278 



NATURE 



[July 19, 1900 



undulation, refleccing a rhythm and possibly significant of a 

 remote pendulum whose rate of vibration is known. If it can 

 be traced to such a pendulum there will result a determination 

 of the rate at which the chronograph scroll moved when that 

 part of the record was made ; and a moderate number of such 

 determinations, if well distributed, will convert the whole scroll 

 nto a definite time scale. 



In other words, if a sufficient number of the rhythms embodied 

 in strata can be identified with particular imposed rhythms, the 

 rates of sedimentation under different circumstances and at 

 different times will become known, and eventually so many 

 parts of geologic time will have become subject to direct calcu- 

 lation that the intervals can be rationally bridged over by the 

 aid of time ratios. 



For this purpose there is only one of the imposed rhythms of 

 practical value, namely, the precessional ; but that one is, in 

 my judgment, of high value. The tidal rhythm cannot be ex- 

 pected to characterise any thick formation. The annual is liable 

 to confusion with a variety of original rhythms, especially those 

 connected with storms. The rhythm of eccentricity, being 

 theoretically expressed only as an accentuation of the pre- 

 cessional, cannot ordinarily be distinguished from it. But none 

 of these qualifications apply to the precessional. It is not liable 

 to confusion with the tidal and annual because its period is so 

 much longer, being more than 2000 times that of the annual. 

 It has an eminently practical and convenient magnitude, in that 

 its phvMcal manifestation is well above the microscopic plane, 

 and yet not so large as to prevent the frequent bringing of 

 several examples into a single view. It is also practically 

 regular in period, rarely deviating from the average length by 

 more than the tenth part. 



From the greater number of original rhythms it is distinguished, 

 just as from the annual and tidal, by magnitude. The practical 

 geologist would never confuse the deposit occasioned by a single 

 storm, for example, with the sediments accumulated during an 

 astronomical cycle of 20,000 years. But there are other original 

 rhythms, known or surmised, which might have magnitudes of 

 the same general order, and to discriminate the precessional 

 from these it is necessary to employ other characters. Such 

 characters are found in its regularity or evenness of period, and 

 in its practical perpetuity. The diversion of the mouth of a 

 great river, such as the Hoang Ho or the Mississippi, might 

 recur only after long intervals ; but from wh»t we know of the 

 behaviour of smaller streams we may be sure that such events 

 would be very irregular in time as well as in other ways. The 

 intervals between volcanic eruptions at a particular vent or in a 

 particular district may at times amount to thousands of years, 

 but their irregularity is a characteristic feature. The same is 

 true of the recurrent uplifts by which mountains grow, so far as 

 we may judge them by the related phenomena of earthquakes ; 

 and the same category would seem to hold also the theoretically 

 recurrent collapse of the globe under the strains arising from the 

 slowing of rotation. The carbon dioxide rhythm, known as yet 

 only in the field of hypothesis, is hypothetically a running-down 

 oscillation, like the lessening sway of the cradle when the push 

 is no longer given. 



But the precessional motion pulses steadily on through the 

 ages like the swing of a frictionless pendulum. Its throb may or 

 may not be caught by the geological process which obtains in a 

 particular province and in a particular era, but whenever the 

 conditions are favourable and the connection is made, the record 

 should reflect the persistence and the regularity of the inciting 

 rhythm. 



The search of the rocks for records of the ticks of the pre- 

 cessional clock is an out-of-door work. Pursued as a closet 

 study it could have no satisfactory outcome, because the printed 

 descriptions of rock sequences are not sufficiently complete for 

 the purpose ; and the closet study of geology is peculiarly 

 exposed to the perils of hobby-riding. A student of the time 

 problem cannot be sure of a persistent, equable sedimentary 

 rhythm without direct observation of the characters of the 

 repeated layers. He needs to avail himself of every opportunity 

 to study the series in its horizontal extent, and he should view 

 the local problem of original versus imposed rhythm with the 

 aid of all the light which the field evidence can cast on the con- 

 ditions of sedimentation. 



