DEATH-RATE AND BACK-TO-BACK HOUSES* (SALFORD).

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been the case during the quinquennial period ending 1895, owing to frequently recurring serious outbreaks of influenza. To the prevalence of influenza, which, directly or indirectly, proved so fatal among all classes of the community, especially during the earlier part of that period, may fairly be attributed the relative check to the continued decline in the death-rate. It must be remembered, however, that although this affection is undoubtedly infectious, we have no reason to suppose that it is in any way associated with insanitary conditions. Nevertheless, as it belongs to the infectious class of disease, we must look upon it as being preventable; but as the cases are frequently of so mild a nature as to escape recognition, preventive measures in the shape of isolation and disinfection are hardly practicable. Fortunately, past experience justifies the conclusion that when we have seen the last of the malady, it is not likely soon to reappear amongst us.

These, then, are some of the questions in which an intelligent Inspector must take an interest, but they are not by any means all. He must make himself familiar with the principles of Sanitation, and acquire a practical knowledge of the various dangers to health met with from day to day, in order to be in a position to protect the public from the effects of their own ignorance and carelessness, as well as from ignorance, carelessness, and fraud on the part of workmen.

In the following pages the principles that ought to guide him in the discharge of his duties will invariably precede detailed description; for it is a common experience that ignorance of the principles with which all details must comply not infrequently results in a useless expenditure of money, and brings discredit on the Sanitary Authority and their officials. It does not follow that the structural alterations called for in one case are equally necessary, or indeed suitable, in all; what may be necessary under certain circumstances, may under others be inexpedient. Again, more than one sanitary appliance may be admissible in a given case, and unless the Inspector appreciates the object that is to be attained, he may condemn a perfectly suitable appliance, simply because, in detail, it does not correspond with what he has seen used under similar circumstances before.

It will not infrequently be the duty of an Inspector to point out to the owner of a property who, it may be recently, has spent a considerable sum in sanitary () work, that, through faulty work, things are no better than they were before, but, in doing so, he must be perfectly confident of his ground, a confi

dence that can only be the outcome of a sound knowledge of the principles of Sanitation.

Inspectors may learn much by extending their field of observa tion to other districts than their own, when opportunity offers, and it is most desirable that they should form societies (or branches of existing Societies of Inspectors) for the discussion of sanitary questions, from time to time, among themselves. Such discussions will tend to keep alive an interest in work which is daily growing in importance, and promote that efficiency which alone, in future, will insure success.

It is important that Inspectors should take a comprehensive view of what is expected of them, and remember that they are workers in a wide field of labour. In order that they may successfully contend against the ignorance that prevails on all sides, they themselves must possess a knowledge of the general laws in operation in Nature, with which man's action must comply if health is to be maintained. Nature is a vast laboratory in which chemical and biological changes are in perpetual operation; no new matter is formed. What exists simply assumes a variety of shapes and conditions in accordance with the circumstances under which it is placed-so growth and decay go on; what are living cells of one animal to-day form the food of other animals or plants to-morrow, or, it may be, become for a time constituents of the inorganic world, to be redissolved and again appropriated by organic matter. These changes involve death and decomposition, and, under certain circumstances, danger to neighbouring life, a danger, however, against which we often possess a remedy in Nature herself, if we study her laws and take advantage of them.

CHAPTER II.

WATER-SUPPLY, DRINKING WATER, POLLUTION OF water.

Water a Prime Necessity of Life.-Water is one of the prime necessities of life, and that all should be provided with a pure supply is by no means the least important of the health requirements. It is a vital element of our food, and assists in the building up of our tissues, of which it constitutes one hundred parts in each hundred and fifty; it preserves the fluidity of the blood; aids in the excretion of effete matter;

and assists in maintaining our bodies at a uniform temperature under varying conditions of heat. It is also essential for drinking purposes, for cooking, and for personal and household cleanliness. By the community at large it is required for public baths, for water-closets, for flushing sewers, for the cleansing of streets, for use of animals, and for manufacturing and various other purposes. It is important, therefore, that the supply should be plentiful as well as pure.

