But that was only a guess, and the matter was of such importance that it needed something more certain than mere assumption. So the Mining Association of Great Britain decided to have a series of experiments which should settle once and for all what part the coal-dust played in these catastrophes, and how best they could be prevented. It was at first thought that an old mine might be utilised for the experiments, but there was the difficulty that such always become wet after work has ceased in them, and so the dust would not behave normally. Moreover, the work would be extremely dangerous and the results difficult to observe. Then a culvert was suggested built of concrete, partly buried in the ground, but that too was dismissed. Finally it was decided to make an imitation mine of steel, using old boiler shells with the ends taken out. The sum of £10,000 was subscribed for the purpose by the coal-owners of Great Britain, and the great work was carried out at Altofts, in Yorkshire, close to a colliery where a terrible disaster occurred in 1886. Here the great tube or gallery was built. Roughly the shape of a letter L, one leg is over 1000 feet long, while the other is 295 feet. The longer leg is 7-1/2 feet in diameter and the shorter 6 feet. At the end of the shorter part a large fan is installed which can force 50,000 to 80,000 cubic feet of air per minute through the structure, so producing the conditions of a well-ventilated mine. The shorter length has several sharp turns in it for the purpose of breaking the force of the explosion along that part, and so shielding the fan from damage, while a tall chimney is provided there, so that, the door being shut to cut off the fan, the gases from the explosion can find a harmless way out. Inside the tube, shelves are fixed along the sides so as to reproduce the effect of the timbering in a real mine, upon the beams of which the dust finds lodgment. Props were put up too, just as they would be in the real mine. Everything, in fact, was done to make the place as perfect a replica as possible of actual underground workings. And then, added to this huge and costly structure, was an outfit of scientific instruments worthy of the important investigations which were to be carried on. To grasp the purpose and working of these we need to remind ourselves of the aims and intentions of the experiments. First of all it was desired to find out how various quantities and qualities of coal-dust behaved. The dust was laid along the floor of the tube and along the shelves. A small gun fired at some point in the tube raised a cloud of this dust just as the gas explosion in the real mine would do. Then another gun was fired to explode the dust-cloud. So far all is quite simple and easy. But to do that would be of no value without the means of finding out exactly what resulted from the explosion. And that is the function of the instruments.