196 



NA TURE 



[June 30, 1887 



value of the book has also been increased by the insertion 

 of several new maps illustrative of the astronomical and 

 meteorological sections. 



Longman^ s New Geographical Reader. Standard VII. 

 (London : Longmans, Green, and Co., 1887.) 



This "Reader" contains lessons on the ocean, currents? 

 tides, the planetary system, and phases of the moon- 

 The subjects are of more scientific interest than those 

 treated in most books on geography, and are arranged in 

 a progressive and readable form. 



The book is divided into sixty lessons, each being 

 followed by a list of some of the words contained in it, 

 with their meanings. 



In the chapters on the ocean the subjects are well 

 selected, and the various depths and currents are illustrated 

 by maps. In the lesson on the tides the differential action 

 of the sun and moon on the water of the earth should 

 have been mentioned. The diagram illustrating neap 

 tides has one bad point, the sun being shown as shining 

 on a part of the moon which is turned away from it. 



In the diagram on page 231, which represents the sun as 

 seen in full daylight from the surface of the moon, the sun 

 is shown with its corona. The fact of the sun being seen 

 from the moon, which has no atmosphere, would not 

 make the corona visible, but would only tend to intensify 

 the light of the sun and the corona proportionally. It is a 

 pity that this illustration should have been put in without 

 any explanation whatever. 



The chapters on the inhabitants of the sea and 

 methods of catching them are very interesting ; also the 

 voyages to the Arctic and Antarctic regions. An 

 appendix is added which contains a summary of the 

 whole book. 



LETTERS TO THE EDITOR. 



[The Editor does not hold himself responsible for opinions 

 expressed by his con-espondents. Neither can he under- 

 take to return, or to cor7'espond with the writers of, 

 rejected manuscripts. No notice is taken of anonymous 

 CO mm unications. 



[ The Editor urgently requests correspondents to keep their 

 letters as short as possible. The pressure on his space 

 is so great that it is impossible otherwise to insure the 

 appearance eveti of communications containing interesting 

 and novel facts.l 



The New Degrees at Cambridge. 



The letter of "Outis" in yesterday's number of Nature 

 (p. 175) is likely, I fear, to convey a false impression as to the 

 new degrees of Doctor of Science and Doctor of Letters, which, 

 by the way, were instituted, for good or for evil, by the Commis- 

 sioners, and not by any "dominant body in the University." It 

 is true that Doctors in the new faculty take precedence after 

 Doctors in Medicine just as Doctors in Medicine take precedence 

 after Doctors in Law, and Doctors in Law after Doctors in 

 Divinity, but this distinction is only of importance when a pro- 

 cession has to be marshalled ; to all intents and purposes the 

 academic rank of all Doctors is the same. 



If it be true, as I believe it is, that the standard for admis- 

 sion to the regular degree of Doctor in Science is only " rather 

 less than that required for admission to the Royal Society," and 

 that the standard for Doctor in Letters is much the same, it 

 follows that the standard for such degrees is much higher than 

 that for any other Doctorate in the University, while that for 

 Doctor in Law is notoriously the lowest of all. 



Since the new degrees were instituted the Council has 

 usually offered the new degrees to those persons selected as 

 recipients of honorary degrees whose claims were essentially 

 scientific or literary, while it has continued to give the honorary 

 LL.D. to persons whose distinction was of a less academic kind. 

 This may have been wise or unwise, but the Council had cer- 

 tainly no idea that in what they were doing they were offering 

 to men of science an honour of a lower grade than that to which 

 they had been accustomed. It is true that, fearing perhaps that 



the less familiar title might at first be not so well understood 

 outside the University, they began by oftering to the recipients 

 the choice of the degree of LL.D. or of Litt.D. or Sc.D., as the 

 case might be ; but I believe that in all cases those who had the 

 choice preferred the literary or scientific degree. 



