new "Meteorological Office" within the agency in 1919. During the 1920s, aviation began to be an important customer, bringing new data and organizational changes. There was also an increased use of telecommunication technologies, including telegraphy and broadcasting. The Stockholm Meteorological and Hydrological Agency also needed to expand its hydrological functions. Sandström then began to study the Gulf Stream, with Pettersson and Ekholm, for its influences on the climate. In 1929, Sandström led an expedition to the Arctic Ocean funded by the state and private foundations. In addition to expeditions and a number of personal boat trips in the Arctic, Sandström traveled several times in winter to Bergen to meet with Professor Bjerknes to discuss the theory of the weather fronts and air masses. His observations provided him with data on the rate of conversion of energy between the atmosphere and the ocean, which will earn him international recognition. Sandström Theorem Sandstrom was principally concerned with the role of heating and cooling in driving ocean currents, and in the larger-scale ocean circulation in general. He asserted that thermal circulation can cause vigorous, steady circulation only if heating occurs at greater depths than cooling. This is known today as Sandström's theorem and represents an attempt at extending the well known result of classical thermodynamic theory that in order for a heat engine to perform positive work over a cycle, the work of expansion needs to occur at greater pressure than the work of contraction. Sandstrom's theorem is therefore technically true, as long as expansion in the fluid is caused by heating and contraction by cooling, and that greater depths occur at greater pressures. There is an ambiguity, however, as to the meaning of the terms 'heating' and 'cooling' in Sandstrom's theorem. So far, heating and cooling has always been interpreted in the literature as being associated with 'surface heating' and 'surface cooling' respectively. In real fluids, however, molecular and turbulent diffusion always cause internal heating/cooling even in absence of external heating/cooling, as long as the temperature of the fluid considered is non-uniform. As is well-known, molecular and turbulent diffusion tends to relax the system toward thermodynamic equilibrium, i.e., toward an isothermal state, which for a statically stable fluid, will warm up the fluid at high pressure, and cool it down at low pressure. Therefore, because of internal diabatic heating/cooling by molecular/turbulent diffusion, the overall heating experienced by a stratified fluid always occurs