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tirely unprotected against the pressure of the water in which it is immersed, and it is important to consider what effect this pressure will have.
In thermometers of the ordinary construction this pressure acts only externally, and produces much greater diminution of the internal volume than when, as in Prof. Lubimoff's thermometer, it acts both externally and internally, a mode of action with which we are familiar in the case of Ersted's piezometer.
From Regnault's experiments it appears that the apparent compression of mercury in glass, when the pressure is thus applied, is .000001234 per atmosphere, whereas the apparentexpansion of mercury in glass for heat is.0000857 per degree Fahrenheit. The latter number is 69 times the former ; it therefore appears that a pressure of 69 atmospheres would be required to falsify the indications of Prof. Lubimoff's thermometer to the extent of 1° F. The actual pressure at the bottom of the well is less than the half of this, and therefore should only produce an error of a few tenths of a degree. This, however, is on the assumption that the glass undergoes no change of figure, a condition which may easily fail of being fulfilled, owing to the want of perfect uniformity in the glass.
Mr. Donaldson has written from Calcutta to the effect that the thermometer which was sent to him has been entrusted to a competent observer, who has taken numerous observations with it, which will be sent shortly.
M. Erman's letter above referred to is immediately followed in the Comptes Rendus' by an account, by M. Walferdin, of some observations, which appear to be very reliable, taken in artesian wells in the basin in which Paris is situated. They were taken with maximum thermometers of the kind invented by Walferdin himself, in which the mercury overflows into a reservoir when the temperature exceeds a certain limit, the thermometers being hermetically sealed in glass tubes to protect them from pressure.
The observations which he first describes were taken in a well, newly sunk to the depth of 263 metres, at St. André, about 50 miles to the west of Paris, and which failed to yield a supply of water. The temperature was carefully observed at the depth of 253 metres by means of two thermometers, which were allowed to remain at that depth for ten hours. Their indications agreed to .03 of a degree Centigrade, and gave a mean of 170.95 C. For the sake of comparison, M. Walferdin observed the temperature at the bottom of a well 75 metres deep, situated at a distance of only 13 metres from the other well, and found it 120.2C., showing a difference of 50.750. in 178 metres, which is at the rate of 1° C. in 30.95 metres, or 1° F. in 56.4 feet. He mentions that he also employed two Six's thermometers (deux thermométrographes) enclosed in copper tubes to protect them from pressure, but both of these gave erroneous indications. The copper case of one was imperfect, and allowed a little water to enter. This one read 10.25 too high, owing probably to the effect of pressure; the other read 2°.15 too low, owing probably to the index being shaken down.
The temperature at the depth of 400 metres in the puits de Grenelle at Paris was observed on two different occasions. The indications were 230.5 on the first and 230.75 on the second occasion; and these M. Walferdin compares with the constant temperature 11°.7 in the caves of the Observatory at the depth of 28 metres. Taking the mean of the two observations, 230.6, we have a difference of 110.9 in 372 metres, which is at the rate of 1° C. in 31.2 metres, or 1° F. in 56.9 feet.
Observations in the well of the Military School, at a distance of 600 metres from the puits de Grenelle, showed a temperature of 16o4 C. at the depth of
173 metres. This gives, by comparison with the Observatory caves, an increase at the rate of 1° C. in 30-85 metres, or 1° F. in 56.25 feet.
These three determinations are in wonderfully close agreement with each other. All three wells are sunk in the chalk of the Paris basin. In the case of the St. André well the thicknesses of the different strata were :
metres. Plastic clay ........
...... 13:52 White chalk ..
263.22 The thermometer which the Committee have been employing for the last three years is a Phillips's maximum, having so fine a bore that the detached column of mercury which serves as the index is sustained in the vertical position by capillary action, and will bear a moderate amount of shaking without slipping down. Numerous instances, however, have occurred in wbich the index has slipped in consequence of jerks or concussions sustained by the thermometer in hauling it up from a depth. During the past six months the Secretary has been in correspondence with Messrs. Negretti and Zambra respecting a proposed modification of the maximum thermometer known by their name, which occurred to him more than a year ago, and was described by him privately to some meteorological friends at the last Meeting of the Association. It was then supposd to be new, but it now appears that Messrs. Negretti and Zambra have made somothing of the kind for the last fourteen or fifteen years. Several changes, however, were necessary before the thermometer was adapted to the uses of the Committee, and the first complete instruments were received in June last. They are enclosed, like the thermometers previously used, in hermetically sealed tubes, for protection against pressure, and they have the advantages (1) of being able to bear more severe jolts without derangement of their indications, and (2) of presenting to view a much broader column of mercury, so as to be more easily read in a dim light.
