higher sound as the rotation increases in rapidity; and it is even according to the height of the sound which the apparatus gives out, that we can judge of the velocity of the rotation. The Intensity of Light.-By the intensity of light is understood the quantity received on a unit of the surface of an illuminated body. This intensity is regulated by the following laws: 1st. The intensity of light received on a given surface is in the inverse ratio of the square of its distance from the luminous source. 2nd. The intensity of light received obliquely on a given surface is proportional to the sine of the angle which the luminous rays make with the illuminated surface.

The first law is proved by reasoning of the same kind as that employed in the demonstration of the first law relating to radiant heat. It may also be proved experimentally, by comparing the shadows projected on ground glass by an opaque rod, illuminated at the same time in one direction by one candle at a given distance, and in another direction by four candles at double that distance; the experiment being arranged as shown in the diagram representing the photometer of Rumford, where the two shadows have the same intensity, and the law is demonstrated.

one of them be more intense than the other, say that which proceeds from the light м, we draw the instrument nearer the other light, until the two bands present the same luminosity. Then, measuring the distance of the photometer from each of the lights, their intensities may be determined by the same rule as before. This small apparatus is of very easy application, and has been adopted in several gas manufactories to compare the intensities of the gas burners. Fig. 250.

comes to help him, and they take 3 days more to finish it: in
what time would they have done the whole, each separately,
or both together? Ans. 18 days, 10 days, and 6 days.
Prob. 50. Divide £149 among A, B, C, and D, so that A
may have half as much again as B, and a third as much again
as B and C together; and D a fourth as much again as A and
C, together. Ans. A £48; B £32; C £4; and D £65.

Prob. 51. There are two silver cups and one cover for both. The first weighs 12 oz., and with the cover, weighs twice as much as the other cup without it; but the second with the cover weighs a third as much again as the first without it: find the weight of the cover. Ans. 63 oz.

Prob. 52. A man could reap a field by himself in 20 hours, but with his son's help for 6 hours he could do it in 16 hours; how long would the son be in reaping the field by himself? Ans. 30 hours.

Prob. 53. A grocer bought tea at 6s. 6d. per lb., and a third as many lbs. again of coffee at 2s. 6d. per lb.; he sold the tea at 8s. and the coffee at 2s. 3d., and so gained 5 guineas by the bargain; how many pounds of each did he buy? Ans. 90 lbs. tea, and 120 lbs. coffee.

Prob. 54. Find a number composed of three digits, each greater by unity than that which follows it, so that its excess above one-fourth of the number formed by inverting the digits shall be 36 times the sum of the digits. Ans. 654.

would amount to £54; and if B's present sum were trebled, it would exceed three times the difference of their original sums, by £6. What had each at first? Ans. A £40, and B £28; or A £28, and B £52; according as A had more or less at first than B.

Prob. 66. Two girls carried between them 25 eggs to market; they sold them at different prices, but each received the same amount upon the whole; at the prices they charged, the first would have sold them all for a shilling, and the second for 13d. How many did they each sell? Ans. 13 and 12.

Prob. 67. A boat's crew rowed 3 miles down a river and up again in 100 minutes; supposing the stream to have a current of 2 miles an hour, find at what rate they would row in still water. Ans. 5 miles an hour.

Prob. 68. A merchant who allows £160 for his annual expenditure, increases his property every year by a fourth part, and at the end of two years is £900 richer than at first: what property had he at first? Ans. £2,240.

Prob. 69. A sold a certain number of tickets at a guinea each, and gave one-third of the produce to B; one-fourth of' the remainder to C; and one-fifth of the last remainder to D; after which he had £210 remaining; how many did he sell? Ans. 500.

