[74] At a meeting held in Glasgow in 1839 to erect a monument to Watt, Dr. Ure observed:--“As to the latent heat of steam,” said Mr. Watt to me, “it was a piece of knowledge essential to my inquiries, and I worked it out myself in the best way that I could. I used apothecaries’ phials for my apparatus, and by means of them I got approximations sufficient for my purpose at the time.” The passage affords a striking illustration of the large results that may be arrived at by means of the humblest instruments. In like manner Cavendish, when asked by a foreigner to be shown over his laboratories, pointed to an old tea-tray on the table, containing a few watch-glasses, test papers, a balance, and a blowpipe, and observed, “There is all the laboratory I possess.” [Illustration: PAPIN’S DIGESTER.] Watt saw that it would be easy to contrive that the cocks should be turned by the machinery itself instead of by the hand, and the whole be made to work by itself with perfect regularity. But there was an objection to this method. Water is converted into vapour as soon as its elasticity is sufficient to overcome the weight of the air which keeps it down. Under the ordinary pressure of the atmosphere water acquires this necessary elasticity at 212°; but as the steam in the digester was prevented from escaping, it acquired increased heat, and by consequence increased elasticity. Hence it was that the steam which issued from the digester was not only able to support the piston and the air which pressed upon its upper surface, but the additional load with which the piston was weighted. With the imperfect mechanical construction, however, of those days, there was a risk lest the boiler should be burst by the steam, which was apt to force its way through the ill-made joints of the machine. This, conjoined with the great expenditure of steam on the high-pressure system, led Watt to abandon the plan; and the exigencies of his business for a time prevented him pursuing his experiments. Watt’s own account of his early experiments will be found appended as notes to Brewster’s edition of the articles ‘Steam and Steam-engines,’ written by Dr. Robison for the ‘Encyclopædia Britannica,’ and afterwards published in a separate form. [Illustration: THE NEWCOMEN MODEL.]