Neither do I think of rhythm seeking as a pursuit to absorb 



the whole time and energy of an individual and be followed 



steadily to a conclusion ; but hope rather that it may receive 



the incidental and occasional attention of many of my colleagues 



NO. 1603, VOL. 62] 



of the hammer, as other errands lead them among cliffs of bedded 

 rocks. If my suggestion, should succeed in adding a working 

 hypothesis or point of view to the equipment of field geolo- 

 gists, I should feel that the search had been begun in the 

 most promising and advantageous manner. For not only 

 would the subject of rhythms and their interpretations be ad- 

 vanced by reactions from multifarious individual experiences, 

 but the stimulus of another hypothesis would lead to the dis- 

 covery of unexpected meanings in straligraphic detail. 



It is one of the fortuna'te qualities of scientific research that 

 its incidental and unanticipated results are not unfrequently of 

 equal or even greater value than those directly sought. Indeed, 

 if it were not so there would be no utilitarian harvest from the 

 cultivation of the field of pure science. 



In advocating the adoption of a new point of view from which 

 to peer into the mysterious past, I would not be understood to 

 advise the abandonment of old standpoints, but rather to emu- 

 late the surveyor, who makes measurement to inaccessible 

 points by means of bearings from dififerent sides. Every inde- 

 pendent bearing on the earth's beginning is a check on other 

 bearings, and it is through the study of discrepancies that we are 

 to discover the refractions by which our lines of sight are warped 

 and twisted. The three principal lines we have now projected 

 into the abyss of time miss one another altogether, so that there 

 is no point of intersection. If any one of them is straight, both 

 the others are hopelessly crooked. If we would succeed we 

 should not only take new bearings from each discovered point of 

 vantage, but strive in every way to discover the sources of 

 error in the bearings we have already attempted. 



THE RELATION OF STIMULUS TO 

 SENS A TION. 

 MOTHING has done more to place on a scientific footing 

 the discussion of the phenomena which the study of matter 

 and energy presents to the eye of reason, than the establish- 

 meut of a doctrine of quantitative equivalence. So much oxygen 

 and hydrogen, so much water ; this amount of energy of 

 chemical separation gone, that amount of sensible heat gained. 

 In a similar way, nothing is likely to do more to give support 

 to the hypothesis that sentience or consciousness is a concomi- 

 tant of certain physiological processes than the establishment of 

 a quantitative relation between stimulus and sensation. 



It has, indeed, long been obvious that some general relation 

 of this kind holds guod. Increased physical pressure is, within 

 certain limits, increasingly felt ; more light gives a higher 

 degree of visual sensation ; the greater the amplitude of the 

 vibrations of a violin-string the fuller and louder the sound. 

 Such statements are, however, indefinite. We want to know- 

 how much the physical increase must be to give just so much 

 increment irt sensation. If we double the strength of the 

 stimulus, do we double also the strength of the sensation ? If 

 not, by how much do we increase it ? Ernst Heinrich Weber 

 sought to express the quantitative relation with some exactness ; 

 Gustav Theodor Fechner and other more recent inquirers have 

 built upon the foundations laid by Weber ; and a provisional 

 law of the relation of physical stimulus to felt sensation has 

 gradually gained wide acceptance. 



Weber's classical experiments dealt with what is termed the 

 " least observable difference." If, for example, a weight of one 

 pound be laid upon the hand, it gives rise to a sensation of pres- 

 sure. If, now, an extra ounce be added no difference is felt, nor 

 is the added weight of two or of three ounces perceptible. The 

 sensation is not increased, and then only just perceivably in- 

 creased, till one-third of a pound is added. This,, then, is said 

 to be the least observable difference. We now start afresh with 

 a load of two pounds, and add, as before, one-third of a pound. 

 But there is no observable difference ; nor is there any felt in- 

 crease in sensation until two-thirds of a pound are added. Start- 

 ing once more with an initial load of three pounds, we find that 

 neither the addition of one-third, nor that of two-thirds of a 

 pound affords any observable difference in the sensation experi- 

 enced. A full pound must be added for the increment to be felt. 

 The least observable differences, therefore, are between 



1 lb. and i -f ^ lb. 



2 lb. „ 2 + 1 lb. 



3 lb. „ 3 -H lb. 



If, then, we extend and generalise the results of such experi- 

 ments, we find that, within certain limits^ to. obtain an orderly 



