THE POPULAR SCIENCE MONTHLY.
all instances it is essential that the land should be well underdrained, and that the sewage should all pass through the soil, and not merely over it; otherwise, as has been shown, it will only occasionally be satisfactorily purified. The catch-water, or, as the committee has termed it, the "supersaturation" principle, is not defensible either on agricultural, chemical, or sanitary principles. An irrigation farm should therefore carry out intermittent downward filtration on a large scale, so that the sewage may be always thoroughly purified, while at the same time the maximum of utilization is obtained.
"It is certain that all kinds of crops may be grown with sewage, so that the farmer can grow such as he can best sell. Nevertheless, the staple crops must be cattle-food, with occasional crops of corn; and it is also certain, from the analysis of the soil, that it has become very much richer, and that the manurial constituents of the sewage accumulate in it. Cattle should be fed on the farm, which leads to a vast increase in the production of meat and milk, the great desiderata of the population producing the sewage. Thus the system of farming must be specialized and capital concentrated, the absence of which conditions has proved a great barrier to the satisfactory practical solution of the sewage question.
"The committee has not been able to trace any ill effects to the health of the persons living around sewage farms, even when badly conducted; nor is there any proof whatever that vegetables grown thereon are in any way inferior to those grown with other manure. On the contrary, there is plenty of evidence that such vegetables are perfectly suited for the food of man and beast, and that the milk given by cows fed on sewaged grass is perfectly wholesome; thus Mr. Dyke, Medical Officer of Health of Merthyr Tydfil, states that, since the abundant supply of milk from the cows fed on irrigated grass, the children's mortality has decreased from 48, 50, and 52 per cent, of the total deaths, to only 39 per cent., and that so far from diarrhoea having been made more prevalent by the use of sewaged cabbages, 'last year the Registrar-General called attention to the fact that diarrhœa was less prevalent in Merthyr than in any place in England and Wales;' and he expressed his belief in 'the perfect salubrity of the vegetable food so grown.'
"With regard to the assumption which has been made that entozoic diseases would be propagated by irrigation, all the evidence that the country has been able to collect, and more especially the positive facts obtained by experiments, are against such an idea; and the committee is of opinion that such disease will certainly not be more readily propagated by sewage irrigation than by the use of human refuse as manure in any other way, and probably less if the precaution be taken of not allowing the animals to graze, but always having the grass cut and carried to them."
Length of Thread of the Silk-worm.—Prof. Riley, of St. Louis, informs us that the calculation, on page 663 of the last volume, as to length of thread and weight of cocoon spun by the mulberry silk-worm, is altogether exaggerated. Instead of the thread being 11 miles in length, it averages not much more than half a mile, and seldom exceeds 1,000 yards; while a single mile, instead of 28 miles of it, would weigh about 151⁄2 grains.
The Constitution of Carboniferous Strata.—At a general meeting of the British Association, Prof. W. C. Williamson delivered an interesting discourse on "Coal and Coal Plants." The speaker said that most men are now agreed as to the vegetable origin of coal, and the drift theory of its accumulation. It was once a vegetable soil, which accumulated under the shade of primeval forests, growing on areas of depression. In time the land sank beneath the sea, and the vegetable elements were buried under layers of sand and mud, accumulations of which again restored the area to the sea-level, when spores of plants once more germinated in a blue mud, and the succession of phenomena which had previously occurred was again renewed. The frequent repetition of these changes, finally, resulted in the accumulation of the thousands of feet composing the vertical series of rocks which are termed the carboniferous strata. Attention had been called by Prof. Huxley to some