No doubt these degrees, like that of LL.D., have been and 

 will continue to be given to men of very different degrees of 

 eminence. It is not every year that the University has the 

 opportunity of enrolling among its honorary graduates a man 

 like Asa Gray ; but I think that, even if he is excluded, the roll 

 of our honorary Doctors in Science and Letters need not fear 

 comparison with that of the honorary Doctors in Law who 

 have received their degrees within the same period. 



Cambridge, June 24. C. T. 



Weight, Mass, and Force. 



The position taken up by "P.G.T." and some others 

 in the discussion on the proper use of the words "weight" and 

 " mass " is similar to that assumed by an astronomer coming for- 

 ward to tell us that we have been calling the stars by their wrong 

 names. 



The following extract from an American technical journal is 

 submitted to the consideration of "P. G. T.," Mr. Hayward, 

 and Mr. Alfred Lodge, in order that they should point out for our 

 benefit where they consider the dynamical language is erroneous, 

 and that they should translate it into the terminology necessary 

 in their opinion to make it correct by using the mathematical 

 terminology of poundals, dynes, moms, poundems, &c. 



"Description of the Strong Locomotive." 

 American yournal of Railway Appliances, March i5> 1887. 



" The weight of the engine in working order is 137,000 lbs., 

 of which 90,000 are on the drivers, 27,000 on the front truck, 

 20,000 on the back truck. The weight of the tender loaded is 

 75,000 lbs. The boiler carries 160 lbs. of steam, which pressure 

 is easily maintained when the engine is pulling the heaviest and 

 fastest trains over the 96-feet grades across the mountains. 



" The engine having 20-inch by 24-inch cylinders, and 62-inch 

 drivers, the traction force is, according to the well-known formula, 

 20^ X 24 4- 62 = 1 54 "8 lbs. for each lb. of mean effective pressure 

 in the pistons. The resistance of modern rolling-stock as de- 

 duced from the most recent experiments both in this country 

 and in Europe is from 12 lbs. per ton of train including engine 

 and tender at speeds of 30 miles an hour to 15 lbs. at 50 miles 

 an hour, above which point the resistance increases in a much 

 greater ratio. 



" Let us suppose then that the engine is hauling a train at 

 30 miles an hour on the level, cutting off at 10 inches of the 

 stroke. From indicator cards taken from the engine under 

 these conditions we find that a mean effective pressure of 100 lbs. 

 is maintained in the cylinders. The traction force exerted will 

 thus be 154 '8 x 100 = 15480 lbs. ; and taking the resistance at 

 12 lbs. per ton, we find the maximum load the engine can 

 pull is 15480 -^ 12 = 1290 tons, and subtracting from this the 

 weight of the engine and tender there remains a weight for the 

 train 1290 — 106 =1184 tons, or the equivalent of no less than 

 59 20-ton cars. 



"Now suppose the engine is running up a grade of 96 feet to 

 the mile (i in 55) at the same speed and cutting off at the same 

 point as before. The resistance to gravity on a 96-feet grade is 

 2240 -=- 55 = 41 lbs. per ton, and this added to the 12 lbs. re- 

 sistance on the level gives 41 ->- 12 = 53 lbs. per ton for the train 

 going up the grade. Under these conditions the load hauled 

 would be 15480 -r- 53 = 292 tons ; or, subtracting the engine and 

 tender, 292- 106 = 186 tons, the equivalent of9| 20-ton cars. 



"Turning now to the question of adhesion, we find that 

 taking the coefficient of adhesion at one-fifth, we have a weight 

 of 18000 lbs., one-fifth the weight on the drivers, as against the 

 15480 lbs. of traction. We need hardly say that the average 

 coefficient of adhesion is usually higher, one-fourth being 

 generally taken in calculations relating to the performance ol 

 locomotives. Under this condition the weight available foi 

 adhesion would be 22222 lbs., or one-fourth the weight on the 

 drivers, and the mean effective pressure in the cylinders would 

 have to amount to 22222 -^ IS4'8 = 143*6 lbs. per square incl: 

 before the wheels would begin to slip or the use of sand become 

 necessary. At the speed of 30 miles an hour and 100 M.E.P 

 (mean effective pressure) this engine would exert about 124c 

 H.P.,"&c. 



