216 



DISCOVERY 



because it is more costly, and tliat the cheaper method 

 for winning the clay, shown in Fig. 3, is now coming 



Fig. 3.— showing .iiXOTHHR JIETHOD UMl'I.i >vi;ii, I.E. GETTING 

 THE CL.4Y BY MECH.\NIC.\I. UIGGKR.S. 



into general use. By this method fossils are broken 

 up and the vertical face cannot be inspected so easily. 



The clay is dug by mechanical diggers and is loaded 

 into tip-wagons, which are drawn up a steep incline, 

 such as is shown in Fig. 2, to the grinding-mills, which 

 are somewhat similar to mortar-mills. It is un- 

 necessary to add any mateiial such as lime or sand, 

 and usually it is unnecessary to add water, the clay 

 itself being sufficiently moist, though not too moist ; 

 most important of all, the clay may be used as soon 

 as it is dug, and need not be stacked in the open for 

 some weeks or months to mature, as is the case with 

 some clays. After being ground and passed through 

 sieves, the clay is sent to the pressing machines and, 

 with a die, is pressed into bricks measuring 9 x 4I X 

 3 inches. These are removed by barrows and stacked in 

 the large modern Hoffmann continuous kilns (shown in 

 Fig. 2) — very familiar objects in some districts. In 

 these kilns the bricks remain for some three weeks, 

 their size being reduced by burning to 8f x 4i X 2| 

 inches. 



The kiln is divided into some sixteen compartments 

 in one of which the green, undried bricks are stacked. 

 Flues and dampers communicate with each compart- 

 ment, and from each compartment with the chimney. 

 Feed-holes in the roof of each compartment are also 

 provided By these devices the hot gases are drawn 



forward from a compartment in which bricks have 

 been burnt, and introduced into a forward compart- 

 ment for drying the green bricks. When these are 

 dry, further heat can be introduced, and increased by 

 the use of fresh fuel for burning them. For this pur- 

 pose a high temperature is required. When the bricks 

 have cooled sufficiently, the compartment is emptied 

 and becomes again available. From this it will be 

 gathered that there is always fire in some part of the 

 kiln and that it goes round in one direction ; hence 

 the name " continuous " kiln. 



The total output from the works round Peterborough 

 is of the order of 8,000,000 to 10,000,000 bricks per 

 week under normal trade conditions. 



Animal Fecundity — I 



By F. H. A. Marshall, Sc.D., F.R.S. 



Reader in Agricultural Physiology in the Uniucrsitij 0/ Cambridge 



The variation in fecundity (i.e. in quantity of off- 

 spring) in different species was explained by Herbert 

 Spencer in a generalisation which he applied to man 

 as well as to the higher animals. He supposed that 

 the capacity to sustain life and the power to produce 

 new individuals are, roughly speaking, in inverse 

 proportion, an idea which was summarised in the 

 dictum that Individuation and Genesis vary inversely. 

 Amid favourable surroundings and in the presence of 

 abundant easily obtained food the necessary expendi- 

 ture of energy by the individual is relatively slight ; 

 in other words, the cost of Individuation is low, and in 

 such conditions the rate of Genesis is correspondingly 

 increased. Spencer cited the Kaffirs, the Boers, and 

 the French Canadians as examples of races fertile 

 among men in which the rate of increase was associated 

 with a nutrition much in excess of the expenditure 

 of energy necessary for maintenance, and he contrasts 

 the fertility of these races with that of others amid 

 less favourable conditions. 



Less Propagation among Active Animals 



Among animals there are countless examples illus- 

 trating the same principle. Spencer cited the hare 

 and the rabbit as species closely allied and living on 

 much the same food, but differing in the amounts of 

 energy spent on movement. The relatively inert 

 rabbit may have six young at a time and four litters 

 a year, whereas- the more active hare has a smaller 

 number of young in a litter and fewer litters. More- 

 over, the rabbit begins to breed when six months old, 

 but the hare not until it is a year old. Thus the 

 fecundity of the rabbit as compared with that of 



