We see from these results, if aneroids, right to begin with, be subjected to a decrease of pressure similar to that to which they were subjected in these experiments:— (1) That a well-constructed large aneroid will not go far wrong down to 24 inches; but after that pressure its reading will be considerably lower than that of a standard barometer, so that a large positive correction will have to be applied. (2) That small aneroids are less trustworthy than large ones, and probably cannot be trusted below 26 inches. (3) That if previous experiments are made upon an aneroid, we are enabled by this means to obtain a table of corrections which, when applied to future observations with the same instrument, will most probably present us with a much better result than had we not verified our instrument at all, and that by this means we may use our instrument down to 19 inches with very good results. Let us now consider the up readings of these instruments, and let us suppose that each instrument is right to begin with—that is to say, while remaining an hour and a half at its lowest reading. These corrections and up readings are exhibited in the following Table: No. of ane roid. 10. 11. 12. TABLE III. Correction for the up readings of the aneroids of Table I. supposed right with standard at lowest reading. 19 in. 20 in. 21 in. 22 in. 23 in. 24 in. 25 in. 26 in. 27 in. 28 in. 29 in. 30 in. 00 +03 +03 +05 +05+02+01-03-04-08-11-18 00 +05 +06 +06 +04 +06 +06 +05 +05 +01 -'02-'02 +02+01 00 +02-01-03-06-11-15-16-19 00 +03 +03-01 +01+02+02 +04 '00 '00 00 +03 +04 +03 +07 +05 +02 '00-'04-'02 -'06-'06 00-05-08-11 00-03-05-08-11-14-'13-'16-'14 0001-02-02-11-12-12-14-21-20-24-26 00 +05+05 +06 +05 +05 +05 +05 +05-01-'11 | — '14 00 +09 +12 +12 +11 +07 +03 +02 00-01-06-06-08-09-06-08 Hence we find the mean up correction for large aneroids :— Supposedright 19 in. 20 in. 21 in. 22 in. 23 in. 24 in. 25 in. 26 in. 27 in. 28 in. 29 in. 30 in. at 19 in..... 00 +03 +03 +03 +03+02+01 In like manner for small aneroids we have the following result : : 19 in. 20 in. 21 in. 22 in. 23 in. 24 in. 25 in. 26 in. 27 in. 28 in. 29 in. 30 in. As before, there are two instances in which the same instrument was twice tried; assuming the mean of the two trials to represent the truth, we find— No. 8. 19 in. 20 in. 21 in. 22 in. 23 in. 24 in. 25 in. 26 in. 27 in. 28 in. 29 in. 30 in. No. 9. 19 in. 20 in. 21 in. 22 in. 23 in. 24 in. 25 in. 26 in. 27 in. 28 in. 29 in. 30 in. - ΟΙ -'02-'03 -'01 --- .02 02 -'OI Mean minus first. determination -'02 -'02-'02 We may learn from these results, if aneroids which have been subjected for at least one hour and a half to the lowest pressures which they register have the pressure increased by means of the gradual introduction of air into the receiver (after the manner already described) : (1) That a well-constructed large aneroid will not go far wrong about 8 inches above the lowest pressure. for (2) That in this respect small aneroids are somewhat less trustworthy than large ones. (3) That if the instrument read be previously tested and its corrections ascertained, we may consider it trustworthy (making use of these corrections) for up readings throughout a greater range than if it had not been so tested. pressure affects I come now to consider whether a rapid change of - It thus appears that if an instrument reads correctly before it is put into the receiver it will read too low immediately afterwards, and that it may be some considerable time before it recovers its previous reading. The instrument cannot, therefore, be safely trusted for absolute determinations if it has been recently exposed to rapid changes of pressure. The experiments hitherto recorded, in which an inch of pressure has been taken away or added every ten minutes, are perhaps analogous to ascents in a balloon, or descents from a mountain; they are not, however, precisely analogous to mountain-ascents, since a longer time than 10 minutes is usually taken to produce a change of pressure equal to 1 inch. At the suggestion of Mr. Charles Brooke, a couple of aneroids were tested in April 1868, with the view of rendering the experiment more analogous to a mountain-ascent. The pressure was reduced by half an inch at a time and at intervals of 30 minutes, the aneroids being well tapped. The following corrections were obtained for down readings (instruments supposed right at 30 inches). These results, when compared with the previous determinations for these same instruments, would seem to show that a somewhat better result is obtained when the exhaustion is carried on more slowly, and hence that the corrections depend, to a considerable extent, on the nature of the treatment received. No. 8 seems to be more