expected. When swallows and swifts glide about at great heights, fine weather will probably continue. Cats, being highly electrical creatures, are readily influenced by coming changes, and usually rub themselves frequently, especially behind their ears, when bad weather is coming. Many other creatures, such as crickets, frogs, bats, &c., may be watched with advantage as indicators of weather. From very ancient dates down even to the present day, many persons are under the impression that the moon's changes are the cause of the changes in the weather, and immediately the latter is spoken of, they refer to the date of the former. Any influence which the moon produces ought to influence a whole district alike, and ought to produce the same effect on so small an area as England. Experience, however, shows that often when it is calm and fine say at Dover, it is wet and windy at Plymouth, showery at Liverpool, fine at Hull, and so on. Forgetting these facts, imperfect observers have often concluded that because it began to rain where they were immediately after full moon, therefore the full moon caused this, whereas a few hundred miles off directly opposite results occurred. Thus a full inquiry appears to have resulted in the conclusion that little, if any, effect can be attributed to the moon. Many kinds of barometers are in use at the present time, and indicate with more or less accuracy the coming atmospheric changes: the best of these is the Aneroid Barometer, which is now made with considerable accuracy. The large Mountain Barometer, which has a long column of mercury, is also a very sensitive instrument. A barometer should be placed in a convenient position, where it may be seen at any time of day and with little trouble. It should be set every morning and evening, and a memorandum made of its height. The average height of the barometer in England, at the sea level, is about 29.95 inches; when we rise 100 feet the barometer falls about one-tenth of an inch, and vice versa. The barometer is in itself a study, and its changes require careful observation. To such as desire a knowledge of this instrument we recommend a study of "The Weather Book," by the late Admiral Fitzroy. The heights of mountains can be obtained with fair accuracy by means of the barometer. Two readings are taken, one at the lower of two stations, the other at the upper; the difference in altitude can then be found by the following rule, where D represents the vertical difference in feet: where L represents the reading at the lower station, u the reading of the barometer at the upper station. Thus, if the reading were 29.800 at the lower, and 29653 at the upper, we should have the following: which gives 136 feet for the difference of level between the two stations. The weather in England is much affected by what we term an average, that is, there are about the same number of wet days every year, and about the same amount of heat each year. Thus, if during January, February, and March, for example, it is very wet and cold, much more so than usual, then we may expect in April or May warmer weather than usual; so that, by remembering past weather, we may predict that which is to come. To the various kinds of clouds have been given names, such as Cirrus, Stratus, Nimbus, and Cumulus. Cirrus is a light fleecy cloud, usually seen after fine clear weather. It is sometimes called “mares' tails," it being like hair or feathers. Stratus is a smoke-like cloud, very common, with ill-defined edges. Rain does not fall from this, though it often conceals the stars or dims the sun. Nimbus is the heavy-looking soft shapeless cloud, which may well be termed the "rain-cloud." Cumulus is a hard-edged, well-defined cloud, the outline of which may be well traced. It usually happens that these clouds are mixed, and may then be described as Cirro-Stratus, signifying a mixture of cirrus and stratus; Cirro-Cumulus, a mixture of cirrus and cumulus; Cumulo-Stratus, &c. Sometimes, for brevity's sake, only the first or first and second letters are used, and C or Ci stands for cirrus, Cu for cumulus, and so on. For a few shillings a barometer and thermometer may now be procured, and observations may be made with these morning and evening, so that, by comparing these with the changes that follow, we may become very accurate foretellers of the weather, and may often save ourselves a wetting, or avoid starting on a journey in waterproof when, half an hour after leaving home, the sun shines brightly on us. SUN-DIALS. The very earliest form of clock or time-measurer was a sun-dial. We read of the sun-dial of Ahaz in Scripture, whilst among ancient nations, such as the Indians, Chinese, &c., sun-dials were common. The principle of the construction of sun-dials depends upon the fact that during twenty-four hours the sun appears to move round a circle part of which is above and part below the horizon, and to return at the end of twenty-four hours to the same point from which it started. If we resided at the North Pole of the earth, the construction of a sun-dial would be extremely simple. We should, when the sun was visible, find it moving round the horizon at a nearly uniform height, and we should then say that, as the sun moved round the horizon in twenty-four hours—that is, round 360°-it would move over 15° in one hour, and over 1° in four minutes of time. Thus, by dividing 360 by 24, we obtain this result. A common circle, therefore, with an upright in the centre, would be our sun-dial. When, however, we are situated in any other part of the world, the lines which are equally distant from each other at the poles will be in some cases farther apart, and in other cases nearer together. The following diagram will aid in making this fact manifest. Suppose P and S to be the north and south poles of the earth; a, b, c, d, e, f, and g, meridians or divisions 15° apart. These lines, as they cut the equator, will be equidistant from cach other, but they will vary in distance when they cut the circle A B C D. A B C D is the circle suitable for a latitude equal to the distance of a from the equator; and the distances apart of the various figures, I., II., III., &c., may be calculated by spherical trigonometry. It being our object, however, to give the most simple means of making a sun-dial, we shall avoid all