Prob. 70. Solve the general simultaneous equations to three unknown quantities, denoting the co-efficients of x, y, and z, Prob. 55. A and B have each a sum of money given them, in the first equation by a, b, and c, and the absolute term by which will support their families for 10 and 12 days respec-d; the co-efficients of the same three quantities in the second tively; but A's money would support B's family for 15 days, equation, by e, f, and g, and the absolute term by h; and the and B's money would support A's family for seven days, with co-efficients of the same three quantities in the third equation 2s. 6d. over; what were the sums? Ans. 258. and 20s. by i, k, and l, and the absolute term by m; and let all the terms in the three equations be positive. Ans. dfl-bhlbgm-dgk+chk—ofm afl-bel+bgi-agk+cek―efi ahl-del+dyi-agm+cem - chi y=afl—bel+bgi—agk+cek—cfi afm-bem+bhi-ahk+dek—dfi afl — bel + bgi—agk+cek—cfi

Prob, 56. A person being asked how many ducks and geese he had in his yard, said, if I had 8 more of each, I should have 8 ducks for 7 geese, and if I had 8 less of each, I should have 7 ducks for 6 geese; how many had he of each? Ans. 120 ducks, and 104 geese.

Prob. 57. A man, woman, and child could reap a field in 30 hours, the man doing half as much again as the woman, and the woman two-thirds as much again as the child; how many hours would they each take to do it separately? Ans. Man 62 hours; woman 93 hours; and child 155 hours.

Prob. 58. From each of a number of foreign gold coins a person filed a fifth part, and had passed two-thirds of them, gaining thereby 35s., when the rest were seized as light coin, except one with which the man decamped, having lost upon the whole half as much as he had gained before; how many coins were there at first, and what was the value of each? Ans. 12 coins; 21s. the value of each.

Prob. 59. A and B start to run a race; at the end of 5 minutes, when A has run 900 yards, and has outstripped B by 75 yards, he falls; but though he loses ground by the accident, and for the rest of the course makes 20 yards a minute less than before, he comes in only half a minute behind B. How long did the race last? Ans. 36 minutes.

Prob. 60. A and B can reap a field together in 12 hours; A and C in 16 hours; and A by himself in 20 hours: in what time could B and C together reap it; and in what time could A, B, and C, together reap it? Ans. B and C in 21 hours; and A, B, and C in 1049 hours.

Prob. 61. Fifteen guineas should weigh 4 oz.; but a parcel of light gold, having been weighed and counted, was found to contain 9 more guineas than was supposed from the weight, and it appeared that 21 of these coins weighed the same as 20 true guineas: how many were there altogether? Ans. 189. Prob. 62. A, B, and C, travel from the same place at the rate of 4, 5, and 6 miles an hour respectively; and B starts 2 hours after A; how long after B must C start, in order that they may both overtake A at the same moment. Ans. 1 hours.

Prob. 63. Two men can do a piece of work in 12 days, and one of them can do half as much again in 24 days: in what time could the other do a third as much again? Ans. 64 days.

x=

2=

LESSONS IN GEOLOGY.-No. LV.

BY THOS. W. JENKYN, D.D., F.R.G.S., F.G.S., &c.
CHAPTER V.

THE CLASSIFICATION OF ROCKS.

SECTION VIII.

THE LIA S.

THE group of rocks which lies under the oolites is called the LIAS, a name which is probably derived from the provincial corruption of the word Layers, on account of the striped or ribband-like appearance of the beds in a lias quarry.

The lias rocks, though classed by some geologists with the oolites, have every claim to be regarded as a separate and independent formation. It is true that in some places, as in the neighbourhood of Bath, the lias appears to pass into the oolite, by means of a sandy marl, which lies geologically between it and the oolite, but which unites in its composition the character of both the upper lias and the inferior oolite, and has also the fossils which are common to both. Yet, as the whole mass s marked by a uniform lithological structure, and contains many organic remains peculiar to its own beds we shall consider it as a distinct group.

I. THE LITHOLOGICAL CHARACTER OF THE LIAS. The lias formation, as developed in Gloucestershire, is divided into four beds.

1. The Upper Lias Shale: with nodular concretions and beds of limestone, abounding with remains of reptiles and fossil

Prob. 64. A messenger starts with an errand at the rate of 3 miles an hour; another is sent half an hour after to over-shells. take him, which he does in 2 hours: at what rate did he ride? Find also in what time he will do it, if he rides 12 miles an hour. Ans. 4 tiles an hour; and 13 minutes.

