occur in still more subtle form in the air. We can 

 not with the naked eye separately see, in either case, the cur- 

 rents that produce the effect, but by Dr. Marey's most inge- 

 nious experiments we are enabled to obtain photographic 

 records from which we can study the forms which offer the 

 least resistance and see why it is. A single illustration, indi- 

 cating the influence of a very slight divergence from the best 

 forms in the case of the air, is here given. 



The experiments of Prof. A.G.Webster, of Clark University, 

 on the propagation, reflection, and diffraction of sound have 

 achieved a result of practical value in the construction of an 

 instrument capable of emitting an accurately measured sound. 

 It is thus possible, in treating persons of defective hearing, to 

 decide with exactness as to the degree of deafness in a sub- 

 ject, and to say if the power of hearing varies at different 

 times. An instrument which furnishes the means of accu- 

 rately determining these points should prove of value in 

 medical treatment. 



