period is 10-45 years; but that it appears to undergo a variation between 8 and 12 years in an interval of 42 years. The small maximum of 1797-7, if a true result, may be expected to repeat itself at some future time, a result which could not fail to aid in the search for the cause of these variations.

The author shows, that according to the long period of 42 years, a maximum should have happened in 1776; but that year Dr Wolf has concluded to be one of minimum sun-spot frequency. That 1776 was really a year of maximum is confirmed by the observations of Van Swinden, who, it is shown, appears to have been the first to obtain a variation due to the decennial period, and to have pointed out the appearance of a law: it is also confirmed by the observations of Cotté at Montmerency.

The ratio of the ranges of the diurnal variation in the years when it is a maximum to that in the years of minimum, is compared for different parts of the world, and found nearly the same in both hemispheres. It is also found that the law of the diurnal variation is the same in the year of maximum as in the year of minimum. The author concludes that the increase of the diurnal variation is not due to a different cause from that which produces the variation at the minimum, and that this cause acts when there are no sun-spots in the same way as, though with less intensity than, when the latter have their maximum frequency and area. The magnetic variations are therefore not due to the sun-spots; the latter appearing only when the common cause produces diurnal variations having at least two-thirds of the maximum amplitude.

The results derived from the sun-spot area are compared with those from the magnetic observations. While a general agreement is found in the decennial variations from year to year, it is evident that the attempt to calculate the amplitude of the diurnal variation from the sun-spot frequency (as has been done by Dr Wolf) must give results frequently deviating widely from the truth, as might be expected from the previous conclusion.

The decennial period of disturbance of magnetic declination at Trevandrum, deduced from hourly observations in the eleven years, 1854 to 1864, is next considered. It is found that the mean disturbance at each hour of the day shows the decennial period; but that the range of the mean value, from the minimum to the maximum

year, is different for each hour, while the maximum and minimum do not happen at exactly the same time for all hours of the day. Secondary maxima and minima are also shown, which vary in their epochs gradually from midnight to noon.

No clear law appears to connect the amount of the maximum disturbance for any hour with that of the minimum for the same hour in the 11 years; the ratio of the first to the second is least for the hours near noon, and greatest for those near midnight. It is found, however, that the maximum and minimum mean disturbance in the diurnal variation for each year, as well as in the decennial variation for each hour, are connected by the following relations :-D being the maximum and D, the minimum disturbance.

The monthly mean disturbance at Trevandrum in each of the years 1854 to 1864 is compared with the monthly mean sun-spot areas deduced by Messrs De La Rue, Stewart, and Lowy, from Carrington's and the Kew Observations, with the following result:-The monthly mean disturbance in the years 1854-56 had a considerable value, and marked variations when there were few or no sun-spots. In 1857 to 1862 there are found several maxima and minima of disturbance and sun-spots which occur simultaneously. In some cases, and especially in June 1862, there is a well-marked sun-spot area maximum without any corresponding change of magnetic disturbance. The cause of the solar disturbance did not then extend its action to the earth at that time.

The author concludes with a notice of the hypotheses proposed to explain the decennial period of magnetic variations and of sunspot frequency, as well as of the cause of sun-spots. It is pointed out that no theory of sun-spot formation can be accepted which does not explain their non (or very rare) appearance every 10 or 11 years, and therefore the cause of the decennial period is bound up with this explanation. A planetary action which disturbs the equilibrium of the solar gases has been proposed; no other seems to present itself, and this the author believes will be found ultimately to be in question; and though he has not himself been able to find any satisfactory evidence in its favour, yet remarkable results have been obtained by Messrs De De Rue, Stewart, and Loewy.

4. On the Parallel Roads of Lochaber. By David Milne

Home, LL.D.
(Abstract.)

The author referred to the papers written on the subject, beginning with that by Dr Macculloch, in the year 1817; and he explained the various theories suggested.

He intimated his adoption of the Lake theory, and expressed his adherence to the view he took in the memoir read by him in this Society in the year 1846, that the blockages of the lakes had been effected by detrital matter.

In support of this view, he pointed out that all over this district of the Highlands there were immense beds of clay, sand, and gravel up to the tops of the hills, at even 2000 feet above the present level of the sea.

These deposits he considered to be undeniable proofs of the prevalence of the sea over this part of the earth's surface to a height of 3000 feet at least.

