But perhaps the most interesting observations and experiments were those made to determine the origin of the ‘dewdrops’ on grass and vegetables. In the first place, it is found that these drops do not appear on all plants. Some are wet, while others growing alongside are dry, though there could be no great difference in their radiating power. Then the leaves do not get wet all over, but only at the edges and on the tips. A closer observation reveals the fact that these so-called ‘dewdrops’ are formed at the end of the minute veins of the leaves and grass, and are not now recognised as dew at all, but moisture exuded from the interior of the plants themselves. Moreover, these drops always appear before the true dew in the evening, and very often are seen when no true dew is formed. They even appear when the vegetables are placed under conditions where condensation of the surrounding water-vapour is impossible, and must, therefore, be due to the vital activity of the plants.
Another observation may be mentioned which clearly shows that moisture rising from below may become condensed on the cooled surfaces of loose material. If the weather is at all cold, the beard and moustaches get covered with moisture; and in very cold climates, the eyebrows, hair, and whiskers get covered with a coating of hoar-frost. The moisture which forms this certainly comes from the body, which is always at a much higher temperature than the surrounding air.
All these observations and experiments have led to the conclusion that moisture is constantly being given off from the earth; and that, except on the rare occasions when a warm moisture-laden wind blows gently over a previously cooled surface, it only returns to the surface of the ground after being condensed into rain, sleet, snow, or hail. Dew is only a portion of the outward current trapped on the exposed and cooled surfaces of the grass and other bodies.
COMRIE EARTHQUAKES.
Regarding earth-tremors or earthquakes, which, curiously enough, seem to be mainly confined in Scotland to Comrie, in Perthshire, a correspondent writing from Comrie kindly favours us with the following notes as to the erection which is there devoted to the registering of earthquakes. Our correspondent says:
I recently visited the building with a view of giving you a few notes as to its history and construction. I may state that about fourteen years ago, the British Association applied to Mr Drummond of Drumearn for leave to erect a house on his property, which he at once granted free of charge, and assisted to defray the cost of erection.
The reason why the British Association selected a site here and erected this earthquake-house at Comrie, was on account of the long-continued periodical shocks that had been felt in Upper Strathearn, particularly from the year 1780 to 1848. About the former date, they had been rather severely felt over the whole district, and damage to some extent done to buildings. On a sheet of water near to Lawers House, the ice was shattered to pieces. Some of the inhabitants at that time kept a record of their occurrence; and we believe the late Sir David Dundas, of Dunira, had a seismometer placed on his estate in Glenlednoch, to the north of Comrie; but there seems to be no evidence to show that it had ever indicated any shock. Coming down to the year 1839, the inhabitants of the village of Comrie were greatly alarmed, about eleven o’clock on the night of the 23d of October, by one of the most violent tremors that had been experienced there; and the good people rushed out of their houses and assembled in the old Secession Church for prayer, which was conducted by the Rev. R. T. Walker, the minister of that church. Many others fled to the hills. But no serious damage was done to property, save some rents in the chimneys. From 1839 to 1847, tremors continued to be more frequent, causing considerable alarm by the movements of furniture and crockery.
The work of erecting the building proposed by the British Association was carried out under the care of the late Dr James Bryce of Glasgow, who resided here for many seasons, and was well acquainted with the locality and its geological formation. The site chosen is a rising ground near Drumearn House, and is built on rock that is supposed to extend a considerable distance westward. The building is stone, and slated, and is about seven feet square inside. The floor is laid with Arbroath pavement, on solid rock, and is overlaid with fine sand, on which are placed two boards, at right angles to each other. These boards are six feet long by nine inches broad, and on each are placed, standing, nine round wooden pins, varying from the fourth of an inch to an inch and a half in diameter, but all of one height (eight inches).
The building is in excellent condition, and the pins or markers are in their places, awaiting the action of an earthquake to record the desired information as to the severity and direction of this now seemingly extinct agency of force in Upper Strathearn. The size of pin or cylinder thrown down, and the direction in which it falls, indicate the strength of shock as well as its direction. Any one who feels interested and may wish to visit the building will readily get access by applying to Mr Drummond.
Many theories have been propounded as to the cause of the earthquakes which have visited this district. The late Mr Patrick M‘Farlane of Comrie, who took a great interest in them, erected a seismometer in the steeple of the parish church of Comrie, which was visited by many of the members of the British Association and others; but so far as we are aware, it never registered any markings. It was a very simple apparatus. The pendulum was of considerable length, and all but rested on a table overlaid with magnesia, which, being light, offered no resistance to the oscillation of the pendulum. A few slight shocks occurred between 1847 and 1877, but these attracted little notice.