2. The Lias Marlstone: sandy, limy, and ferruginous, rich in fossil wood, ferns and shells.

3. The Lower Lias Shales: with interlaminations of sands, Prob. 65. If A's money were increased by half of B's, it clays, and limestone nodules.

This tabular statement shows that the lias, as a geological formation, is a clayey and limy deposit, sometimes having a great abundance of marls, and sometimes rich in limestone, especially in the lower beds of the group. As a mass it may be said to consist of stratified marls, blue or gray; of various clays; and of a peculiar limestone; the thickness of the whole beds varying from 500 to 1000 feet.

4. The Lias Limestone: in laminated strata, with partings of however, like No. 15 of the same woodcut, abounded, and clay, of striped ribband-like appearance. have been found fossil plentifully at Lyme Regis. Large Ophira, also, have been found there, and in the shales of Gloucestershire and the marlstones of the Yorkshire coast. 3. The beds of the lias are rich in fossil shells of a great variety of species, but particularly of the Nautilus, the Ammonite, the Terebratula, the Belemnites,-the last of which are so remarkable for their ink bags. The Belemnite was an animal very much like the cuttle fish, except that it had other apparatus like that of the nautilus and the ammonite. Around its head it had eight arms, and each arm was provided with fifteen or twenty hooks, which were used to pierce the flesh of fishes, that it might have firm hold on its prey. It lived in a shell; and within the cavity of the shell, there was an oval bag or bladder containing black fluid, like ink, which the animal could squirt out through a tube or quill. When the animal was alarmed or pursued by an enemy, it let out this In the south of England, the lias first develops itself ink to darken the water, and thus effect its escape from its at Lyme Regis, in Dorsetshire, whence it passes through pursuer. The very ink itself now petrified, and the bag which Somersetshire to Gloucestershire. About six miles below contained it, and also the tube or pen by which the ink was Gloucester, one branch of it turns to the west, and in a tor-shot out into the water, are all preserved in some of the specituous direction passes through Monmouthshire, to the lower mens of this fossil.

The lias may generally be known by its external aspect in a quarry, or in a railway cutting; for everywhere the rocks appear as an alternation of thin beds of limestone, weathered into a light brown colour, and each bed being separated from the other by seams of dark-coloured clay. The prevailing colour of the lias limestone is blue, and hence the name "blue lias" but among its lower series there are beds of a yellowish white colour, which is called "white lias."

"

parts of the vale of Glamorganshire, where it supplies the 4. The greatest marvels of the liassic fossils, are the remains famous hydraulic or Aber-ddawon lime; the other branch of gigantic saurians or lizards, a species of marine reptiles. takes a north-eastern direction through Gloucestershire, Wor- Two monsters of this class have been called, one the Ichthyocestershire, Warwickshire, Leicestershire, Lincolnshire, and saurus, from xus, ichthys, fish, and oavpa, saura, lizard, and Nottinghamshire, to Whitby in Yorkshire, where it disappears means a fish-lizard; and the other the Plesiosaurus, from under the moorlands of the coast. There is a remarkable λeσLov, plesion, akin to, and oavpa, saura, a lizard—lizardoutline of the lias on the road between Wem and Witchurch, in Shropshire.

In England, the lias is one of the most extensive of the secondary rocks, and is also one of the most useful. Several of the lias beds afford valuable slabs for paving the floors of houses and flagging the sides of streets. As some of these slabs can take a tolerable polish, they are frequently worked for ornamental purposes. The remarkable stones which, when polished, present diminutive trees and shrubs as if growing in clay, and hence called "landscape stone," or Cotham marble, are found in detached masses of this formation. Lias lime has the valuable property of hardening or setting under water, as in the foundation of bridges, wharfs, and sea walls. In some places the lias clays are very bituminous, and contain iron pyrites, the decomposition of which produces the alum shales of Whitby on the Yorkshire coast. This abundance of bitumen in the lias shales has led many persons to fruitless sinkings for coal.