The instrument is to be used in a vertical position, with the bulb uppermost. Between the bulb and the stem there is a contraction, through which the mercury will not pass except under pressure. It is set by holding it with the bulb end lowest, and tapping this end on the palm of the hand, till the part between the contraction and the bulb is full of mercury. It can then be held with the bulb up, and the mercury in the stem will run down to the lower end, from which the graduations begin. In this position, the top of the column indicates the temperature of setting, which must be lower than the temperature intended to be observed.
The instrument is then to be lowered into the bore to any required depth, and allowed to remain there for about half an hour, to ensure its taking the temperature of the surrounding water. The expansion of the mercury in the bulb with heat will force a portion of the liquid through the contraction, and subsequent cooling in hauling up will not cause any of it to return. The portion which has thus escaped from the bulb into the stem will usually be found remaining close to the contraction, when the thermometer has been hauled up. The instrument must then be gently inclined, so as to make the bulb end slightly the lowest, when the mercury in the stem will all unite into one column, which will run down to its place on again raising the bulb. The head of the column will then indicate the required temperature.
Report on Observations of Luminous Meteors, 1870–71. By a Com
mittee consisting of JAMES GLAISHER, F.R.S., of the Royal Observatory, Greenwich, ROBERT P. GREG, F.R.S., ALEXANDER S. HERSCHEL, F.R.A.S., and CHARLES BROOKE, F.R.S., Secre
tary to the Meteorological Society. The object of the Committee being, as in the previous year, to present a condensed Report of the observations which they have received, and to indicate the progress of Meteoric Astronomy during the interval which has elapsed since their last Report, the reviews of recent publications relating to Meteoric Science which will be found in the sequel are preceded by a statement of the results obtained by the observers, who have during the past year contributed a valuable list of communications on the appearances of luminous meteors and regular observations of star-showers to the Committee. The real heights and velocities of thirteen shooting-stars obtained by the cooperation of Mr. Glaisher's staff of observers at the Royal Observatory, Greenwich, during the simultaneous watch for meteors on the nights of the 5th to 12th of August last, are sufficiently accordant with the real velocity of the Perseids (as already previously determined by similar means, in the year 1863) to afford a satisfactory conclusion that the results of direct observation are in very close agreement with those derived from the astronomical theory of the August meteor-stream. Shooting-stars were observed to be more than usually frequent on the nights of the 17th of August and 24th of September last, accompanying on the latter night a rather brilliant display of the Aurora. On the nights of the 18th-20th of October last the sky was so generally overcast as to conceal the view of any meteoric shower which may have taken place on that well-established meteoric date. But on the mornings of 13th-15th of November last a satisfactory series of observations of the November star-shower (so far as its return could be identified) recorded at the Royal Observatory, Greenwich, and at several other British stations, concurs with very similar descriptions of its appearance in the United States of America in showing the rapid decrease of intensity of this display since the period of greatest brightness, which it attained in the years 1866 and 1867. Notices of the extreme brightness with which it was visible in the following year (1868) are extracted from astronomical and meteorological journals kept in Switzerland and Scotland. A short view of the sky on the night of the 12th of December last was obtained at Birmingham, where the accurate divergence of the meteors observed by Mr. Wood from the radiant point in Gemini of the December meteors sufficed to verify the periodical return of that meteoric current. The state of the sky was not favourable for observations of meteors on the first two nights of January ; but during two hours, when the sky was clear, on the night of the 20th of April last, the well-known group of April meteors was noted, on the periodical date, diverging in considerable numbers, and with the characteristic features of brightness, and leaving a persistent streak from the direction of a nearly fixed centre in the constellation Lyra. One meteor of the shower, simultaneously observed at Birmingham and Bury St. Edmunds, afforded sufficiently accurate materials for calculating its real distance from the observers, and the length and velocity of its visible flight relatively to the earth. The combined observations of the regularly recurring meteor-showers during the past year having at present proved successful in contributing some valuable materials to their history, the Committee propose to resume during the coming year a systematic watch for their return, and to provide observers
of the regular star-showers of August and November, and those of smaller interest and abundance in January, April, October, and December, with suitable maps and instructions to enable them to obtain, without unnecessary pains bestowed in preparations or expense, the most careful and complete records of their extraordinary displays. In order that the operations of the Committee may thus continue to be systematically directed towards the objects which have acquired important interest from the discovery of the astronomical connexion of shooting-stars with the orbits of comets, introducing the strictest methods of inquiry into the laws of their appearance, the Committee earnestly desire the renewal, in the coming year, of the support which, since its first formation, by their correspondence and cooperation, observers have hitherto freely contributed to the British Association.
