280 REPORT — 1851. 



mercurial wave requires to be known, in order that the one half of the time 

 of production aud transit may become a constant, which will enter into the 

 result of every observation made with the instrument, it was necessary rigidly 

 to determine this time. 



It was conceived, in the first instance, that as the length of the trough was 

 in direction with the line of transit of the elastic wave producing the dis- 

 turbance, the mercurial wave would be produced by the stroke of the end of 

 the trough against the mercury, and that the time of mercurial wave pro- 

 duction and transit might be obtained by providing another much longer 

 trough of precisely equal transverse section and equal depth of mercury, and 

 whose length should be a giveu multiple, say 10 times that of the trough of 

 the seismoscope. Then by producing a longitudinally traversing wave of 

 translation in this trough, by means of a wood plunger at one end, or of a 

 blow, and observing how many times in 60" this wave traversed the length 

 of the trough, the time of the wave in the seismoscope trough could be got 

 by the following formula : — 



Let the length of the shorter canal or trough = 1 . 



That of the longer a multiple of it = w. 



The whole time of transit of the wave in the shorter trough, i. e. 



the time both of production and transit . = T. 



The true time of transit only ^ t- 



The whole time of transit in the longer trough = t'. 



Then 



t'--T=(n-l)t, 

 and 



, n — l 



Therefore, if R=the time of producing the wave, 

 T-t=R. 



On trial, however, it was at once found that the effect of a blow, either on 

 any part of the seismoscope or upon any support thereof, in whatever direc- 

 tion, was to produce not a mere longitudinal wave only, but a combination of 

 waves emanating at the same moment from all sides of the trough, and again 

 receding towards them, and so coming to rest, that in fact all sides of the 

 trough vibrated and transmitted the vibration. 



After several other methods had been attempted, it was found that upon 

 placing the instrument upon a perfectly firm and immoveable base, such as 

 the solid rock, and on gently tapping the latter with a rod of iron or metal 

 while observing the cross wires, it was possible so to time the recurrence of 

 the blows, that a recurrent disturbance could be kept up, so as just to keep 

 the cross wires out of view. 



If the number of blows in a minute was reduced one or two beats, the 

 cross wires reappeared at certain determinate intervals; and if the number 

 of blows was increased, they remained invisible, except at certain intervals 

 (or beats), until the increased number became a multiple of that number 

 which completely kept them out of view. 



It was therefore obvious, that by ascertaining this number correctly by 

 repeated observations, thenumber of pulses, divided into the time=60", would 

 give twice the time of producing and transmitting the wave in the mercurial 

 trough, viz. the time of its production and advance, and recession and extinc- 

 tion, and that half this again would be the time of i transit sought. 



Accordingly a great number of such observations were made with the in- 



