tion of these foreign substances in the air- | with the greatest possible advantage. A passages of the lungs, and the consequent committee of the Ladies' Sanitary Associexclusion of oxygen from the blood. ation has raised funds by which it has been Even the dust of ordinary rooms, from enabled all the fine summer weather to carpets, furniture, clothes, curtains, and send parties of poor children into the other things, becomes a source of impurity parks. Of the danger of keeping children of air in our houses, and adds to the de-in-doors I had a good illustration a few struction of health which goes on from weeks ago. the presence of carbonic acid.

One of the most common causes of impurity of air from these particles is the unconsumed carbon in the atmosphere of towns and cities. It is these particles which blacken linen and all white furniture, and the wool of sheep's backs. It exists in such quantities in London, that the air may be filtered through fine muslin, and pure carbon collected in considerable quantities. It is possible to strain the air of a house, and get rid of all these particles. I know one gentleman in the city who uses a steam-engine on the premises for the purpose of forcing the air through metallic sieves before it enters his house. The consequence is, that directly you enter his door, the air has all the purity of that at the sea-side or the mountain-top; and instead of the oppression which all London air gives, you feel invigorated. This shows what may be done, even in the heart of the city. The fact is, air is like water: you may contaminate it by suspending impurities in it; but when these are withdrawn it remains as pure as ever.

I had occasion to compare the health of two streets, one a street with well-to-do artisans and small tradesmen, the other a tumble-down street where lodged the very poor. To my great surprise, the children of the very poor were less sickly and died less than those of their better-off neighbors. On examining the mothers of these families, I got what I think was a satisfactory explanation. The mothers of the poor children confessed that their children were seldom or never in-doors; but few of them went to school, and they consequently spent their days in the street. The more opulent class kept their children out of the street and sent them to school. Of course, no rule can be laid down as to the number of hours people ought to keep in the open air, but there can be no doubt of the soundness of the advice-" Get as much as you can." Get it for yourselves, get it for your neighbors. Let the government, let corporate bodies, let munificent individuals do what they can to tempt men and women into the parks of great towns and neighboring The question then comes, if impure air fields. Above all, let there be attractions. is so dangerous, how are we to render the sufficient to draw men and women from air we breathe pure? How can we get the public-house, from the dancing-saloon, fresh air? In the first place, every one and other vicious places, where, in addition should be impressed with the fact that the to the poisoning atmosphere, there is the open air must always be more pure than poisonous drink and poisonous morality. the air of houses, or any confined space Would that in England a taste for light whatever. The atmosphere in Cheapside refreshments could be given to the popu is infinitely purer than any inhabited draw-lation, so that tea and coffee, with honest ing-room at the west end of London. As nutritious viands, could be substituted for far as fresh air is concerned, a party of the present system of drinking beer and ladies and gentlemen would be more healthfully occupied in looking at the omnibuses from the curbstones in Fleetstreet than in the most elegant diningroom in Belgravia.

The night air of Houndsditch is freer from carbonic acid than the sleepingrooms of Mayfair. Hence the importance of getting as much into the open air as possible. Children, provided they are warm, can not be too much in the open air. It is a most merciful act to take little children from their close homes into the open parks; and this has been done in London

gin-a system that annually destroys hecatombs of our hard-working, honest, intelligent artisans. It is especially on those whose occupations are sedentary, and to whom fresh air is most necessary for health, that this destructive habit entails its greatest evils.

A more difficult thing to do is to keep the air of houses fresh. The multitudinous things it involves, and its apparent simplicity, are the great difficulties with which this practice has to contend. We call the act ventilation, and most intelligent people believe their houses are

ventilated. If they did not they could | it is precisely those who take least exer

not rest a moment. They would not lie down in their beds at peace one night if they thought the evils I have spoken of as resulting from want of fresh air were coming on their families. Nevertheless, I will put this question to them: Do you believe for one moment that with your closed windows and doors, with your brick drains or your cesspools, with your dust-bins, and your dirty (I mean no ill compliment-it is too true) furniture, that the air of your rooms is pure? The air of London is dirty and impure enough, but what is it as it passes from your window crevices, the key-holes of your doors, and the tiles of your house? Dirtier and more impure than ever. If you say it is not impure, you are wrong; if you know it is impure and talk of the ventilation of your house, it is cant.

cise or go out least that suffer most.

The same arrangements in houses that secure the influx of oxygen from without, and the efflux of the carbonic acid from within, also secure the escape of those solid particles which are so injurious when contained in any considerable quantity in the air. It is a well-known fact, that you may so dilute the poison of various fevers, as they escape from the bodies of those attacked, that no one shall be injured by it. If you place one patient with fever in a large ward, no other patient gets the disease; but if you place several fever patients in the same room, then every person that enters may catch the fever. So it is with the poisons of drains and cesspools. If they be well diluted in the open air nobody suffers, but let them con

dangerous, but no wise man would allow his drains or cesspools to leak into his house. But how many men in a thousand see to these things? how many women? how many servants? My experience tells me very few. This accounts for the faint odors and sickening smells that so often salute you in the houses of the rich as well as the poor; of the medical man, who has yet to learn how to apply the laws of physiology to the maintaining the health of his own household, as well as the poor mechanic, who is alike ignorant of the cause of the unhealthiness of his family, and powerless to remove it if he did. And yet, how angry people look if you tell them their houses are "nuisances, injurious to health." They believe in fresh air, they talk of the advantages of fresh air, but they have yet to learn how little they have of it at home, and how much more of it they need if they would secure the health and strength their Creator intended they should enjoy.

