RESEARCHES AND EXPERIMENTS. 



723 



at a rate which threatened at no distant day to 

 cumber the whole ground. The only use that 

 could then be found for this material was in 

 the construction of marine works, and a small 

 fraction was thus disposed of. Now this slag is 

 made into bricks, paving -blocks, building-sand, 

 concretes, and the like, and the demand is ful- 

 ly equal to the supply, even when the furnaces 

 are in full blast. Streets and crossings are 

 paved with slag blocks, and river walls, water- 

 courses, and similar constructions consume vast 

 quantities annually. Bricks of this material 

 are largely employed for building in London, 

 and blocks for paving. But perhaps the most 

 scientific system yet introduced for utilizing 

 slag is its employment for glass-making : the 

 slag is run direct from the iron-furnace into the 

 glass-furnace, there mixed with other materials, 

 and used for making bottles and other articles 

 of coarse glassware. 



Utilizing Exhaust Steam. The apparatus 

 contrived by Mr. James Atkinson for utilizing 

 the waste heat of exhaust steam by employing 

 it to raise the temperature of the feed- water 

 supplied to boilers will, it is claimed, save at 

 least 20 per cent, of the cost of producing the 

 steam, besides increasing the durability of the 

 boiler. Mr. Atkinson's heater consists of a 

 number of straight tubes, screwed into a tube- 

 plate, which forms the base of an inclosed 

 cylindrical vessel containing the tubes and the 

 water to be heated. These heating tubes are 

 closed at their upper ends, but are open at the 

 bottom to the exhaust steam, for which a short 

 passage is provided. Small circulating tubes 

 draw any air out of the heating tubes which 

 would prevent them from being filled with 

 steam. The latent heat of a portion of the 

 exhaust steam is transmitted through the heat- 

 ing tubes to the feed-water, which is forced 

 through the heater, and passes into the boiler 

 at a temperature of from 210 to 212. For 

 this contrivance it is claimed that while it is 

 in no way inferior to others, as proved by 

 practical results attained, it possesses perfect 

 freedom from back-pressure on the engine, 

 greater facilities for cleaning out and examin- 

 ing, a better arrangement for purifying the 

 feed- water, increased durability, greater sim- 

 plicity in general construction, and consequent- 

 ly greater cheapness in manufacture. The same 

 principle maybe applied to heating liquids, air, 

 or other gases, for manufacturing purposes; 

 also rooms and buildings. 



The Paces of the Horse. A few years ago a 

 rather excited discussion was carried on in the 

 English newspapers concerning the position 

 of the legs of the horse when in motion, the 

 occasion of the controversy being the exhibi- 

 tion in the National Academy of Miss Thomp- 

 son's celebrated painting of " The Charge at 

 Balaklava." The action of the horses in this 

 piece was by many of the art critics declared 

 to be unnatural and impossible. On the other 

 side it was maintained that the horses were 

 represented in natural postures, ascertained by 



systematic observation, while the conventional 

 postures of all previous artists were condemned 

 as untrue to nature. Thus the matter stood, 

 neither side acknowledging defeat, till Mr. 

 Muybridge, a photographer of San Francisco, 

 brought into the dispute the evidence of facts, 

 which appears to have been a surprise to both 

 of the parties, and indeed to every one who has 

 seen the beautiful series of instantaneous pho- 

 tographs produced by Mr. Muybridge. That 

 gentleman has perfected an automatic electro- 

 photographic apparatus for recording the ac- 

 tion of the horse in motion. In taking the 

 negatives he employs a series of cameras op- 

 erated by electricity, and so placed as to fix 

 with absolute accuracy the several phases in 

 the continuous action of the horse while mak- 

 ing one stride. The exposure for each nega- 

 tive is about the two-thousandth part of a sec- 

 ond. Six different positions, showing the va- 

 rious motions of the horse's legs in walking, 

 have been photographed. From these photo- 

 graphs it would appear that the walking horse 

 always has two feet on the ground, and for a 

 brief interval in each stride three feet. Twelve 

 positions representing the various motions in 

 trotting have in like manner been photo- 

 graphed. In none of these pictnres do we rec- 

 ognize anything like the conventional figure 

 of a trotting horse in motion. M. J. Marey, 

 author of a work on animal locomotion (" An- 

 imal Mechanism "), and who has shown rare 

 ingenuity in studying the paces of the horse, 

 has expressed his admiration of Muybridge's 

 instantaneous photographs of the horse, and is 

 confident that in like manner the naturalist 

 will be enabled to determine the true motions 

 of birds, insects, and all other animals. 



Population- Density and Death-Rate. How 

 density of population affects death-rate is very 

 clearly shown by Dr. Farr. There are in all 

 England and Wales 619 sanitary districts, and 

 these, being classed according to their respec- 

 tive death-rates, form eighteen groups. A study 

 of these eighteen groups shows that mortality 

 increases as the density of population. Thus, 

 in the ten years, 1861-'70, at one end of the 

 scale the annual deaths per 1,000 of the pop- 

 ulation are 15, 16, 17; at the other end of the 

 scale the deaths are 31, 33, 39. The acres to 

 the person in the former three are 12, 4, and 

 3; in the latter, 1-01, '05, and -01. The inter- 

 mediate rates of mortality are 18, 19, 20, 21, 

 22, 23, 24, and 25, and the acres to a person 

 are 4, 3'3, 2'9, 2-1, 1*1, "05, '02. Now, exclud- 

 ing the London districts, about which there is 

 some difficulty, there are seven groups of dis- 

 tricts where the mortality is 17, 19, 22, 25, 28, 

 32, and 39. In these districts the number of 

 persons per square mile is 166, 186, 379, 1,718, 

 4,499, 12,351, and 63,823. Thus in Liverpool, 

 which is the densest and unhealthiest district 

 in England, there were 63,823 per square mile, 

 of whom 39 per 1,000 died annually. Or the 

 same facts may be differently stated thus : The 

 nearer people live to each other, the shorter 



