Mr Edgeworth on a Machine for Measuring Distances. 95 



tained by shutting the gas entirely off when it is not used, 

 by means of a stop-cock placed near the main door where the 

 pipe enters the house. This, we believe, is already adopted 

 in Edinburgh ; and as it is next to impossible that gas could 

 escape unperceived to any dangerous extent while it is burn- 

 ing from various apertures, this main stop-cock must be held 

 to be a great security. Lest the servant or the master of the 

 house should neglect to put out this stop-cock when the lights 

 are put out, various contrivances might be adopted for prevent- 

 ing any such negligence. A light, for example, issuing from 

 a small jet, might be so arranged that it could only be put out 

 by turning the main stop-cock ; a reflector might be so plac- 

 ed on the stop-cock as to indicate, during the day, that it had 

 been left open ; and various other simple contrivances might be 

 resorted to for reminding the master of the house of his duty. 

 3. The most important safeguard of all, and the only one 

 which we think really useful both to those who use, and to those 

 who do not use gas, is one of Sir Humphry Davy's safety 

 lamps, which should be lighted whenever there is any suspicion 

 that gas has been making its escape either from the pipes of 

 the house, or leaking into the cellars from the main pipes in 

 the streets. With this lamp we may advance with perfect se- 

 curity into rooms filled with the explosive mixture, and thus 

 protect ourselves against every possible risk. 



Art. XVII. — Description of a Machine applied to a Gig, for 

 Measuring Distances. By William Edgeworth, Esq. 

 C. E. and M. R. I. A. Communicated by the Author. 



There have been a variety of machines applied to carriages, 

 for counting the revolutions of the wheel, and thus measuring 

 the distance travelled over ; but all that I have ever heard of, 

 wear rapidly and get out of order, as the machinery is placed 

 on or near the axletree, on parts of the carriage that are sub- 

 ject to jar. 



For this reason, I was determined to suspend on the body 

 of the carriage, which was on springs, all the machinery re- 

 quisite for registering the revolutions of the wheel. I availed 



