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not be of immediate necessity in the conservative lines of astronomical inquiry.

DAVID P. Todd.

LETTERS TO THE EDITOR. Correspondents are requested to be as brief as possible. The writer's name is in all cases required as proof of good faith. A method for determining the unit of light. IN all photometric work hitherto undertaken, one of the main difficulties has been to obtain a satisfactory standard of light, -one which will be always constant, and which can be accurately duplicated. Heretofore, all experiments in this direction have been failures. The plan here suggested contemplates, not the employment of a unit quantity of light, but the employment of a certain effect produced by that unit quantity as a standard. In other words, it makes not the light, but the photometer, the constant.

This photometer must, then, be some device for measuring radiant energy. But, for photometric purposes, we wish only to measure that portion of the energy which has a wave-length readily visible to the human eye. With the great differences in color of our modern sources of illumination, it is absolutely impossible to state an exact equivalence between the yellow light of a candle-flame, and the blue light of an electric arc. For really accurate work, we can compare only light rays of the same wave-length. As the human eye is most sensitive to light from that portion of the spectrum between the D and E lines, in the following plan I have selected that region of the spectrum to be used exclusively for the comparison of the brilliancy of the various lights. In all probability, the total brilliancy of an incandescent body does not increase in a ratio exactly proportional to the increase in brilliancy of the yellow rays; but this difference, within practicable limits, is probably so small as to be entirely negligable. And we have the advantage of being able to state an accurate arithmetical ratio between the lights, instead of what must be at best a mere general comparison of the relative effect of the two lights upon our eyes.

Briefly stated, then, the method I would suggest consists in moving the light to be measured towards the slit of a spectroscope, until a certain effect is produced upon a screen so placed as to receive the yellow rays. When this effect is produced, the spectroscope is receiving the standard amount of light from the source; and the brilliancy of the source can then be determined by measuring its distance from the slit.

In attempting to apply this method, the difficulty which at first arises is, to obtain an effect which can be measured with accuracy. By permitting the spectrum of a light to fall upon suitable screens, three classes of effects may be obtained; namely, heating, visual, and chemical. Of these, the second is evidently unsuited for the purpose of obtaining a standard. The third is too uncertain, and not susceptible of sufficient accuracy, so that the first alone remains. Of the two practicable heat methods of measuring radiant energy, the thermopile is the more sensitive; but the bolometer responds the quicker to changes of temperature, and has the narrower surface. The latter instrument has, therefore, been selected for this application of the method. The unexposed arm of the bolometer has a slight additional adjustable resistance thrown into its circuit, so that, when the instrument is not in use, the

needle of the galvonometer will have a certain deflection dependent on the strength of the battery-current employed. When the light to be measured is placed in front of the slit of the spectroscope (which should be quite broad), the deflection will be diminished. As the light approaches the slit, the deflection will decrease, and finally become zero, at which time it is giving out the standard light. Its brilliancy can now be read off from its position upon a scale placed in front of the slit and parallel to the collimator.

This photometer might also be used to adjust the position at which an incandescent electric or other lamp should be placed in order to furnish a constant supply of light. This source could then be used as a unit in an ordinary photometer.

WM. H. PICKering.

An American Silurian scorpion. The American scorpion' from the water lime group of New-York State, described by Professor Whitfield on pp. 87, 88, is undoubtedly a young specimen of Eusarcus scorpionis (Grote and Pitt: Bulletin of the Buffalo society of natural sciences, vol. iii., pp. 1, 2), so named by an error, and which will be redescribed as Eurypterus scorpionis in the forthcoming vol. v. of the society's bulletin.

The enclosed is a sketch of the youngest specimen in my possession, drawn full size: the largest I have,

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Of course it would be rather hazardous for me to say that the American scorpion, described in a former number of Science, was not the young of Eusarcus scorpionis (Grote and Pitt), in as positive language as Mr. Julius Pohlman says it is; for our knowledge of the embryonic features and development of the Eurypteridae is yet too little to allow of many positive assertions, where not accompanied by absolute evidence. Still I must say that I do not believe it to be the young of that or any other Eurypterid. The form of the limbs, the existence of the nipper-shaped palpus and of an apparently true mandible, resembling so much those of the Scorpionidae, are features which we should scarcely look for in an embryonic or undeveloped form of Eurypterus. If Mr. J. Pohlman had seen the photographs of the specimen instead of the rude cut, or had examined the specimen itself, I think he would have expressed a very different opinion. R. P. WHITfield.

