by sparks. On the other hand, the phenomenon of electricity displayed on the lakes, and on the very dry plains of elevated plateaux, does not seem to me of the same nature. Finally, the surprising fact of the so-called galloping electricity, coursing over prairies, observed by M. Quiquerez, near Courtamon, may be regarded as a variety of lightning; a miniature lightning, resulting from the fact that the electrified cloud was grazing the earth and discharged itself over the whole surface by a thousand sparks, which were seen to run along the ground. It is probable that these phenomena should be divided into several categories, the causes of which are not identical. Some proceed from a static tension, others from a series of discharges having a certain analogy to lightning. The phenomenon of the chant of batons or staves, in other words, the resonance or bourdonnement of the soil, constitutes still another species. It has been observed only on the summit of mountains or of culminant peaks; never, as far as I know, on plains or at the bottom of valleys. It supposes a continuous dynamic action, or an efflux of the fluid towards the clouds by the most salient terrestrial conductors, sensibly different from static tensions and abrupt discharges. If we collate the observations which have been above indicated, we shall distinguish therein several common features:

1st. The efflux of electricity by the culminant rocks of mountains is produced under a clouded sky, charged with low clouds, enveloping the summits or passing at a small distance above them, but without the occurrence of electric discharges above the place whence the continuous efflux is proceeding. It would seem, therefore, that when this efflux takes place, it sufficiently relieves the electric tension to prevent lightning from being formed.

2d. In all the cases observed, the summit of the mountain was enveloped by a shower of hail or sleet, which leads to the supposition that the continuous efflux of electricity from the ground towards the clouds is not unconnected with the formation of the vapor and probably also with that of the hail.

At the Piz Surley and at the Nevado de Toluca there fell a sleet or snow resembling rice, and at the pass of the Jungfrau the snow fell whistling like hail, which seems to indicate that it was rather sleet which was falling than snow.

Doubtless we should take into account the higher temperature of the valleys, where the hail, proceeding to melt, turns into rain; but still we do not think that in the particular cases which we have just indicated the phenomenon of rain falling in the valley and that of the sleet of the isolated mountain peaks relate to a condensation taking place in accordance with identical laws through the whole extent of the sky. Thus, in particular, during the observation of June 22, 1865, I saw on the horizon all the spires of rocks, although isolated and far remote from one another, enveloped by a powdery sleet which continued for a long time, while in the rest of the sky all condensation had ceased, and in the valley there fell a copious shower of rain, but of very short duration.

Moreover, the phenomenon which passed around the summits of the rocks was quite different from that which deluged the valleys. Around these lofty pyramids there were columns of a fine sleet of great rarity; in the valley a heavy and drenching rain, such as the thin sleet of the summits would not have produced had it been converted into water. Around the elevated projections, therefore, by which the electricity was flowing off, the condensation presented the special character of being little abundant, powdery (fine sleet), and more persistent than in the rest of the sky.

The electric phenomenon which has been described, and which we term the electric resonance of the mountains, seems not to be rare in high regions, without, however, being very frequent. Among the guides and hunters whom I have interrogated on the subject, some had never observed it; others had heard it but once or twice in their lives. But it is proper to add that it is precisely on those days when menacing skies repel adventurers from the highest altitudes that the phenomenon manifests itself.

EXPERIMENTS ON ANEROID BAROMETERS MADE AT THE KEW

OBSERVATORY.

BY B. STEWART, LL. D., F. R. S.

(From the Proceedings of the Royal Society, London.)

[The frequent use of the aneroid barometer in meteorological observations and in topographical surveys in this country will render the following paper from the proceedings of the Royal Society of London interesting to many of the readers of the Smithsonian Report.-J. H,]

In judging of the value of an instrument, says the report, such as an aneroid, it is not the mere extent of difference between its indications and those of a standard barometer that ought to guide us; but it is rather the constancy of its indications under the various circumstances to which it may be subjected, that determines its value. An aneroid may differ from a standard barometer at the ordinary pressure, and to a greater extent at other pressures; but provided these differences can be well ascertained and remain constant, such an instrument ought to be regarded as valuable, just as much as a chronometer of known constancy, but of which the rate is wrong.