centrate themselves in a room and deI know of no means by which a house struction takes place. I say safety is secan be naturally ventilated without super-cured by ventilation in houses otherwise intendence, or machinery. The system of pumping into public buildings warm pure air, and pumping out the impure air, is to be commended, as it secures by the same machinery both warmth and pure air. Whether any thing of this kind can be done for private houses is at present very questionable. In the meantime, houses ought to be built so that an ordinarily intelligent person, who understands that hot air ascends and goes out at the upper apertures of a room, and that cold air comes in from below, can so arrange that there is a perpetual flow of air through the room without creating cold by draught. This can generally be done in rooms where the window sashes come down from the top in two sides of a room, or in one side where a door opens at the other. But, alas! how many houses are thus constructed? Not one in a hundred in town or country. When they are so constructed, the sashes are not let down from the top. The bed - rooms, which have been closed up all night, are indulged with a small quantity of fresh air by a little opening from below. The consequence of all this closing of doors and windows is sickness. The children are ill in the nursery, the servants are ill in the kitchen, and the master and mistress are ill in the drawing-room. The source of this sickness is easily understood, if you recollect how large a portion of time the inhabitants of houses spend in-doors, and

But I must bring my sermon to an end. I have thrown these few remarks together as free from technical terms as I could, in the hope of calling the attention of the intelligent readers of the Popular Science Review to a subject still greatly neglected. The more I see of the interior of our households, especially in London, the more I am convinced this subject is not fully understood. I have named only a few of the diseases which arise from de

From The Leisure Hour.

THE REVIVAL OF

di and the reputation his discoveries had
already obtained for him, he was appointed
to the chair of mathematics in the Univer
sity of Pisa, though only in his twenty-
fifth year.

of an extensive and solid literature, which | pursuits they opened up to him. Soon
afterwards bore its fruit in the elegance afterwards, through the influence of Ubal-
and purity of his style. His leisure hours
were applied to the cultivation of music
and drawing, in both of which arts he ex-
celled, and which he continued to cultivate
through life. His talents early became so
manifest, that at the age of eighteen his
father resolved, at whatever sacrifice, to
give him the benefit of a university educa-
tion. He was accordingly entered as a
student in the University of Pisa, on the
5th of November, 1581.

Here, besides his studies in medicine, with a view to his profession, he attended a course of the peripatetic philosophy as it was then taught; but to the physical theories of Aristotle, which were then implicitly assented to and taught, he would not bring his mind to consent, without the conviction of reason and experiment. Beginning even then in the academical discussions to combat the firmest supporters of the Aristotelian dogmas, he obtained the reputation of possessing an obstinate and contradictory disposition.

He then applied himself to the investigation of the true laws of motion by real experiments. He demonstrated that gravity acts on all bodies alike; and that bodies of unequal weights will fall through the same space in equal times, any slight dif ferences being due to the resistance of the air. It was at that time assumed, on the authority of Aristotle, that a body ten times as heavy as another would fall through one hundred yards while the other fell through ten. And though the experiment was tried from the tower of Pisa, and both bodies reached the ground at almost the same instant, (the small dif ference, as Galileo rightly observed, being due to the unequal resistance of the air,) the witnesses of the experiment were not convinced.

Such was the opposition raised against the new system of philosophy, that in 1592 Galileo was obliged to resign his chair at Pisa and return to Florence. At this time he made acquaintance with two enlightened Florentine gentlemen, Salviati and Sagredo, and through the influence of the latter he obtained the chair of mathematics at Padua, which was conferred on him for six years. It was in grati tude for these benefits that Galileo afterwards gave the names of Salviati and Sagredo to the two interlocutors in his dialogues, who support the philosophical and common-sense sides of the question.

About this period, in the year 1582, Galileo made the first, and perhaps one of the most valuable of his discoveries. Happening one day to be in the metropolitan church at Pisa, he remarked the regular and periodic movement of a lamp sus pended from the roof. As the lamp was swinging to rest, after being lighted, he observed the equal duration of its oscillations, whether great or small, and this he confirmed by repeated experiments. He immediately perceived the use to which this phenomenon might be applied for the exact measurement of time, and this idea having remained in his mind, he employed it fifty years afterwards (in 1633) for the During this time enjoying greater freeconstruction of a clock intended for astro- dom than at Pisa, he wrote several mathenomical observations. This seems, how-matical and mechanical treatises, and made ever, not to have been a very perfect or successful instrument, and the great invention of the pendulum clock, afterwards of such service to astronomy, is due, not to Galileo, but to Huygens. The idea, how ever, so successfully caught and applied by Huygens, was in his time already afloat, and seems to have been first introduced to the notice of philosophers by Galileo. At this time Galileo had no knowledge of mathematics, and was first impelled to them by conceiving that they could be of assistance in drawing. But once having entered on the study of Euclid and arithmetic, he abandoned every thing for the

several important discoveries and inven-
tions. He invented the thermometer, and
the proportional compass or sector. It
was probably, also, during this time that
he first became inclined to adopt the Co-
pernican system of the world, in which
the discoveries of the telescope afterwards
confirmed him, and of which, at a later pe-
riod, he became so conspicuous an asser-
tor. In 1599 his commission as professor
was renewed for another period of six
years. In 1604 an unknown star of extra-
ordinary brightness having suddenly ap-
peared in the constellation of Serpentarius,
Galileo demonstrated by observations that