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The geology of natural gas.

I do not wish to enter into a further discussion of the interesting question of the geology of natural gas, in anticipation of the results of a special investigation which has just been commenced in the oil and gas regions by Mr. John F. Carll for the State geological survey, but, in reply to Prof. I. C. White's criticism of my letter on this subject, I desire to state a few facts in support of my conclusion, that the 'anticlinal theory' alone is insufficient to account for the existence of natural gas, in all localities in the Pennsylvania and adjoining gas regions. In order to clearly understand this communication, reference should be made to Science, June 26 and July 17.

In the first place, it is important to know that the general statements contained both here and in my letter of July 17, refer, not only to all the gas regions of Pennsylvania which, with possibly the exception of the Erie district, are geologically connected with the oil-fields, but also to those other gas localities in New York, Ohio, and West Virginia which are not in the vicinity of producing oil-wells. The facts relating to the geology of natural gas, now in the possession of any one geologist, are not sufficiently numerous or connected to permit of the deduction of any ultimate theory; and it is only possible, for the present, to deduce special geotectonic conditions under which natural gas has so far been exploited. Some of these conditions are so varying and apparently antagonistic, that it is only possible to differentiate any one of the general laws controlling the occurrence of natural gas by a comparison of the individual facts obtained from innumerable well-drillings.

The facts given here will serve to elucidate my previous article, and I hope will prove to be sufficient to clearly define the few conclusions at which I have arrived, from field observations extending over a period of ten years, and from numerous studies in conjunction with Mr. Carll, of the results of his surveys, which are more thorough, complete, and valuable than any examinations which have ever been made bearing on the geology of both petroleum and natural gas.

The general structure of the strata drilled through by the gas-wells in the vicinity of Pittsburgh) now considered the most important gas district) is the same as that of the strata in the different parts of the Devonian and carboniferous series pierced by the oilwells at the Smith's Ferry (30 miles N. 60° W. from Pittsburg) and the Slippery Rock (34 miles N. 20° W. from Pittsburgh) districts, where in both districts heavy oil is obtained from the base of the coal meas

ures, and amber oil from the Berea grit; in the Thorn Creek (25 miles N. 5° E. from Pittsburgh), and south end of the Clarion, Butler, and Armstrong (28 miles N. 20° E. from Pittsburgh) districts, where oil is obtained from the Venango (Devonian) sands; and in the Pleasant Unity (30 miles S. 65° E. from Pittsburgh), Dunlap Creek (31 miles S. 12° E. from Pittsburgh), Whiteley Creek (45 miles S. from Pittsburgh), and Dunkard Creek (48 miles S. from Pittsburgh) districts, where oil is obtained from the Mahoning sandstone (lower barren coal measures) and overlying strata. The discovery of oil at Mount Nebo, about eight miles north-west of Pittsburgh, and the several small oil-wells reported to have been obtained in close proximity to the Washington (Chartiers Creek) gaswells, together with traces of oil found upon special examination in the gas from wells which are supposed to produce absolutely dry gas (the gas obtained from the Carpenter well on the Daum farm, Westmoreland county, was supposed to be free from oil or water: when, however, the gas was confined under a pressure of a hundred and sixty pounds to the square inch, water was precipitated), the existence of natural gas, either in or near all the productive oil-pools, under geological and physical conditions similar to those found to obtain in what are frequently spoken of as 'natural-gas regions proper,' are all sufficient reasons for considering the districts producing either oil or gas exclusively (?) one in a geological sense.

