166 



RESULT OF THE ASTKOiNOxMEK ROYAL'S RECENT PENDULUxM EXPERIMENTS. 



[1855. 



moisture take place, as should be the case, at times just pre- 

 ceding and following the setting of the sun, and at least 7 or 8 

 hours after its rising. In the observations of Lieutenant Hood, 

 made in the voyage of Captain Franklin, between the 1st of 

 February and the 31st of iMay, the greatest declination took 

 place at 8 and 9 o'clock in the morning, and the least at an 

 hour after noon. Thus, as is seen, the times of the maxima 

 and minima are widely variable in those high latitudes, where 

 there are great differences in the length of the day, and also 

 in temperature, and therefore considerable electric disturbances 

 of the air. 



It is a singular fact, sometimes noticed, that when an ob- 

 server is in the midst of an aurora, so to speak, the action on 

 the needle may be null. This was remarked by Mr. Forster, 

 at Port Bowen, beyond 65° N., the latitude of Forts Franklin 

 and Enterprise, where Dr. Richardson had on the contrary ob- 

 served the action of the needle. In fact, a needle in the 

 interior of the circle formed by the aurora about the magnetic 

 pole, is no longer under the influence of the currents which 

 circulate around it and not above or below, and it ought there- 

 fore to experience only a variable and irregular action. 



I have said that the aurora was probably of daily occurrence, 

 and varied only in intensity. These differences in intensity are 

 the reason for its being not always perceptible, and also for 

 its less frequency remote from the magnetic poles. As to the 

 differences ot number for each month, they are attributable to two 

 causes — but especially to the unequal length of the nights, for 

 there should be fewer in the shorter nights. Thus in May, June 

 and July the fewest are seen, because the days are the longest, 

 while in the nine others, and especially in March, September 

 and October, they are most numerous. This preeminence of 

 these three months above others, of still shorter days, can be 

 due only to this, that the auroras are most frequent at the 

 times of the equinoxes, and especially the autumnal equinox. 

 This is readily understood if we consider that the vernal 

 equinox is the time when the sun transfers to the northern 

 hemisphere its powerful influence either direct or indirect in 

 the devtlopement of electricity ; and that the autumnal should 

 be followed wnth a large condensation of the vapours accumu- 

 lated in the atmosphere during the months of summer — a 

 condensation which, as already explained, facilitates the 

 neutralisation of the two electricities, developed in large 

 quantities during the summer, and augments consequently the 

 intensity of the discharge at the pole. 



It has been pretended that in the appearances of the aurora 

 borealis there are secular variations ; in other words, that there 

 are epochs comprising a certain number of years during which 

 auroras are particularly frequent, and others in which they are 

 rare. This opinion does not appear to me to be based on 

 documents sufficiently exact to be admitted. There may be a 

 difference in different years, as there is a difference in 

 temperature and humidity. But this is far from making out 

 a periodicity in auroras : to establish such a periodicity, there 

 ought to be the collected observations of a century, from ob- 

 servers at least as good, and as favourably situated with 

 reference to the magnetic poles, as those now engaged : and 

 this we have not. We need not therefore dwell longer on this 

 point, only remarking that if really such a periodicity exists, it 

 might be connected with the change in the magnetic poles, 

 which are the centers of the aurora, and which according to 

 the surface about them would more or less facilitate the electric 

 circulation ; for it is evident that the naked soil would afford 

 more ready circulation than a surface covered with a great 



thickness of ice. But, I repeat it, the fact of the periodicity 

 is far from proved. 



Recapitulation. — 1. All observations agree in demonstrating 

 that the aurora borealis is a phenomenon taking place in our 

 atmosphere, and that it consists in the production of a lumin- 

 ous ring whose centre is the magnetic pole, and having a 

 diameter more or less large. 



2. Experiment demonstrates that in causing in highly 

 rarified air the reunion of the two electricities near the pole of 

 an artificial magnet, a small ring of light is produced similar to 

 that which constitutes the aurora, and having a like movement 

 of rotation. 



3. The aurora is consequently due to electric discharges 

 taking place in the upper regions between the positive elec- 

 tricity of the atmosphere and the negative electricity of the 

 earth — the electricities being separated by the direct or indirect 

 action of the sun, jirincipally in the equatorial regions. 



4. As these electric discharges take place constantly, though 

 with varying intensity, depending on the state of the atmos- 

 phere, the aurora should be a daily phenomenon, more or less 

 intense, and consequently visible at greater or less distances, 

 and only when the night is clear — which accords precisely 

 with observation. 



5. The phenomena that attend the aurora, such as the 

 presence and form of the cirro-stratus clouds, and especially 

 the disturbances of the magnetic needle, are of a kind to de- 

 monstrate the truth of the electric origin attributed by the 

 author to the aurora — an hypothesis with which these phe- 

 nomena correspond even in their minutest details. 



6. The aurora australis, according to the few observations 

 on it which have been made, presents exactly the same 

 phenomena as the aurora borealis, and is explained in the 

 same manner. 



Result of the Astronomer Eoyal's Ke:ent Pendulum 

 Experiments; Harton Pit, South Shields. 



Addressed by Professor Airey to Mr. James Mather. 



Royal Observatory, Greenwich, Dec. 2, 1854. 

 My Dear Sir, — It will be, I am sure, matter of satisfaction 

 to you to know that the result of the computations of the pen- 

 dulum vibrations gives the highest confidence in the certainty 

 of the results to be deduced from them. The comparison of 

 the rates of the pendulums before and after their interchanges 

 shows, that there is no evidence of their having undergone any 

 mechanical change whatever, and almost positive evidence 

 against their having undergone any change amounting, in its 

 effect on their vibrations, to l-20th part of a vibration in a 

 day. The immediate result of the computations is this, sup- 

 posing that a clock was adjusted to go true time at the top of 

 the mine, it would gain 2 J seconds per day at the bottom. Or 

 it may be stated thus, that gravity is greater at the bottom of 

 the mine than at the top by l-19190th part. To go a little 

 further into the interpretation. If there had been no coal 

 measures or rocks of any kind between the top and the bottom, 

 but merely an imaginary stand to support the pendulums, the 

 gravity at the top icould hare lieen less than at the bottom by 

 i-8406th part nearly. But it is less by only l-19200th part. 

 And what is the cause of the difference ? It is the attraction 

 of the shell of matter, whose thickness is Included between the 



