David Kinnebrook, who had observed the transits of stars and planets very well in agreement with me all the year 1794, and for the greater part of the present year, began from the beginning of August last to set them down half a second of time later than he should do according to my observations; and, in January of the succeeding year he increased his error to eighttenths of a second. As he had unfortunately continued a considerable time in this error before I noticed it, and did not seem likely to get over it and return to a right method of observing, therefore, though with reluctance as he was a diligent and useful assistant to me in other respects, I parted with him.

"The error was discovered from the daily rate of the clock deduced from a star observed on one of two days by him and on the other by myself, coming out different to what it did from another star observed both days by the same person, either him or myself.

"I cannot persuade myself," he continues, "that my late assistant continued in the use of this excellent method (i. e. Bradley's eye and ear method) of observing, but rather suppose he fell into some irregular and confused method of his own, as I do not see how he could have otherwise committed such gross errors."

This record made by Maskelyne is the first account we have of any observation of a personal equation. Maskelyne considered that the discrepancies between his and his assistant's observations were due to some faulty method on his assistant's part and did not concern himself with investigating the matter further. The incident of Mr. Kinnebrook's dismissal was, however, mentioned in a history of the Greenwich observatory published twenty years later (1816) and here it attracted the attention of Bessel (Friedrich Wilhelm Bessel, 1784-1846), the Königsberg astronomer.

It was hard for Bessel to see how an assistant, who would have every reason for bringing his observations into agreement with those of his superior, should have so persistently shown this constant error. To test the matter for himself he compared his own results with those of other astronomers. In December,

1820, he observed ten stars on alternate nights with Dr. Walbeck, determining the rate of the clock as Maskelyne and his assistant had done. When they first compared their results they found a difference not of 0.8 seconds, but of 1.1 seconds. Being trained astronomers, they took particular precautions on the following two days, but with very little difference in the result, for the discrepancy was nearly one second (average for eight days 1.041 seconds). So careful were they that Bessel wrote, "We ended the observations with the conviction that it would be impossible for either to observe differently, even by only a single tenth of a second." Still Bessel was not satisfied. Walbeck was less experienced in transit observations than he, and so similar comparisons were made either directly or indirectly with many of the best astronomers of Europe.

It became recognized therefore that there is in each observer a tendency to observe star transits in a characteristic way which may differ in time results from other observers equally well trained (the relative personal equation), and that these time results differ by a more or less constant amount from the true time measured (the absolute personal equation). What applies in observing star transits by the eye and ear method applies more or less generally in all cases where a person reacts in any way to a certain stimulus. This "constant error" is analogous to the constant error of the chronometer and in a similar way it may be determined within certain limits of variation and allowed for.

In the present day the royal astronomer would determine experimentally the personal equation of any second Mr. Kinnebrook and instead of dismissing him would make corrections accordingly.

You will see, therefore, that in some senses the human being is an instrument of precision, having, as do scientific instruments of precision, a certain degree of sensitivity, a certain interval of uncertainty and certain constant and variable errors. You see, too, that the analogy of the galvanometer, the scale-pans, etc., is not merely an analogy.

The psychophysicist can study the reactions (judgments,

choices, discriminations, etc.) of this human instrument and determine the limits of its sensitivity and the probable error of its results.

Nor is the scientist working in his laboratory the only one interested in these problems. They are fundamental in many of the important problems of human society and yet also appear in many of the insignificant happenings of the day. Thousands of dollars are spent annually in giving musical instruction to persons utterly incapable of acquiring a musical education. Psychophysical methods like those developed by Professor Seashore in the laboratory of psychology at the University of Iowa can determine whether an individual is capable of acquiring a musical education. We have a problem in psychophysics when a man buys a spool of silk to match a sample his wife has given him. He does not wish to take home a shade just noticeably different. But whether it is a matter of thousands of dollars as in one case or of ten cents in the other, these problems are interwoven with all the activities of our every-day life.

A MILLION DOLLARS A DAY FOR LIGHT

BY C. E. CLEWELL

Assistant Professor of Electrical Engineering

Consider, if you will, what it is which largely distinguishes the present age from past ages, or even the present century from past centuries. What is it which limited for centuries the highly developed civilization, for example in Greece, to a small group here and there, whereas today, civilization in a highly developed state is world wide and not limited by space conditions to any particular or localized world group or small number of world groups? What has caused the ocean liners to replace the simpler craft in use prior to the time of Robert Fulton, and why has the loom been taken from the home and put into the factory? Or again, why has transportation been so transformed that the stage coach has been replaced by the railway express almost, if not entirely within the memory of many still living?

Professor Charles F. Scott of Yale University, in a recent address, has aptly answered some of these questions when he states that "Many factors have contributed to the broadening in the world's civilization, among which have been modes of travel, modern industry, the invention of the steam engine, electricity and so on." He goes on to point out, however, that the steam engine is the underlying basis of it all, for it is representative of manufactured power, and it has largely done away with the limitations which were imposed in past ages by the sole use of animal power.

The results, which are so familiar to all, have revolutionized the mode of civilized life. Manufactured power has reduced the distance from Philadelphia to New York from several days to several hours; our navigation has encircled the world by means of the fast liners of gigantic proportions compared with the older sailing craft; we have elevators in our office build

ings and stores; moving stairways; trolleys; automobiles; and our foodstuff and other supplies often come from points of remote location. Imagine, if you can, a snow storm or a flood of such severity as to cut off temporarily all the channels of manufactured power; bringing transportation to a standstill, cutting off the food supply and telephone service, and you have, in thus severing the normal channels of manufactured power, a paralysis of the community.

However, with all these transforming influences, brought about fundamentally by the coming of the steam engine, there have been limitations to the use of power from such a source. For example, while it has been possible to develop power economically in large quantities by the steam engine, it has been difficult to transmit this power with any degree of flexibility. Thus we find through New England and elsewhere, even today, factories and mills whose very architectural lines have been dictated by this difficulty of transmitting power readily in mechanical form over any considerable distance. Reference is here made to those cases where a tower occupies the center of the structure, the mill taking the form of two wings extending to each side away from the central tower. Here the base of the tower contained the engine room, and from the engine, located at this point, the leather belts extended upwards through the tower and transmitted the mechanical power of the engine to the shafting on the various floors. Here, then, we see a typical case of the engine and the work it was to do standing as a unit in their space relation to each other.

Who would have dared in these earlier days, that is, say sixty or seventy-five years ago, to have suggested the wisdom of locating the engine in a separate building, possibly a mile or more distant from the mill, so as to be nearer a railroad and thus better situated with respect to a coal supply. No, the engine room and the mill were a unit and it was necessary that they remain a unit for the time.

Again, the steam engine, while capable of high economy of operation, was not only rather inefficient in small sizes, but actually unsuited to many uses. For example, who would