Gas-wells are not entirely confined to narrow belts (one-fourth to one mile wide) along the crests of anticlinal folds, nor are those which have apparently been found in synclines necessarily in the vicinity of subordinate crumples or anticlinal rolls which are so frequently found in extensive basins. The "dip of the gas-sand and the position of the anticlines and synclines" is the third of the five principal geological and physical conditions, which I have already enumerated (Science, July 17), which seem to influence the occurrence of natural gas, and in special cases would seem to be the most important consideration. Most of the saddles and basins in western Pennsylvania have a progressive dip along their axial line toward the south-west; and a well, drilled half a mile to the north-west or south-east of a given point on the crest of an anticline, will encounter any given stratum at the same elevation as a well drilled immediately on the crest of the same anticline at a distance southwest from the given point, the distance in each case being dependent upon the intensity of the dip in the three directions. The anticline along which the famous Murraysville gas-wells in Westmoreland county have been drilled is an instance.

About ten miles north-east of the village of Murraysville, two large gas-wells have been obtained about three miles apart (north-west and south-east), one on Beaver Run, the other on Pine Run. The total dip of the Upper Freeport coal-bed, from the Beaver-run well to the Pine-run well, is two hundred and fifteen feet, or at the rate of seventy feet per mile toward the north-west. The former well is found in close proximity to the anticlinal axis along which the great Murraysville wells are obtained, farther to the south-west; while the latter well is near the synclinal axis. The extension of the general direction of this anticlinal line to the north-east of the Beaver-run well crosses the Conemaugh River near the mouth of Roaring Run, where a well was drilled, evidently on account of the existence of the anticline at that point; but no gas was found. The Apollo well, about three miles northeast of the Pine-run well, along a line parallel to the structural lines of the district, found no gas. In the

case of the Roaring Run and Apollo wells, it may be possible that no porous stratum, which could serve as a gas reservoir, was pierced by the drill: this, as already stated (Science, July 17), is the first necessary condition of the existence of gas.

The Ridgway gas-well is located in a syncline, and not on a subordinate anticline, as has been suggested, but at a point where there is a certain regular dip of about 10 toward the west, on the side of the syncline. The Kane gas-wells - including the large one at Kane, which is now supplying the residents of the town with light and fuel, and the famous Kane geyser (gas) well-are both in a syncline, the southeast dip, in the one case, and the north-west dip, in the other case, toward the centre of the basin, being less than fifty feet per mile; and the south-west dip along the axis of the basin being from fifteen to twenty-five feet per mile. The great McMullen & Hallet gas-well, commonly known as the Mullen snorter,' is not in the vicinity of any anticline. The gas-sand at this well is nearly horizontal, having a dip of about eleven feet only in a direction S. 15° W.

The gas-wells found in the vicinity of the city of Erie are located in a region where no anticlines or synclines have been discovered. The dip of the rocks here is toward the south-west, at the rate of about twenty feet per mile, from recent surveys: or from the surveys made nearly fifty years ago, by the First geological survey, as pointed out by Professor Lesley, the average dip was estimated to be fourteen feet per mile. Gas-wells have been drilled in the vicinity of Fredonia, New York, one as early as 1821. Gas is still obtained here; and, as far as the structure has been made out, no anticlines exist in the vicinity of the Fredonia wells.

While these few facts would seem to be enough to show that all gas-wells, either in the vicinity of productive oil territory, or at considerable distances removed therefrom, are not necessarily in the vicinity of anticlines, many instances might be cited, particularly in the gas regions recently developed in Pennsylvania, to show that some of the largest and most productive wells are either on or in the vicinity of anticlinal crests. I am free to admit, as I have already done, that the position of anticlines and synclines have an important bearing upon the location of profitable gas-wells; but I cannot believe that, in view of our present knowledge, the anticlinal theory' is sufficient to account for all occurrences of natural gas. As to whether it will be possible for facts still to be recorded to give any geologist an adequate basis for the formulation of an ultimate theory, we must await the results of Mr. Carll's present investigation. CHAS. A. ASHBURNER, Geologist in charge Penn. surv.

907 Walnut Street, Philadelphia, Aug. 24.

Annuaire géologique universel.