On the Continent, especially in the north and south coast of France, in Switzerland, and in Germany, the lias is largely developed, but with some variations from the English type. In the neighbourhood of the Vosges mountains, in France, the lias consists of a yellow and quartzose sandstone, containing mica, a few clayey nodules flattened, and small quartz pebbles of a white and black colour. In Westphalia and Bavaria its lower beds are so sandy as to form a good freestone for building materials.

II. THE ORGANIC REMAINS OF THE LIAS.

like.

The Ichthyosaurus was a monster lizard, being occasionally more than thirty feet in length. Our next engraving, fig. 8, p. 292, will give you a good idea of the structure and the size of these animals.

The general outline of the Ichthyosaurus is supposed to have resembled a porpoise, but with considerable modifications; for while it had the mouth of the porpoise, it had also the teeth of a crocodile, the head of a lizard, the paddles of a whale, and the back-bone of a fish. Its teeth were very numerous, conical, sharp, and striated like those of the crocodile. Its eye was enormous like a wheel, fourteen inches in diameter, which must have given it an immense extent and power of vision. Its jaws and teeth show that the animal was carnivorous, or lived on flesh; and the expansion of its jaws must have been great and terrible, as the length of its jaw is sometimes six feet. Its whole structure was adapted to aquatic life, but also destined to breathe atmospheric air, like our present whales. Ten species of this lizard have been found in the lias.

Some of the fossil remains of the Ichthyosaurus have presented us with a specimen of the stomach and the abdominal region of the animal, as represented in the annexed engraving, fig. 9, p. 292.

The stomach of this monster is loaded with coprolitic matter and with fish scales, all of which have been converted into stone. The specimen represented, in our last figure is in the geological museum of Oxford. The length of its stomach seems to have been nearly co-extensive with its trunk. The capacious stomachs of these reptiles remind us of the crocodile

The lias of England is distinguished from all the liassic-for as human bodies have been found whole in the stomachs beds of Europe, by the unrivalled assemblage of organic ren ains that it has supplied, some of which are of the most extraordinary character.

1. The vegetable fossils consist of gigantic reeds and different species of wood. The strata, sometimes, exhibit the reeds standing erect as they grew, but generally broken off, and parts of them lying flat and compressed, and leaving casts in the sandstone. Fragments of the wood of coniferous plants have been found at Whitby, and several species of zamia, and ferns, at Lyme Regis. These fragments of wood are in many instances charred, in others converted into stone, coated with agate, or penetrated with the oxide of iron. At Whitby, trunks and branches of trees appear in the alum shale, having the bark and softer parts converted into jet, while impressions of leaves, like those of the palm, are found in the sandstone and ironstone.

2. The lias does not abound in corals; though in some parts of Warwickshire, beautiful masses of Astrea, like No. 11, in fig. 7, of the last lesson, have been found. Encrinites,

of crocodiles, the jaws and the form of the teeth of the Ichentire. The fossil contents of their stomachs, also, have the thyosaurus indicate that these animals gorged their prey bones of smaller ichthyosauri-bones derived from individuals that must have been several feet in length; we infer that these reptiles were exceedingly voracious, that their stomachs were of prodigous size, and that they devoured even their own young.

The Plesiosaurus was an animal, as its name implies, more akin to a lizard than to a fish. It had the head of a lizard, the teeth of a crocodile, the body of a serpent, the tail of a quadruped, the paddles of a whale, like the Ichthyosaurus, but it was distinguished by a neck of enormous length. This long neck enabled it, probably, to dart its head suddenly upwards to catch birds or insects that came within its reach, or to plunge it downwards to pounce on the fish in the water below it.

The habits of these two monsters were different, though both of them were adapted to aquatic life. The Plesiosaurus lived in shallow creeks, and bays, and estuaries, and floated

on the surface of the water, but the Ichthyosaurus inhabited no time to decay. This is particularly and strikingly proved the depths of the ocean.