On Eiicke's Comei. 275 



difficulty, and are analogous to those to which pieces of artil- 

 lery are submitted before they are delivered from the arsenal, 

 would give all the guarantee desirable, and leave to the free 

 disposition of industry a species of steam engines which have 

 the incontestable advantage over the old ones of producing a 

 given quantity of moving force, with an economy of 30 or 40 

 per cent, in fuel. 



It is to this prodigious advantage, which manufacturers 

 could not fail to appreciate, that must be attributed the truly 

 extraordinary rapidity with which the employment of high and 

 mean pressure engines is propagated. He concludes by noti- 

 cing the increase of Mr. Woolf 's engines both in England and 

 France ; the very high pressure ones of Oliver Evans in North 

 America, and the importation of Trevithick's engines to Peru. 

 The high pressure engines he considers to be most desirable 

 in places like Paris, where the fuel is expensive*. 



LXIII. On Encke's Comelf. 

 TT is well known that the orbits of most of the comets are 

 extremely eccentric, and approach so much to the form of a 

 parabola, that it is usual to compute the elements of the orbit 

 upon the supposition that they move in this curve ; and it is 

 found in general that the observations made on those bodies 

 are well represented by this hypothesis, which leaves however 

 the time of the revolution wholly indeterminate. In fact, there 

 are but very few comets, in which any considerable ellipticity 

 of the motion has been perceived, and only two instances in 

 which the periodical revolution has been determined to any 

 great degree of accuracy, namely, Halley's comet, which ap- 

 peared for the fifth time in 1759, according to his prediction, 

 with a periodical revolution of seventy five years ; and Olbers' 

 comet, which was obsei'ved for the first time in 1815, with 



* The comparative advantages of high and low pressure steam is an in- 

 teresting question. It involves these three considerations, viz. the difference 

 between the specific heats of steam under different pressures, — simplifying 

 the apparatus and reducing the expense of its construction ;— and lastly,iii 

 applying the fuel with the least loss of heat. If there be a considerable 

 difference in the specific heat of steam under a high pressure, it will be an 

 advantage to work with high pressure steam, and the higher the pressure 

 the greater the economy. The ordinary methods of comparing engines do 

 not show tlie advantage of simplicity of construction ; to the exjjenditure 

 of fuel, the interest on the capital expended in erecting tlie engine, and the 

 yearly expenditure for rejjairs, should be added. In the saving of fuel by 

 judicious application of iieat, the greatest advances have been made of late 

 years. A great deal of that economy which is ascribed to high pressure en- 

 gines, is due to the improvement in the art of ajjph'ing heat. 



f From the North American Review, No. XXXiV. 



M m 2 such 



