OF ARTS AND SCIENCES. 479 



then it was possible to introduce unity, harmony, and precision into 

 all the physical sciences by making the familiar units of measurement 

 universal. As other forms of energy (mechanical, electrical, mag- 

 netic, chemical, capillary, radiant, and gravitation) can be converted, 

 directly or indirectly, into heat-energy, heat has become a universal 

 standard of energy, current everywhere in science, and redeemable. 

 Hence it has become of prime importance to determine the mechanical 

 equivalent of heat, — the amount of heat, for example, which corre- 

 sponds in energy to a given mass falling through a given height in a 

 given latitude. In this way heat and all its dependencies will be 

 measured by the units of ordinary work. For more than forty years, 

 physicists in different countries, and by various methods, led by 

 Joule, have been engrossed with this measurement, reaching results 

 which have slowly but happily converged towards a common agree- 

 ment. 



Professor Rowland, after an historical and critical review of the 

 methods and results of older cultivators in this rich field, has turned 

 up the soil anew, deepening the furrows. 



The fruits of his long and patient labor were made known to the 

 Academy in 1879, in Volume XV. of the Proceedings. New appara- 

 tus was devised ; the comparative merits of mercurial and air ther- 

 mometers were discussed ; and the various constants of science which 

 enter into the case were re-examined. The research is a model of 

 ingenious and conscientious experimentation, and was not published 

 until it had received from its author the same severe criticism which 

 he had applied to the work of others. That his final conclusion 

 harmonizes so well with the best of Joule's, increases our confidence 

 in both. A larger discrepancy might have given a greater show of 

 originality ; but science would have paid for the novelty by a loss of 

 security, and another revision of the whole subject would have been 

 entailed upon it. 



When Newton announced his dynamical theory of the solar system, 

 as simple as it was comprehensive, it made slow headway against 

 the fanciful hypothesis of Descartes, which was intrenched in all the 

 universities of Europe. And yet Newton's theory reposed upon a 

 firm mathematical foundation; while that of Descartes submitted to 

 no quantitative tests, and contradicted all the known laws of me- 

 chanics. The history of astronomy from that time almost to the 

 present moment tells of ever new victories achieved by the combined 

 attacks of the telescope and mathematical analysis in the province of 

 celestial mechanics, presenting the law of gravitation as supreme 