When the sea retired, so as to expose to atmospheric action the higher parts of the country, there would be depressions in the surface of the land, where lakes would be formed. These lakes would continue at high levels, till the streams issuing from them cut through the detritus. In some cases, the process of erosion would be so gradual, that the lakes would subside without producing any conspicuous beach-lines on the mountain sides. In other cases, the removal of the blockages would be on a large scale, owing to the looseness of the detritus; and if these removals were separated from one another by a considerable interval of time, beach-lines of a permanent character would be formed on the sides of the mountains enclosing the valleys.

The author referred to the existence in this district of the Highlands, now, of several lakes at high levels, which were kept up by detrital blockages. He instanced, in particular, Loch Earba, in the Lochaber district, at a height of about 1150 feet, which was kept up by such means, and on whose banks there was evidence that the lake had once stood 30 feet higher than at present. Near Kingussie there was Loch Gwynae, at a height of 1015 feet above the sea, on whose sides there were traces of five terraces, the highest of which is 132 feet above the present surface of the lake.

VOL. IX.

Y

He referred to the ample means of cutting through and removing detrital matter possessed by streams and rivers, mentioning particularly the enormous cliffs of detritus cut through by the Rivers Spean and Spey.

He next proceeded to discuss the theories of other geologists.

With regard to the theory that the parallel roads were formed by the sea, he adduced arguments to show, that this view was impossible, inasmuch as the "roads" should in that case have all stood at the same level; whereas, in the different glens, they stood at different levels. Moreover, it had been found, that old river courses existed, by which the water in Glen Gluoy flowed into the water in Glen Roy, and that the water in Glen Roy flowed through Glen Glaster into Loch Laggan, a state of things utterly fatal to the marine theory.

With regard to the blockage of the lakes having been formed by ice, the following objections were stated:

1st, The improbability that some of the glens were filled with water, whilst others were filled with ice, the temperature of those glens being all much the same, in consequence of nearly equal altitudes above the sea.

2d, The impossibility of getting a glacier to come to the exact spot, where the lakes stopped, to form barriers several miles long, so solid and permanent in structure, as to prevent the escape of the water from lakes above 300 feet deep.

The author coucluded by referring to the numerous examples in the Lochaber district, of boulders perched on tops of hills, and of rocks smoothed and striated. These phenomena had been ascribed by some geologists to the action of land-ice. But, coupling with these high-perched boulders, the occurrence of kaims or eskars on the sides of the hills (above the parallel roads), and therefore formed before the Lake period, the author was inclined to ascribe these phenomena to one agent-viz., a sea loaded with ice, when the land was submerged, and to a strong current in the sea, from the north-west, which swept over the submerged land, and through such valleys as Glen Spean and Glen Roy. The lakes, he referred to the period when the land was rising out of the sea. Their beaches were formed on the marine detritus;-which also for a time dammed back the lake waters.

Monday, 5th June 1876.

D. STEVENSON, Esq., C.E., Vice-President, in the Chair.

The following Communications were read:

1. Physical Observations in Northern Asia. By
Professor G. Forbes.

2. On Parallel Motions. By the Rev. John Wilson, M.A., Bannockburn Academy. Communicated by Professor Kelland.

It has been well said that the transmission of force is an essential condition in machinery." It is no less true that directness in transference is important; that the fewer links in the chain binding driver and follower together, the less likely is the machine to be put out of gear. There is no question here as to the comparative values of the different modes of conveying motion from a prime mover, rolling contact, sliding contact, wrapping connectors, or linkwork. Each has its own excellencies; each its special advantages; and one is to be preferred to another only according to the nature of the work to be done.

I. Watt's Parallel Motion.-The general problem is the "commutation of circular with rectilinear motion." The importance of the question began to be felt soon after the introduction of the steam-engine; and Watt, in 1784, patented an invention which not only had the credit of being the earliest, but up to recent times, the most reliable and accurate parallel motion in existence. This system was a great advance on the huge chains and arches which were affixed to the working beam of the engine for the purpose of obtaining the desired motion; and it has proved to be sufficiently accurate for all practical purposes. The construction is simple, consisting of three bars: two, rotating round fixed centres, and connected at their other extremities to the third bar. A point in this bar, either within or without the points of junction, accord