The circumstances which may be supposed to affect the indications of an aneroid may be classed under three heads, namely:

1. Time.

2. Temperature.

3. Sudden variations of pressure.

1. Time. As to the influence of time, no definite experiments were made. 2. Temperature. A good aneroid is generally compensated by its maker for the effects of temperature, and the question to be investigated is, to what extent such compensations are trustworthy. I record the results of subjecting six aneroids, each four and one-half inches in diameter, made by two different makers, to a very considerable range of temperature.

These results are, on the whole, very satisfactory, and appear to show that a well-made compensated instrument has its indications comparatively little affected by a very considerable temperature change. It ought always to be borne in mind that an aneroid is not capable of being read to the same accuracy as a standard barometer, and that the 1-100th of an inch is a very small quantity. These temperature experiments were made at the ordinary atmospheric pressure. unable to say what effect a change of temperature would have at a diminished

pressure.

3. Sudden changes of pressure.-For the purpose of investigating the influence of sudden changes of pressure upon the indications of aneroids, I applied to some of the best-known makers of these instruments for the loan of several, and through their courtesy in lending me a sufficient number, and for a sufficiently long time, I have been enabled to investigate this influence at some length. In the fol lowing experiments the instruments were, to begin with, suspended vertically, at the usual atmospheric pressure. They were tapped before being read. The pressure was then lowered an inch, and the instrument allowed to remain 10 minutes at this pressure before being read, after having been again well tapped. The pressure was thus reduced an inch every time, being allowed to remain 10 minutes at each stage; the instrument was always well tapped before being read, by means of an arrangement contrived for this purpose by Mr. R. Beckley. The exhaustion was carried downwards to 19 inches, and the instrument was allowed to remain an hour and a half at its lowest pressure; the air was then admitted an inch at a time, the previous arrangement as to time and tapping being followed. Separating the results of the experiments into two sets, one comprising large (four inch to four and a half inch) aneroids, and the other small instruments, wo find the mean down correction to be as follows, each aneroid being supposed right at 29 inches:

Mean correction of two large aneroids....

Mean correction for four small aneroids

29 in. 28 in. 27 in. 26 in. 25 in. 24 in. 23 in. 22 in. 21 in. 20 in. 19 in.

.00

.00 +.02 +.03+.04+.04+.07.11.14.19 .00 +.01.02 +.03+.07+.07+.09.12.17+.23

+.25

+.25

We see from these results, says Dr. Stewart, that 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. Readings of these instruments under increasing pressure, after remaining an hour and a half at the lowest reading, were recorded.

The mean corrections for up readings are exhibited in the following table, each aneroid being supposed right at 19 inches:

19 in. 20 in. 21 in. 22 in. 23 in. 24 in. 25 in. 26 in. 27 in. 28 in. 29 in. 30 in.

For two large aneroids.... .00 +.03+.03+.03 +.03+.02 +.01 .00.03.06.08 -.11 For four small aneroids.... .00 .00 +.01 +.02.01 -.01.02.04 -.07.10.15 16

We may learn from these results, says Dr. Stewart, that 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 for about 8 inches above the lowest pressure.

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.

come now to consider whether a rapid change of pressure affects an aneroid after the experiment has been completed.

The following table will exhibit the results obtained in this direction :

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 10 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 one 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.

From all these experiments, Dr. Stewart concludes as follows:

A good aneroid of large size may be corrected for temperature by an optician, so that the residual correction shall be very small.

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 six inches of pressure. A large aneroid is more likely to be correct than a small one.

The range of correctness of an instrument used for mountain ascents may be increased by a previous verification, a table of corrections being thus obtained. If an aneroid has remained some time at the top of a mountain, and be supposed correct to start with, then it will give good results for about eight inches of increase of pressure.

A large aneroid is more likely to be correct than a small one

If the aneroid has been previously verified, it is likely to give a better result. 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.

[I have subjected many aneroids to the test of a sudden diminution of pressure under the receivers of an air-pump, but have never found one of which the index would return to the same point when the original pressure was restored. I have not, however, had recourse to tapping, which Mr. Stewart has found essential. The instrument in its present condition cannot be relied on to indicate absolute pressure, though it may be used in many cases with good effect in determining differences of pressure of limited extent.-J. H.]

23 S