4.2 x 69 x 7.5
Wood and iron
INDEX 1788 : N.II.256
Tubulus ferreus [vulgo pistolla] ligno armatus, qui cochleis aptatur Parallelepipedo ferreo Machinae n.º 152. ut furmus detineatur. Is tubulus ita dirigitur, ut glans plumbea a pulvere pyrio inde explosa ex aquae superficie resiliat ad angulum quinque vel sex graduum. Arca magna lignea 10. pedes longa, ferreis bracteis intus contecta, quae aqua repletur, et inde operitur velamento.
Small iron tube ("pistol") mounted on wood adapted with screws to an iron parallelepiped from Machine n°. 152 in order to keep it secure. This tube is placed so that a lead bullet fired from it with gunpowder rebounds from the surface of the water at a 5 or 6 degree angle. Large wooden chest 10 feet in length covered with iron leaves which is filled with water and is covered with gauze.
This pistol was made for a quite spectacular experiment which showed that a bullet fired over the surface of water is deflected by it when the angle of inclination of the shot, measured above the horizontal is very small, 5 or 6 degrees. If it is larger, the bullet touches the surface of the water and penetrates into the liquid, changing direction, the same way a ray of light does when it is refracted.
The movable weapon is secured to a graduated quadrant, sitting on a support which permits measurement of the angle of the direction of the bullet with the vertical by means of a plumb line which hangs from the vertex of the quadrant.
In Nollet's book, Leçons de Physique Expérimentale, he describes the experiment in action. The bullet was fired over a tub of water, whose surface was covered with gauze stretched on a wooden frame. Placed vertically on the border of the tub was another framed gauze.
The weapon, which was loaded with gun powder, fired a lead bullet that crossed the vertical gauze, reached the horizontal gauze, which it also crossed, and penetrated the water, then hitting a plank which was submerged perpendicularly in the tub. The proof that the bullet changed direction as it penetrated the liquid was the mark that was left on the plank.
In another experiment, the bullet was fired at a 5 or 6 degree angle above the horizontal. It was proved that the bullet, on reaching the surface of the water is deflected by it, then crosses the gauze in front of it, in a direction symmetrical to that of the direction of incidence in relation to the norm.
The weapon used in Coimbra was, as dalla Bella said, an iron tube mounted on wood, which is commonly known as a "pistol". The weapon still exists, but the wooden tub which was 3.25 meters in length, no longer exists.
From Colégio dos Nobres, catalogue n.º 246.
Carvalho, Rómulo de, História do Gabinete de Física da Universidade de Coimbra, Universidade de Coimbra, Biblioteca Geral, Coimbra, 1978, pp. 418-421.
Nollet, Jean-Antoine, Leçons de Physique Expérimentale, Paris, 1764, Vol. I, Pl. 2, Fig. 4 , Pl. 3, Fig 7.