constant under different treatment than No. 9. : From all these experiments we may perhaps conclude as follows:(1) A good aneroid of large size may be corrected for temperature, by an optician, so that the residual correction shall be very small. (2 a) If an aneroid correct to commence with be used for a balloon- or mountain-ascent, it will be tolerably correct for a decrease of about 6 inches of pressure. (26) A large aneroid is more likely to be correct than a small one. (2 y) The range of correctness of an instrument used for mountainascents may be increased by a previous verification, a table of corrections being thus obtained. (3a) If an aneroid have remained some time at the top of a mountain, and be supposed correct to start with, then it will give good results for about 8 inches of increase of pressure. (36) A large aneroid is more likely to be correct than a small one. (3) If the aneroid has been previously verified, it is likely to give a better result. (4) After being subjected to sudden changes of pressure, the zero of an aneroid gradually changes; so that under such circumstances it ought only to be used as a differential and not as an absolute instrument-that is to say, used to determine the distance ascended, making it correct to begin with, or to ascertain the distance descended, making it correct to begin with-it being understood that the instrument ought to be quiescent for some time before the change of pressure is made. Before concluding I ought to mention that most of the experiments herein described were undertaken and executed in a very careful manner by Mr. T. W. Baker. "On an Easy Method of measuring approximately the Intensity of Total Daylight." By Roger J. Wright, Esq. time An easy method by which the amount of light may be at any measured and registered appears to be still wanting. I would suggest the following plan, by which I believe the desired object may be attained. A B is a rod of solid metal, terminated by a heavy base, which keeps the rod in a perpendicular position. CD is a hollow tube, blackened inside, of such a diameter as exactly to fit and slide over A B. The extremity, B, of the rod A B is painted of a snowy white, with a jet-black spot in the centre, as shown in the figure. On A B is marked the scale, beginning with zero at A. The tube is pushed over the rod till the extremity C coincides with the zero line at A. The method of using this instrument is as follows:- Draw the tube gently up the rod, at the same time looking steadily at the black spot before mentioned. It will be found, as the tube ascends, that the black spot will gradually disappear, and ultimately vanish in the gloom; it will also be found that on different days, and differ ent hours of the same day, the point at which the black spot vanishes will vary with the intensity of the light. This point is read off on the graduated scale, and thus we are enabled to measure the intensity of the light at any required time. In taking an observation, it would be well to state whether that portion of sky round the zenith from which the cone of rays proceeds be clear or cloudy. It will be seen that the result obtained by this method is not scientifically correct, as it will be affected by the eyesight of the person who makes the observation, but only in a slight degree. The method of measuring light, as just described, has been known to me for upwards of three years. The hope that I should some day be enabled to make the instrument scientifically correct has hitherto prevented me from making it public. As I understand that it is highly desirable to have some means of estimating the changes in the light which will occur during the total eclipse of the sun in August next, I no longer feel justified in keeping in the background an instrument which may possibly be of some slight assistance. THE XI. Intelligence and Miscellaneous Articles. ON THE DISPERSIVE POWER OF GASES AND VAPOURS. BY M. CROULLEBOIS. HE measurement of the dispersive power of gases, which has long been obscurely foreseen, has not yet been supplied by any physicist by the aid of a convenient and accurate experimental method. The illustrious mathematician Cauchy, in the month of August 1836, even sent to the Academy a memoir which concluded, as a necessary consequence of his able theory of light, that this dispersion had no existence. Arago announced that in this there was an error of fact, and he promised to publish a memoir on this subject containing numerous delicate measurements; but this memoir never appeared, and it has not been met with in his papers. We are lost in conjectures as to the method employed by this illustrious physicist to measure the dispersion of colours in elastic fluids. Doubtless |
