It had not, however, escaped the notice of Mr. Watt, that there existed in steam another source of power besides that of atmospheric pressure. The experiments of his learned friend, Dr. Black, of Glasgow, as well as those of the French chemists, and of Papin, in the instance of his digester, had ascertained the laws of its expansive force, and amongst other interesting facts, those subservient to our present purpose; viz. That after water has reached the boiling point, 212° of Fahrenheit, the caloric which enters it no longer becomes latent, but sensible in the steam, which thereupon acquires expansive force to an unlimited degree: that this force increases geometrically; or, that every accession of about 30° of heat, nearly doubles its power at those stages of progression; that when the pressure at a high temperature is taken off, or the steam allowed to flow, there is an instantaneous and rapid production of steam; a fact which proves there can be no necessity of a large space for the steam to form in above the water, provided it be sufficient to prevent water from issuing with the steam, and, therefore, that boilers of a small cylindrical form are best. It may be a fair question, why Mr. Watt did not further employ this principle of expansive force? We may readily conceive of several motives to the contrary. Watt and Bolton's engines were in great demand; they gave entire satisfaction, and the work they performed saved so much labour as to afford the purchase at a high price. The public had gained immensely by this better form of the engine, and Mr. Watt enjoyed the benefits of the patent he had obtained; and, at a later period, this preference was increased by an accident which happened to Trevethick's engine, though caused by gross mismanagement, that would have been equally fatal to any other. From an investigation, by a committee of parliament, into the causes of the several fatal explosions of steam-engine boilers within a few years, published in Tillock's Magazine, vol. 1., it appears that in every instance the accident was fairly attributable to neglect or mismanagement. Many competent persons were summoned to give their opinions; and through the contrariety of their testimony, the prevalent opinion appears to have been, that cast-iron boilers cannot be safe; that as many engines of high steam as of low are now used in England, but that the high are much the most economical in fuel and cost; that they are more safe, if properly constructed; it being argued by some, that boilers for steam of 100 pounds to the inch, are easily made of strength to sustain 500 pounds; this excess being much greater than in those constructed for low steam, makes them comparatively the safest, as the safety valves are less liable to be accidentally prevented from venting the steam.