A contemporary and acquaintance of Harvey was Robert Boyle, one of the most important of the early scientific investigators, who was an avowed disciple of Bacon, and followed his methods with conscientious care. His work covered a large field, but it was concerned mostly with the action of gases. He is best known by "Boyle's Law," which is usually expressed as follows: "When the volume of a mass of gas is changed, keeping the temperature constant, the pressure varies inversely as the volume; or the product of the pressure by the volume remains constant." While it has been found that this law is not absolutely true with all gases at all temperatures and pressures, its departure from accuracy are very small, and these are now definitely known. With certain tabulated corrections, this law is the basis on which most of the calculations for steam engines, air engines and gas engines are made. It is usually expressed by the formula p v = p´ v´ = constant. Boyle is said to have "discovered" this law, and Harvey is said to have "discovered" the circulation of the blood. Doubtless they did: but if they had done no more than "discover" these things, no one else would have been the wiser, and the world would have been no richer. What these two men did that made us wiser and the world richer, was to make inventions of definite character, and give them to the world in such manageable forms, that "persons skilled in the art can make and use them." In 1620, the spirit thermometer, as we know it now, was invented by Drebel. It is by some ascribed to Galileo. An interesting controversy has been waged as to which was actually the inventor. The facts seem to be that Galileo did invent a thermometer in which the height of water in a glass tube indicated approximately the temperature. The tube was long and ended in a bulb at the top. The bulb being warmed with the hand of Galileo, and the open lower end of the tube being immersed in water, and then the warmth of the hand removed, water rose in the tube to a height depending on the warmth of the air in the bulb. The height of the water therefore varied _inversely_ as the temperature. The defect of the instrument was that it was a barometer as much as it was a thermometer; because the varying pressure of the atmosphere caused the water to rise and fall accordingly, and thus falsify the thermal indications. Drebel realized this, and closed both ends of the tube.