The undersigned being mentioned, under the name of Dr. Svedonius, amongst the collaborators in the above-named work recently published by Dr. Dagincourt in Paris, and two articles on Sweden and Norway appearing in the same, signed in my name, of which I had no knowledge until after their publication, I do hereby declare that the said articles are not composed by me, but are uncritically compiled from two pamphlets printed in the years 1874 and 1878, and are, consequently, now substantially antiquated pamphlets, with the authorship of which I had nothing whatever to do. These pamphlets, together with several others on the same subject, I have, at the re

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Probable period of gestation in the 'horned toad.'

On the 15th of May last I captured a very fine specimen of an adult female Phrynosoma Douglassii. The fact having long been known to me that these reptiles are capable of sustaining prolonged fasts without any apparent inconvenience, I determined to test the question for my own satisfaction and information. Accordingly, this specimen was placed where it was impossible for it to secure any food. One month after its incarceration it was taken out to be examined. No particular change was noticeable; the barest traces of emaciation could be seen in the limbs; but the creature upon being teased puffed itself up, as they do, and made short leaps with open mouth at my finger. It also ran nimbly about my study.

It was replaced in its limited quarters, and another month passed by without its having taken a particle of nutriment. Its eyes now had a slight sunken appearance, and some shrinkage of the limbs could be detected. I dipped it in water for a moment, and once more introduced it to its narrow prison. At this stage of the proceedings my chief surprise arose from the fact that the body of the animal still retained its rotund contour, and was, if any thing, plumper than at the time of the inauguration of the experiment.

Upon this date it had passed no excrementitious matter for nearly three weeks.

My surprise was great, when, in looking into the box on the afternoon of the 10th of the present month, to find strewed about the bottom of it no less than seven newly-born young. These were all dead, and enveloped in their membranes, which latter also enclosed a bright yellow yelk about as large as a small pea. At the time, circumstances prevented me from making any further examination; but, two hours later, my astonishment was at its pitch, when I found fourteen more young had come to light. Two of these were without the membranes and yelk, but every one of the twenty-one was dead.

Upon examining the mother, it was at once evident that her labor had not terminated; and, indeed, within the next ten minutes she was delivered of three more young ones. These were all born tail first: two of them were living, and had to be simply freed from their envelopes, the yelks having been absorbed. The remaining one was like the majority of the others, and lived but a moment or two.

As I write these lines I have before me twentytwo of the young in alcohol, two live and active little fellows of the same brood, and the mother-lizard, who, though she has lost much of her original activity and flesh during her three months' test, looks for all the world as fully capable of enduring many more days of it.

Taking all the circumstances I have related into consideration, I believe it will be found that about one hundred days is the period of gestation of this viviparous reptile.

It will be of interest to state, in the present connection, that other lizards endure these fasts as well as Phrynosoma; for I have a large Sceloporus, un

dergoing the test, that has suffered but very little, not having partaken of any food whatever for over a month.

I had a live Sceloporus consobrinus about my room here nearly two months, but one day it was missed, and ten days afterwards it was found in a dark corner. Nothing remained of it but the skin, enclosing a perfect skeleton and seven eggs. These latter had firm white shells, and were each of an elliptical form. R. W. SHUFELDT. Fort Wingate, N. Mex., Aug. 12.

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Some of these, I suspect, are caused by the vowel in the name of the color and the proper name being the same. Lydia, perhaps, may wear the dress of the first owner of the name I ever saw. The others I cannot account for.

The months stand in a circle: December, January, and February grouped close together on the upper, or right hand; March and April curve around; May has a little more room; June, July, August, and September are wider apart; October and November correspond to March and April on the other side. The winter months are in the shade; the summer ones in a strong light. F. M. SLACK.

THE LICK OBSERVATORY.

To German parents in Lebanon county, Penn., in the year 1796, was born a son, who received the name James Lick. As a boy, he learned the piano-maker's trade in Philadelphia, where, in youth and early manhood, he led a varied life, engaging in divers occupations, from the making and selling of furniture and pianos, to the managing of a theatre. When about thirty-five years old, he went to South America, where he resided chiefly at Buenos Aires, acquiring property to the extent of about forty-five thousand dollars, with which sum, in 1847, he emigrated to the site of the present San Francisco, and invested it in real estate. In a quarter of a century he found himself worth a fortune nearly one hundred times as

great, which, by the execution of a deed of trust, he placed under the control of a board of trustees, of which Mr. Richard S. Floyd is now the president.

