THE EKGLISH TELEGRAPH 633 
Visible to the others. The powers of the stations in this 
Vespect are exceedingly various. The station on Putney 
iieath, communicating with Chelsea, is generally rendered 
feeless during easterly winds by the smoke of London, which 
iills the valley of the Thames between this spot and Chelsea 
•ospital ; or, more commonly between the shorter distance 
the Admiralty and Chelsea. Dead flats are found to be 
Universally unfavourable ; and generally stations are useless 
*>early in the proportion of tlie miles of dead flat looked 
®ver. On the contraiy, stations between hill and iiill 
inoking across a valley, or series of valleys, are mostly clear, 
®nd water surfaces are found to produce fewer obscure days 
^an land in any situation. The period least favourable oi 
same day is an hour or two before and after the sun’ 
Passage of the meridian, particularly on dead levels, whee 
*^6 play of the sun’s rays on the rising exhalations rendes 
*^istant vision exceedingly obscure. The tranquillity of the 
feorning and evening are ascertained to be the most favour- 
*ble hours for observation. 
A message from London to Portsmoutli, is usually transmit- 
in about fifteen minutes ; but, by an experiment tried for 
^Qe purpose, a single signal has been transmitted to Plymouth 
®ttd back again in three minutes, which, by the Telegraph 
Jt>ute, is at least five hundred miles. In this instance, 
however, notice had been given to make ready, and every 
feptain was at 'his post to receive and return the signals. 
Die progress was at tire rate of one hundred and seventy 
Diles in a minute, or three miles per second, or three 
’Jconds at each station j a rapidity truly wonderful 1 The 
English telegraph consists of a large frame, in which are 
placed and worked six shutters, marked in the plate a, b, c, 
e, f, by means of ropes pulled in the manner of bell 
*^''Pes. The number Cf signals produced by it is sixty-three 
'^by which are represented the ten digits, die letters of the 
Alphabet, many generic words, and all the numbers which 
^n be expressed by sixty-three variations of the digits. 
^ be signals are sufficiently various to express any three or 
our words in twice as many changes of the shutters. 
, The observers at these telegraphs are not expected to 
!^®6p their eye constantly at the glass, but look only every 
_ Ve minutes for the signal to make ready The telescopes 
Dolland’s Achromatics, which possess no recommenda- 
£ e 