Sources of Supply. From whatever source our water-supply may be obtained, it is dependent for replenishment upon the rainfall. From the surface of the land, rivers, lakes, and oceans, evaporation takes place through the agency of the sun's heat, and the atmosphere has the power of retaining moisture, in the form of invisible vapour, in quantities varying with the temperature. The higher the temperature, the greater capacity the atmosphere has for retaining moisture in this invisible form. Should the temperature fall, which it may do from a variety of causes, a point is at length reached which is called saturation point, when the atmosphere contains as much moisture as it can possibly hold in the form of vapour; and if the temperature should be further reduced, a portion of the moisture is condensed into fine globules, and becomes visible as mist. Clouds are simply mist, and their circumscribed and apparently solid appearance is the result of their being viewed from a distance.

By the cooling of the atmosphere still farther, the globules of moisture coalesce into larger particles, until a point is reached, at which, by reason of their weight, they can no longer be retained in suspension, and are deposited in the form of rain. If the point of saturation of the atmosphere should not be reached— that is, if condensation should not occur-until the temperature is below freezing point (32° F.), then the deposit occurs in a solid form, and falls to the ground as snow.

At

The average annual rainfall in Great Britain is about 30 inches, varying from 20 inches on the East Coast to as much as 70 inches, or more, on the West Coast of Scotland and Ireland. certain localities in the English lake district 150 inches is not an unusual amount. One inch of rainfall a day, or even more, is not an infrequent occurrence in this country; in some countries there may be much more, and in these the annual fall may amount to 400 inches or more.

*

After having fallen, a certain portion of the rain water is lost by evaporation, another portion runs off the surface and the re

*For a fuller account of the causes of the rainfall, &c., see Elementary Meteorology, by R. H. Scott (International Scientific Series).

mainder penetrates into the soil, the extent to which each takes place being regulated by a variety of circumstances, such as the amount of the rainfall, the temperature, the slope of the surface, and the porosity—that is, the openness of the soil. In winter a larger quantity penetrates or runs off the surface in the direc tion of the natural drainage than in summer, when, by reason of the increased temperature, evaporation is very rapid, and when it requires a heavy shower of rain to overcome the absorbent properties of the warm dry surface, so as to enable any portion to reach the water-courses, especially in flat districts.

Subsoil Water.-That portion of the rainfall which penetrates the surface continues to descend until, sooner or later, it meets with an impervious stratum, where its downward course is diverted in one direction or another, in accordance with the natural fall of the stratum in question. This fall is usually in the direction of the general surface fall-that is, towards the natural water outlet of the area. It must be remembered that the pace at which this subsoil water travels is very slow indeed, by reason of the obstruction offered to its progress by the soil through which it has to travel, and, for this reason, it varies with the density or looseness of the soil in question. Now it is this subsoil water which we tap in sinking what we call our surface wells; it, therefore, contributes largely to our domestic watersupply. As regards its qualities for this purpose, more will be said presently.

It will be understood from the above description that the depth of a surface well will vary according to the distance from the surface of the impervious stratum upon which the subsoil water rests.

It may here be stated that this has an important bearing upon the dampness of a locality, and for this reason it is of the utmost importance, especially in the case of low-lying districts, that no obstacle should be offered to the natural flow of the stream or water-course, which is the ultimate outlet of the water in question. The placing of weirs along the course of rivers, for the purpose of back-pounding the water, in order that it may be used as a motive power for mills, is a custom which, unfortunately, still prevails throughout the country.

Deep Water.-Beneath the impervious stratum just mentioned, we come upon other porous strata, and in them supplies of water which have percolated downwards from distant points where these strata reach the surface (Fig. 1). By sinking deep wells into such a stratum at its lower part we tap this supply, and, if the surrounding country should be much higher