Notices of the appearance of twenty-two fireballs and small bolides have during the past year been received by the Committee, fourteen of which were compared to the apparent size and brightness of the moon, and the latter include three detonating meteors of the largest class. Descriptions of some of the largest of these meteors are contained in the accompanying list and in the following paragraphs of the Report. No notice of the fall of an aërolite during the past year has been received, although the occurrences of large meteors during the months of autumn and spring, preceding April last, were more than ordinarily frequent. Of one of these, which appeared with unusual brilliancy in Cornwall, Devonshire, and the south-western counties of England on the evening of the 13th of February, it is possible to estimate, at least approximately, the locality and the real elevation of its flight. Careful observations of such phenomena when they appear are, however, again recommended by the Committee to all observers who may have the necessary astronomical skill, and the rare opportunity to note their brilliant courses by the stars.
In the discussion of some papers on Meteoric Astronomy which follow the foregoing observations, it will be seen that in the hands of its talented originator, Prof. Schiaparelli, the cosmical theory of periodical shooting-stars has received fresh and valuable illustrations, and the apparently inexplicable grouping of radiant-points for several successive days in the neighbourhood of a general centre of divergence, if notexplained, appears to depend upon effects of planetary disturbances of a single meteoric stream from which the parasitic radiant-points have been derived. The discussion of such examples is simplified, and their complete explanation is, perhaps, not beyond the reach of the persevering application with which skilled astronomers in every country are now bent on the solution of the complicated and intricate geometrical problems presented to them by the distribution and features of the known radiant-points of shooting-stars. To a brief description of this interesting memoir are added, at the close of the Report, some notices of works which have recently appeared on the more general branches of meteoric science.
I. METEORS DOUBLY OBSERVED. 1. A Table of the real heights of sixteen shooting-stars doubly observed in England during the meteoric shower of August 1870, independently of the observations recorded at the Royal Observatory, Greenwich, was presented in the last volume of these Reports. A comparison of the observations made on that occasion at the Royal Observatory, Greenwich, with those recorded at the other stations, enables the real paths of thirteen meteors (ten of wbich are new to the former list), seen by Mr. Glaisher's staff of observers, to be satisfactorily determined; and the real heights and velocities of the meteors thus identified, together with the particulars of the observations from which they are concluded, are entered in the Table opposite. .
The accompanying diagram (drawn on the same scale as that in the last Report) readily exhibits to the eye the actual heights at appearance and disappearance (or the heights of the centres of the visible paths of the meteors Nos. 1, 4, 9) above the earth's surface. The last vertical line on the right represents (as in the last Report) the average height at first appearance and that at disappearance of all the meteors regarded as identified in the present list, of which the approximate heights of those points have been satisfactorily ascertained. The resulting average heights are :
At first appearance. At disappearance.
Heights at appearance and disappearance of thirteen shooting-stars simultaneously observed at the Royal Observatory, Greenwich, and at other stations in England, August 6th-11th, 1870 (Nos. 1, 4, 9 are calculated heights at the centres of the real paths.)
The present average heights are somewhat less than those observed in the year 1863; but they agree more closely with the general average height at first appearance, 70.05 miles, and that at disappearance, 54:22 miles (as given in the Report for 1863, footnote on p. 328), of nearly all the shooting-stars