calculations which may require mathematical knowledge, and deal only with the subject in an easy, practical manner. Thus we will suppose a sun-dial is required to be made for any part of Great Britain or Ireland, and we will proceed to make one. First examine a map of the county, and look down the side and find what is the latitude of the place on which your sun-dial is to be placed. Suppose this latitude to be 52. Take a piece of cardboard like the annexed, and about the position s make a mark, and draw a line, S O, parallel to the sides of the card. The size of this card may be about eight inches or a foot square. Next take a common protractor, or instrument for setting off angles, and from S on each side of S O set off angles of 12°, such as are represented by the lines S T and S R. Again, from the line so set off angles of 24° 37' in the same manner as the preceding, and draw these lines as shown by S P and S L. Again, set off 38° 25', and 53° 57', and 71° 20' on each side of s o, and finally 90°. Upon referring to the table at the end of this article, these angles will be found opposite 5230 of latitude. If, however, the latitude were different from 5240, we should set off the angles shown opposite this latitude, instead of those we have mentioned: thus for 5120 of latitude 11° 51' would be the first angles set off, and 24° 19' the next two. P R L Having marked off the card, we next cut a triangle in the annexed form, making the angle C A B equal to the latitude of the place for which the sun dial is to be constructed. This triangle may be made of cardboard, brass, tin, &c., according as the dial is made of cardboard, tin, &c. с Fix the triangle on the dial so that the point a coincides with the point s on the dial, and the line A B coincides with the line s o, and take care that B C is exactly perpendicular to the dial. The dial is now completed, and is ready to be fixed in its position, which, for a dial constructed in this manner, is a horizontal position. A horizontal stand, a window-sill, or a post firmly fixed in the ground would do, care being taken that this foundation is horizontal. The line so must now be placed exactly north and south, and this may be accomplished in either of the following ways: A Set a watch by the nearest railway station to true Greenwich time; find from a map the difference in longitude between Greenwich and where you want to place your sun-dial. Suppose this difference to be 1° 16′ west. Multiply 16' by 4, and it becomes 64 seconds of time—that is, I m. 4 s.; also multiply 1° by 4, and it becomes 4 minutes of time, because degrees become minutes, and minutes of arc become seconds, according to the proportion of 24 hours for 360°: thus for 1° 16' difference of longitude there will be 5 m. 4 s. difference of time. The sun at twelve o'clock is due south, and it then casts a shadow from an upright object due north. To find when it is south, we allow for the difference of time due to longitude and for the clock being before or after the sun. In many almanacks (Hannay's and others) the allowance between sun time and clock time is given; and this, as well as the allowance due to longitude, being made, we obtain twelve o'clock by the sun at our position, and can then place the line S O so that the shadow of the triangle is over it. Example. On the 21st of November, 1867, I wanted to set a dial north and south by the sun at Liverpool. Having set the watch to Greenwich time by the railway, I found that there was 11 m. 30 s. difference of time due to the longitude, Liverpool being west of or after Greenwich. There were 14 m. 2 s. difference of time between the sun and a clock on the 21st of November, as shown by an almanack, the sun being before the clock. Therefore at Greenwich the sun would be south 14 m. 2 s. before twelve. At Liverpool it would be twelve o'clock when it was 11 m. 30 s. after twelve by Greenwich; but the sun would be south at Liverpool 14 m. 2 s. before it was twelve o'clock there. Hence, at 2 m. 32 s. by the watch set to Greenwich the sun would be south at Liverpool, and the sun-dial might then be placed north and south. Another method is to place a straight stick exactly upright by means of a plumb-line on a piece of level ground; at about nine a.m. mark where the shadow of the top of this dial comes to, and mark this spot: about three p.m. the shadow will reach the same length, and we can tell when it is the same length by marking out a part of a circle, the point of the stick in the ground being the centre and the length of the shadow the radius. The shadow gets shorter up to twelve o'clock, and then longer again till it reaches the part of the circle. Halve the distance between the two shadows of equal length, and join the point thus found with the point of the stick, and this line will be north and south, and the sun-dial may be placed north and south by its aid. When the sun-dial has been placed in position, it will then indicate sun time, and we must add or subtract the allowance due for the difference between sun time and clock time, in order to find the true local time. The following table, for the 1st and 15th of each month, shows the allowance to be made to sun time in order to obtain true time. 15th Ist " 14 24 15th 9 11 4 2 5536 6 4 20 neither add subtr. 4 44 subtr. 2 58 According to different years, the allowance for seconds will vary slightly. The dates between those given will require a proportional allowance to be made. Beyond the 90° or six hours on the dial, two additional angles may be set off for seven and eight hours p.m., these angles being equal respectively to those for five and four hours. When the sun-dial is completed and placed in position, as well as figured, it will be somewhat in the following form: VI Various other sun-dials may be constructed, but that described is the most simple, and is generally useful. Such a dial, fixed in the grounds or near the house in country localities, is a very good check on the clocks, and serves as a time-corrector. We have calculated the angles for several places in England, drawn the diagram on cardboard, and given it to a local `ironmonger to cut in brass. The dial has been erected at an expense of a few shillings, and has not only been a useful instrument as regards time, but has been an interesting ornament in the grounds. |