162 EEroRT— 1895. 



4. Observations of Frof. J. Milne and Prof. T. Gray. 



In the author's experiments, which were commenced in conjunction 

 with Professor Thomas Gray in 1881, and continued at various times 

 during the next four years, the object was not simply to determine the 

 rate of transmission of earth waves, but also to determine their general 

 chai'acter. Usually the movements resulting from the fall of a heavy 

 weight, or the explosion of dynamite or gunpowder, were recorded by 

 seismographs. The weights employed varied from 1,7101b. to 2,000 1b., 

 while the charges of dynamite, which were exploded in holes 8 or 10 feet 

 in depth, seldom exceeded 2 lb. Although the ground in all cases 

 excepting one was soft, tlie resultant vilirations up to distances of about 

 600 feet were sufficiently large to be recorded as clear diagrams by 

 bracket and other seismographs. 



At various stations usually in a straight line joining them with tlie 

 focus of the explosion, seismographs were installed, which wrote their 

 movements on the smoked surface of a long plate of glass, the motion of 

 which was controlled by clockwork. One seismograph was placed so that 

 it wrote the movements parallel to the line of installation. These are 

 called normal vibrations. A second seismograph was an-anged to 

 record the movements at right angles to such a direction. These 

 are called transverse vibrations. A vertical lever seismograph was 

 occasionally employed to give the vertical motion. A fourth pointer 

 actuated by an electromagnet in connection with a short pendulum 

 swinging across mercury gave a broken line marking small but equal 

 intervals of time. 



By the depression of a contact key, the receiving plates at all the 

 stations were set in motion, the pointers of the seismographs drew fine 

 straight lines on the smoked surfaces, while the pendulum indicated 

 intervals of time. A few seconds later a second contact was made and 

 the charge exploded, and the seismographs gave open diagrams of the 

 resulting vibrations. When the earth motion had ceased, all tlie plates 

 were stopped and were ready to receive a second diagram without any 

 readjustments. One observer controlled all the stations, and the only 

 errors due to human interference may have arisen from slight differences 

 in the sensibilities given to the recording instruments. Tliis, however, 

 disappears when velocities were determined, not from the commencement 

 of a disturbance, but from the sharp commencement of the violent vibra- 

 tions or from the intervals of time between the appearance of particular 

 waves at the different stations. 



Observations were also made with seismographs having single indices 

 by observing the disturbance created in similar dishes of mercury, and 

 with other arrangements. 



The results of observations made I'especting velocity of propagation 

 were as follows : — 



1. The velocity of transit of vertical vibrations near to an origin 

 decreases as a disturbance radiates. Normal vibrations, although they 

 have shown a decrease in velocitj' between the second and third stations, 

 have also shown a decided increase. This latter observation has been 

 marked with the transverse motions. 



2. Near to an origin the velocity of transit varies with the intensity 

 of the initial disturbance. 



3. In different kinds of grounds, with different intensities of initial 