Mr. Lick died at the age of eighty years. His chief scientific bequest was the sum of seven hundred thousand dollars, for the erection of a great observatory at a mountain elevation. He was anxious to secure the highest elevation consistent with ready accessibility; and Lake Tahoe, nearly eight thousand feet above sea-level, was about the first site which came prominently to his notice. The proposed locality was visited, investigated, and rejected ; and the site of Mount Saint Helena, an eminence much nearer San Francisco, was visited by Mr. Lick in person. Early in 1875 Mr. Thomas E. Fraser suggested Mount Hamilton, in the county of Santa Clara, as a desirable site; and, on his recommendation, Mr. Lick decided upon this eminence for the permanent location of the great observatory. Mount Hamilton is situate in the Pacific coast-range, about fifty miles south-east of San Francisco, and thirteen miles in a direct line from San José, the nearest city. A telephone-line, and an excellent mountain road, now connect the two.

Al

Mount Hamilton has a treble-pointed summit, about forty-five hundred feet high; and no mountain within a radius of one hundred miles approaches this elevation. The two extreme peaks of the general summit are nearly a mile distant from each other, in a northeast, south-west direction. The southernmost peak is bare of all woody growth, and its lines converge to form an angle slightly acute. though about a hundred and twenty-five feet lower than the northern summit, this peak was chosen by the trustees for the location of the observatory, on the advice of Professor Newcomb and Mr. Burnham; as it presented the greater advantage in point of accessibility, configuration, and a minimum of obstruction to the view south, east, and west. The first work was to cut down this apex; and about forty-five thousand tons of rocks were removed, leaving an irregularly oval plateau, about four hundred and fifty feet in length, and with an extreme breadth of about two hundred and twenty-five feet. The lands about the mountain, which are set aside for observatory purposes, comprise a government reservation of about fifteen hundred acres, to which the trustees have added a hundred and sixty acres by purchase.

The first astronomer who visited the site of the projected observatory was Mr. Sherburne

W. Burnham, who in the autumn of 1879, on the recommendation of Professor Holden and Professor Newcomb, was invited by the Lick trustees to make a systematic study of the suitableness of the atmospheric conditions for observational research. In October he was joined by Professor Newcomb, who remained for a few days upon the summit, to advise with regard to the proper location of the buildings and instruments. Mr. Burnham devoted two months' time to the measurement of close double stars. During the period, Aug. 17 to Oct. 16 inclusive, he found,

First-class nights, 42; medium nights, 7; cloudy and foggy nights, 11.

The summer of 1881 marked great progress. The transit of Mercury in the latter part of that year was observed with the twelve-inch equatorial and the four-inch meridian transit instrument in their permanent quarters, Professor Holden and Mr. Burnham securing complete series of satisfactory contact-observations. During the period Oct. 20 to Nov. 9, Professor Holden found fourteen nights which were perfectly clear, with at least average conditions of vision; and one of them was exceptionally fine.1

In the summer of 1882 the results achieved on the mountain were even more important than during the year previous. The construc

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There was not in the whole time a single poor night when it was clear.

In the spring of 1880 Capt. Floyd spent several weeks in Washington, accompanied by Mr. Fraser, whom the trustees had appointed superintendent of construction of the observatory. There they were in daily consultation with Professor Newcomb, Professor Holden, and other astronomers, with regard to the plans of construction on the mountain, and the final instrumental equipment of the institution. Under their direction, the architect's plans for the main building were prepared at this time, and the work of construction was at once begun.

tion of the main building was rapidly carried forward, and the problem of water-supply for all future purposes was shown to be effectively solved. Springs of excellent water had been discovered about four hundred feet below the summit, and a reservoir large enough to hold eighty-five thousand gallons was built on the apex of the middle peak. A year later, the trustees took the additional precaution of providing a second reservoir of nearly seventy thousand gallons capacity, in which the rains are collected from the slate roofs on Observa

1 It is to be remembered that this was at the season of the year when the change from the summer to the rainy season was impending, and when the inequality of the temperature between the day and the night was something near the maximum.

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